CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a divisional of prior application Ser. No. 12/985,876, filed Jan. 6, 2011, now U.S. Pat. No. 7,962,818, issued Jun. 14, 2011;
     Which was a divisional of prior application Ser. No. 12/840,928, filed Jul. 21, 2010, now U.S.; 7,890,829, issued Feb. 15, 2011;   Which was a divisional of prior application Ser. No. 12/563,775, filed Sep. 21, 2009, now U.S. Pat. No. 7,793,182, issued Sep. 7, 2010;   Which was a divisional of prior application Ser. No. 11/954,403, filed Dec. 12, 2007, now U.S. Pat. No. 7,613,970, issued Nov. 3, 2009;   Which was a divisional of prior application Ser. No. 11/293,061, filed Dec. 2, 2005, now U.S. Pat. No. 7,328,387, issued Feb. 5, 2008.   Which claims priority from Provisional Application No. 60/634,842, filed Dec. 10, 2004.   

     This application is related to U.S. patent application Ser. No. 10/983,256, filed Nov. 4, 2004, titled “Removable and Replaceable Tap Domain Selection Circuitry,” and U.S. patent application Ser. No. 11/258,315, filed Oct. 25, 2005, titled “2 Pin Bus.” 
    
    
     BACK GROUND OF THE DISCLOSURE 
     This disclosure relates in general to IC or core signal interfaces and particularly to IC or core signal interfaces related to test, emulation, debug, trace, and function operations. 
     DESCRIPTION OF THE RELATED ART 
       FIG. 1  illustrates an IC or embedded core circuit  100  containing functional circuits  102 , IEEE 1149.1 (JTAG) circuit  104 , and emulation, debug, and/or trace circuit  106 . The functional circuit  102  communicates externally of the IC or core via bus terminals  103 . The 1149.1 circuit communicates externally of the IC or core via bus terminals  108  and internally to the functional circuit  102  via bus  114 . The emulation, debug, and/or trace circuit communicates externally of the IC or core via bus terminals  110  and internally to the functional circuit  102  via bus  112 . As seen, the 1149.1 circuit  104  comprises data registers  116 , instruction register  118 , mux  122 , falling clock edge FF  124 , tristate buffer  128 , and test access port (TAP) controller  120 . The 1149.1 circuit  104  has external terminals on bus  108  for a test data input (TDI)  132 , a test mode select (TMS)  134 , a test clock (TCK)  136 , a test reset (TRST)  138 , and test data output (TDO)  140  signals. The data registers  116  comprise a set of serially accessible registers, some providing input and output to functional circuit  102  via bus  114 . The registers can be used for performing boundary scan test operations on functional bus terminals  103 , performing internal scan testing of the functional circuit  102 , and/or supporting debug, trace, and/or emulation operations on the functional circuit  102 . As indicated, a power up clear (PUC) circuit  130 , which is a circuit for resetting or initializing a given circuit upon application of power, may be used instead of or in combination with the TRST terminal to set the state of the Tap  120  in the 1149.1 circuit  104 . 
       FIG. 2  illustrates an IC  200  containing four JTAG circuits  104 . One JTAG circuit  104  is associated with non-core circuitry in the chip and is referred to as the Chip Tap Domain  202 . The other JTAG circuits  104  are each associated with circuitry of an embedded core and are referred to as Core Tap Domains  204 - 208 . The Tap domains  202 - 208  are shown in Tap domain region  201 . The JTAG circuit  104  bus terminals  108  of each Tap domain  202 - 208  may be coupled to chip terminals  212 - 220  via a Tap Domain Selection circuit  210 . Once coupled, the JTAG circuit  104  of a selected Tap domain  202 - 208  may be accessed via chip terminals  212 - 220  for test, debug, trace, and/or emulation operations by an external controller. A variety of Tap domain selection circuits  210  that could be used in this example are described in a referenced paper entitled “An IEEE 1149.1 Based Test Access Architecture for ICs with Embedded Cores” authored by Whetsel and presented at the IEEE International Test Conference in November of 1997. 
     When using a Tap Domain Selection circuit as shown in  FIG. 2  it is best to remove the TDO tristate buffer  128  of JTAG circuits  104 , if possible, to allow the flip flop  124  of the JTAG circuit  104  to directly drive the TDO signal on the interface  108  between the JTAG circuit  104  and the Tap Domain Selection circuit. This practice prevents floating (i.e. tristate) TDO signal lines inside the IC/core. 
       FIG. 3  illustrates an IC or embedded core circuit  300  containing functional circuits  102 , JTAG circuit  302 , and emulation, debug, and/or trace circuit  106 . The IC  300  is identical to IC  100  of  FIG. 1  with the exception that JTAG circuit  302  is different from JTAG circuit  104 . The difference is that the JTAG circuit  302  includes a flip flop (FF) in the TCK path to the Tap  120 . The D input of the FF is coupled to the TCK signal  136 , the Q output of the FF is coupled to the TCK input of the Tap  120 , and the clock input of the FF is coupled to a functional clock (FCK) output  306  from function circuit  102 . The Q output of the FF is also output as a return clock (RCK) output on terminal  308  of bus  310 . The difference between bus  108  of  FIG. 1  and bus  310  of  FIG. 3  is the additional RCK signal  308 . The use of FF  304  in JTAG circuit  302  forces the TCK signal from an external controller to be sampled by the FCK  306  before it is allowed to be input to the Tap  120 . The RCK output  308  to the external controller indicates to the external controller when the TCK signal has been sampled by the FCK. For example, if the external controller sets TCK  136  high, the RCK signal  308  output will go high when the FCK  306  clocks the TCK into FF  304 . When the controller sees a high on RCK, it can set TCK low and again wait for the RCK to indicate when the low on TCK has been clocked into the FF  304  by the FCK  306 . This method of operating the JTAG circuit  302  allows the external controller to synchronize the operation of the TCK signal to the frequency of the FCK signal, using the handshaking operation provided by the RCK signal. This TCK handshaking technique, while not compliant to the IEEE 1149.1 standard, is being designed into embeddable cores provided by ARM Ltd. Thus the technique must be adopted in ICs that use embedded cores from ARM Ltd. 
       FIG. 4  illustrates an IC  400  containing four JTAG circuits  302 . One JTAG circuit  302  is associated with non-core circuitry in the chip and is referred to as the Chip Tap Domain  402 . The other JTAG circuits  302  are each associated with circuitry of an embedded core and are referred to as Core Tap Domains  404 - 408 . The Tap domains  402 - 408  are shown in Tap domain region  401 . The JTAG circuit  302  bus terminals  310  of each Tap domain  402 - 408  may be coupled to chip terminals  412 - 422  via a Tap Domain Selection circuit  410 . Once coupled, the JTAG circuit  302  of a selected Tap domain  402 - 408  may be accessed via chip terminals  412 - 422  for test, debug, trace, and/or emulation operations by an external controller. The Tap domain selection circuit  410  is similar to the Tap domain selection circuit  210  of  FIG. 2  with the exception that it includes additional circuitry for coupling the RCK  308  output of a selected Tap domain  402 - 408  to the RCK chip terminal  422 . 
     SUMMARY OF THE DISCLOSURE 
     In a first aspect of the present disclosure, a method and apparatus is described in  FIGS. 5-30  for addressing, instructing, and accessing Tap Domains in ICs or core circuits using a reduced number of signal terminals. In a second aspect of the present disclosure, a method and apparatus is described in  FIGS. 31-34  for accessing a target Tap domain in an IC or core circuit using a reduced number of signal terminals. In a third aspect of the present disclosure, a method and apparatus is described in  FIGS. 35-36  for reducing the number of IC or core signal terminals involved with emulation, debug, and trace operations. In a fourth aspect of the present disclosure, a method and apparatus is described in  FIGS. 37-40  for reducing the number of IC or core signal terminals involved in function I/O operations. In a fifth aspect of the present disclosure, a method and apparatus is described in  FIGS. 41-49  for selectively using either the 5 signal interface of  FIG. 41  or the 3 signal interface of  FIG. 8 . 
    
    
     
       DESCRIPTION OF THE VIEWS OF THE DRAWINGS 
         FIG. 1  illustrates an IC or core with a standard JTAG circuit Tap Domain. 
         FIG. 2  illustrates an IC or core having plural standard JTAG circuit Tap Domains and Tap Domain selection circuitry. 
         FIG. 3  illustrates an IC or core with a non-standard JTAG circuit Tap Domain. 
         FIG. 4  illustrates an IC or core having plural non-standard JTAG circuit Tap Domains and Tap Domain selection circuitry. 
         FIG. 5  illustrates an IC or core including the addressable Tap Domain Selection circuit of the present disclosure. 
         FIG. 6  illustrates more detail view of the addressable Tap Domain Selection circuit of the present disclosure. 
         FIG. 7A  illustrates the operation of the Tap Domain Selection Circuit of the present disclosure in response to first, second, and third protocols. 
         FIG. 7B  illustrates sequences of first, second, and third protocols of the present disclosure. 
         FIG. 8  illustrates a detail view of the Addressable Tap Domain Selection Circuit interfaced to plural Tap Domains. 
         FIG. 9  illustrates a detail view of the Address circuit of the present disclosure. 
         FIG. 10  illustrates a detail view of the Instruction circuit of the present disclosure. 
         FIG. 11  illustrates a detail view of the Tap Linking circuit of the present disclosure. 
         FIG. 12  illustrates the Reset, Address, and Instruction Controllers of the present disclosure. 
         FIG. 13  illustrates a detail view of the Hard and Soft reset controllers and sequences of the present disclosure. 
         FIG. 14  illustrates the state diagram of the Address and Instruction controller of the present disclosure. 
         FIGS. 15A and 15B  illustrate detail views of the Address and Instruction controller of the present disclosure. 
         FIG. 16  illustrates the state diagram of the standard IEEE 1149.1 Tap controller. 
         FIG. 17  illustrates the connection between an external controller and the circuitry of the present disclosure existing in ICs or core circuits. 
         FIG. 17A  illustrates the connection between an external controller and the circuitry of the present disclosure existing in stacked die circuits. 
         FIG. 18  illustrates the connection between an external controller and a circuit containing the present disclosure that is interfaced to standard legacy JTAG circuits in ICs or cores. 
         FIG. 19  illustrates the connection between an external controller and a circuit containing the present disclosure that is interfaced to standard legacy JTAG circuits in ICs or cores, and to ICs or cores that include the circuitry of the present disclosure. 
         FIG. 20  illustrates the TDI/TDO connection between I/O buffers of the present disclosure existing in an external controller and in target ICs or cores. 
         FIG. 21  illustrates the TMS/RCK connection between I/O buffers of the present disclosure existing in an external controller and in target ICs or cores. 
         FIG. 22  illustrates the data input circuit of I/O buffers of the present disclosure. 
         FIG. 23A-23D  illustrates the operation of the output buffer of the I/O circuits of the present disclosure existing in an external controller and a target IC or core. 
         FIG. 24  illustrates the four cases of signal flow between the I/O buffer of an external controller and the I/O buffer of a target IC or core. 
         FIGS. 25-28  illustrate different sequences of performing first and second protocols of the present disclosure. 
         FIG. 29  illustrates the sequence of performing a second protocol, then a third protocol, then a first protocol according to the present disclosure. 
         FIG. 30  illustrates the sequence of performing a second protocol, then a first protocol according to the present disclosure. 
         FIG. 31  illustrates an interface between an external controller and a standard JTAG circuit within an IC or core. 
         FIG. 32  illustrates a reduced interface between an external controller and a standard JTAG circuit within an IC or core according to the present disclosure. 
         FIG. 32  illustrates an interface between an external controller and a non-standard JTAG circuit within an IC or core. 
         FIG. 33  illustrates a reduced interface between an external controller and a non-standard JTAG circuit within an IC or core according to the present disclosure. 
         FIG. 35  illustrates an interface between an external controller and emulation, debug, and trace circuits within an IC or core. 
         FIG. 36  illustrates a reduced interface between an external controller and emulation, debug, and trace circuits within an IC or core according to the present disclosure. 
         FIG. 37  illustrates a functional interface between first and second functional circuits of an IC or core. 
         FIG. 38  illustrates a reduced functional interface between first and second functional circuits of an IC or core according to the present disclosure. 
         FIG. 39  illustrates a functional interface between a master functional circuit in a first IC or core and slave functional circuits in second and third ICs or cores. 
         FIG. 40  illustrates a reduced functional interface between a master functional circuit in a first IC or core and slave functional circuits in second and third ICs or cores according to the present disclosure. 
         FIG. 41  illustrates an Addressable Tap Domain Selection circuit similar to that of  FIG. 8  with the TDI, TDO, TMS, and RCK signals coupled via buffers to externally accessible signal terminals. 
         FIG. 42  illustrates a group of target devices on a board or other substrate, each target device including the Addressable Tap Domain Selection Circuit and its associated 5 pin TCK, TDI, TDO, TMS, and RCK interface, as well as Tap Domain Region. 
         FIG. 43  illustrates the legacy target devices of  FIG. 18 , each including the standard IEEE 1149.1 TRST, TCK, TMS, TDI, and TDO terminals, and optionally the non-standard RCK terminal. 
         FIG. 44  illustrates an Addressable Tap Domain Selection circuit that has been designed in an IC or core to selectively use either the 5 signal interface of  FIG. 41  or the 3 signal interface of  FIG. 8 . 
         FIG. 45  illustrates an example design of the Interface Select Circuit of  FIG. 44 . 
         FIG. 46  illustrates an example of the configuration of the Interface Select Circuit when it is in the 3 signal interface mode. 
         FIG. 47  illustrates an example of the configuration of the Interface Select Circuit when it is in the 5 signal interface mode. 
         FIG. 48  illustrates a group of target devices on a board or other substrate, each target device including the Addressable Tap Domain Selection Circuit of  FIG. 44  and its selectable 3 or 5 pin interface. 
         FIG. 49  illustrates the legacy target devices of  FIG. 18 , each including the standard IEEE 1149.1 TRST, TCK, TMS, TDI, and TDO terminals, and optionally the non-standard RCK terminal and the Addressable Tap Domain Selection Circuit of  FIG. 44  operating in either the 3 or 5 signal interface mode. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 5  illustrates an IC  500  including the test, debug, trace, and/or emulation architecture of the present disclosure. The architecture includes a Tap domain region  522  comprising individual Tap domains  502 - 508 . Each Tap domain  502 - 508  includes a JTAG circuit  510 , which can be either the conventional JTAG circuit  104  or the modified JTAG circuit  302 . Each JTAG circuit  510  is coupled to an Addressable Tap Domain Selection circuit  514  via buses  512 . If a JTAG circuit  510  is a conventional JTAG circuit  104 , its bus  512  will be the same as bus  104 . If JTAG circuit  510  is a modified JTAG circuit  302 , its bus  512  will be the same as bus  310 . 
     Addressable Tap domain selection circuit  514  is coupled to external IC terminal signals TCK  516 , TMS/RCK  518 , and TDI/TDO  520 . The TCK  516  signal is the same as the TCK  214  signal shown in  FIGS. 2 and 4 , with the exception that, in addition to operating as a clock input to the IC  500  from an external controller, the TCK  516  of  FIG. 5  can also be operated as a data input and a control input from the external controller, according to a first protocol defined by the present disclosure. The TMS/RCK  518  signal is a signal defined by the present disclosure to operate as a signal that can serve as either an input signal to the IC  500  from an external controller or as a simultaneous input/output between the IC  500  and the external controller. Similarly, the TDI/TDO  520  signal is a signal defined by the present disclosure to operate as a signal that can serve as either an input signal to the IC from an external controller or as a simultaneous input/output between the IC and the external controller. 
       FIG. 6  illustrates in more detail the connections between the Addressable Tap Domain Selection circuit  514  and the Tap Domains  510  in Tap domain region  522 . Selection Circuit  514  is coupled externally of the IC via signal terminals TCK  516 , TMS/RCK  518 , and TDI/TDO  520 . As seen, pull up elements, pull down elements, or other state holding elements  602  such as bus holders are preferably connected to these terminals to allow them to be set to a known state when they are not externally driven. Selection circuit  514  is coupled to the Tap domains  1 - 4  in Tap region  522  via TDI  1 - 4  signals  132 , TDO  1 - 4  signals  140 , TMS  1 - 4  signals  134 , RCK  1 - 4  signals  308 , TCK signal  136 , and TRST signal  138 . 
     In this example, the Tap region  522  is assumed to contain four Tap domains  510  with all four Tap domains  510  being modified Tap domain  302  types. Thus each of the four Tap domains  510  will have a RCK  308  output ( 1 - 4 ) to the Selection circuit  514 . In another example, the Tap region  522  may contain four Tap domains  510 , each being conventional Tap domain  104  types, which would eliminate the need for the RCK signal connections to the Selection circuit  514 . In still another example, the Tap region  522  may contain mixtures of modified Tap domains  302  requiring RCK signal connections and conventional Tap domains  104  not requiring RCK signal connections. Also while this example shows four Tap domains  510  in Tap region  522 , a lesser or greater number of Tap domains  510  ( 104  or  302  types) may exist in Tap region  522 . 
     The purpose of the Addressable Tap Domain Selection circuit  514  is to allow for an external controller coupled to terminals  516 - 520  to input an address to the Selection circuit  514  of the IC then load an instruction into the Selection circuit  514  of the IC. The loaded instruction may provide a plurality of control functions within the IC, at least one control function being to control which one or more Tap domains  510  in Tap region  522  is selected for access by the external controller. 
     In applications of the present disclosure, a plurality of ICs may be coupled, at some point, to an external controller via terminals  516 - 520 , as depicted in  FIG. 17 . Each Selection circuit  514  of each IC will have a local and a global address that enables it to input an instruction. The local address, as defined by the present disclosure, is an address capable of uniquely identifying one Selection circuit  514  within a given IC from any other Selection circuit  514  within the same or different IC. The global address is defined as an address that commonly identifies all Selection circuits  514  within any number of ICs. All the Selection circuits  514  of ICs will input the address from the external controller, but only the Selection circuit  514  having an address that matches either the local or global address input will be enabled to further input the instruction. Thus Selection circuits  514  not matching the address input will not input the instruction. These non-addressed Selection circuit  514  will be placed in an idle condition until the next address and instruction input sequence occurs. 
       FIG. 7A  illustrates the high level operation of the Addressable Tap Domain Selection circuit  514  in response to first, second, and third protocols applied to the Selection circuit  514  via terminals TCK  516 , TMS/RCK  518 , and TDI/TDO  520 . The first protocol uses terminals TCK  516  and TMS/RCK  518  to; (1) move the Selection circuit  514  from the Tap Domain Access state  708  to either the Hard Reset state  702  or Soft Reset state  704 , (2) move between the Hard Reset state  702  and the Soft Reset state  704 , (3) move from the Address &amp; Instruction input state  706  to either the Hard  702  or Soft  704  Reset states, or (4) remain in either the Hard  702  or Soft  704  Reset state. The second protocol uses terminals TCK  516 , TMS/RCK  518 , and TDI/TDO  520  to move the Selection circuit  514  from the Hard or Soft reset states into the Address &amp; Instruction input state  706  or, if in the Address &amp; Instruction input state  706 , to remain in the Address &amp; Input state  706 . The third protocol uses terminals TCK  516 , TMS/RCK  518 , and TDI/TDO  520  to move the Selection circuit  514  from the Address &amp; Instruction Input state  706  into the Tap Domain Access state  708  or, if in the Tap Domain Access state  708 , to remain in the Tap Domain Access state  708 . 
     Entry into the Hard reset state  702  fully resets all circuits in both the Selection circuit  514  and the Tap domains  510  in Tap region  522 . Entry into the Soft reset state  704  does not fully reset the Selection circuit  514  or Tap domains  510 . The Hard and Soft reset states  702 - 704  serve as starting points for communication sessions using the second protocol in state  706 . The Hard and Soft reset states  702 - 704  also serve as ending points for communication sessions using the second protocol in state  706  and using the third protocol in state  708 . Entry into the Address &amp; Instruction input state  706  starts a communication session using the second protocol for inputting the above mentioned address and instruction. Entry into the Tap Domain Access state  708  starts a communication session using the third protocol for accessing the selected Tap Domain(s)  510 . 
       FIG. 7B  illustrates examples of “starting and stopping” sequences of first, second, and third, and sequences of first and second protocols. 
     Protocol sequence A  712  illustrates the sequence of; (1) initially performing a first protocol to enter into or remain in the Hard Reset state  702 , (2) switching from performing the first protocol to performing the second protocol to cause entry into the Address &amp; Instruction input state  706  to input an address and instruction, the instruction in this case selecting one or more Tap Domain(s)  510  for access, (3) switching from performing the second protocol to performing the third protocol to enter the Access Tap Domain state  708 , for accessing the Tap domain(s)  510  selected by the loaded instruction, and (4) switching from performing the third protocol, after the Tap domain access has been completed, to performing the first protocol to enter the Hard Reset state  702 , which terminates the protocol sequence and resets the Selection circuit  514  and the Tap Domains  510 . 
     Protocol sequence B  714  illustrates the sequence of; (1) initially performing a first protocol to enter into or remain in the Hard Reset state  702 , (2) switching from performing the first protocol to performing the second protocol to cause entry into the Address &amp; Instruction input state  706  to input an address and instruction, the instruction in this case selecting one or more Tap Domain(s)  510  for access, (3) switching from performing the second protocol to performing the third protocol to enter the Tap Domain Access state  708 , for accessing the Tap domain(s)  510  selected by the loaded instruction, and (4) switching from performing the third protocol, after the Tap domain access has been completed, to performing the first protocol to enter the Soft Reset state  704 , which terminates the protocol sequence. 
     Protocol sequence C  716  illustrates the sequence of; (1) initially performing a first protocol to enter into or remain in the Soft Reset state  704 , (2) switching from performing the first protocol to performing the second protocol to cause entry into the Address &amp; Instruction input state  706  to input an address and instruction, the instruction in this case selecting one or more Tap Domain(s)  510  for access, (3) switching from performing the second protocol to performing the third protocol to enter the Tap Domain Access state  708 , for accessing the Tap domain(s)  510  selected by the loaded instruction, and (4) switching from performing the third protocol, after the Tap domain access has been completed, to performing the first protocol to enter the Soft Reset state  704 , which terminates the protocol sequence. 
     Protocol sequence D  718  illustrates the sequence of; (1) initially performing a first protocol to enter into or remain in the Soft Reset state  704 , (2) switching from performing the first protocol to performing the second protocol to cause entry into the Address &amp; Instruction input state  706  to input an address and instruction, the instruction in this case selecting one or more Tap Domain(s)  510  for access, (3) switching from performing the second protocol to performing the third protocol to enter the Tap Domain Access state  708 , for accessing the Tap domain(s)  510  selected by the loaded instruction, and (4) switching from performing the third protocol, after the Tap domain access has been completed, to performing the first protocol to enter the Hard Reset state  702 , which terminates the protocol sequence and resets the Selection circuit  514  and the Tap Domains  510 . 
     Protocol sequence E  720  illustrates the sequence of; (1) initially performing a first protocol to enter into or remain in the Hard Reset state  702 , (2) switching from performing the first protocol to performing the second protocol to cause entry into the Address &amp; Instruction input state  706  to input an address and instruction, and (3) switching from performing the second protocol to performing the first protocol to enter the Hard Reset state  702 , which terminates the protocol sequence and resets the Selection circuit  514  and Tap Domains  510 . 
     Protocol sequence F  722  illustrates the sequence of; (1) initially performing a first protocol to enter into or remain in the Hard Reset state  702 , (2) switching from performing the first protocol to performing the second protocol to cause entry into the Address &amp; Instruction input state  706  to input an address and instruction, and (3) switching from performing the second protocol to performing the first protocol to enter the Soft Reset state  704 , which terminates the protocol sequence. 
     Protocol sequence G  724  illustrates the sequence of; (1) initially performing a first protocol to enter into or remain in the Soft Reset state  704 , (2) switching from performing the first protocol to performing the second protocol to cause entry into the Address &amp; Instruction input state  706  to input an address and instruction, and (3) switching from performing the second protocol to performing the first protocol to enter the Soft Reset state  704 , which terminates the protocol sequence. 
     Protocol sequence H  726  illustrates the sequence of; (1) initially performing a first protocol to enter into or remain in the Soft Reset state  704 , (2) switching from performing the first protocol to performing the second protocol to cause entry into the Address &amp; Instruction input state  706  to input an address and instruction, and (3) switching from performing the second protocol to performing the first protocol to enter the Hard Reset state  702 , which terminates the protocol sequence and resets the Selection circuit  514  and Tap Domains  510 . 
       FIG. 8  illustrates the Addressable Tap Domain Selection circuit  514  in more detail. The Selection circuit  514  includes a TDI/TDO I/O circuit  802 , a TMS/RCK I/O circuit  804 , Reset, Address &amp; Instruction controllers  806 , an address circuit  808 , an instruction circuit  810 , and a Tap Linking circuit  812 . The I/O circuits  802  and  804  each include an output buffer  814 , a resistor  816 , and a data input circuit  818 . 
     The output buffer  814  of I/O circuit  802  has an input coupled to the TDO output signal  820  from Linking circuit  812 , an output coupled to one lead of resistor  816 , and a 3-state control input coupled to the output enable  1  (OE 1 ) signal  822  from Linking circuit  812 . The other lead of resistor  816  is coupled to the TDI/TDO terminal  520 . The data input circuit  818  has a first input coupled to the TDI/TDO terminal  520 , a second input coupled to the TDO signal  820 , and an TDI output signal  824  coupled to inputs of the Address circuit  808 , Instruction circuit  810 , and Linking circuit  812 . 
     The output buffer  814  of I/O circuit  804  has an input coupled to the RCK output signal  826  from Linking circuit  812 , an output coupled to one lead of resistor  816 , and a 3-state control input coupled to the output enable  2  (OE 2 ) signal  822  from And gate  846 . The other lead of resistor  816  is coupled to the TMS/RCK terminal  518 . The data input circuit  818  has a first input coupled to the TMS/RCK terminal  518 , a second input coupled to the RCK signal  826 , and an TMS output signal  830  coupled to inputs of the Linking circuit  812  and Controllers  806 . 
     The Reset, Address, and Instruction Controllers  806  has inputs coupled to TCK terminal  516 , TMS signal  830 , an Address Match (AM) signal  838  output from Address circuit  808 , and to a function reset and/or power up clear signal  844 . The Controller  806  outputs instruction control (IC) signals  832  to Instruction Circuit  810 , an address clock (AC) signal  834  to Address Circuit  808 , a hard reset (HR) signal  836  to Instruction Circuit  810  and to the TRST input of Tap Domains  510  in Tap Region  522 , and an Enable signal  842  to And gates  848  and  850 . 
     And gate  850  inputs the Enable signal  842  and the TCK  516  signal and outputs a TCK  136  signal to Tap Domains  510  in Tap Region  522 . When Enable signal  842  is high, And gate  850  couples TCK signal  516  to TCK signal  136 . When Enable is low, TCK signal  136  is forced low. 
     And gate  848  inputs the Enable signal  842  and a signal  846  from instruction output bus  840  and outputs the OE 2  signal  828  to output buffer  814  of I/O circuit  804 . When Enable signal  842  is high, And gate  848  couples instruction output signal  846  to the OE 2  signal  828 . When Enable is low, OE 2   828  is forced low, disabling output buffer  814  of I/O circuit  804 . If the Tap Domain  510  selected for access is a conventional Tap Domain, i.e. no RCK, the loaded instruction will output a low on instruction signal  846  to disable output buffer  814  from outputting RCK signals  826  onto TMS/RCK  518  when Enable signal  842  is set high. If the Tap Domain  510  selected for access is a Tap Domain that uses the RCK signal, the loaded instruction will output a high on instruction signal  846  to enable output buffer  814  for outputting RCK signals  826  onto TMS/RCK  518  when Enable signal  842  is set high. 
     The Linking Circuit  812  is coupled to the I/O circuits  802 - 804  and to the Controllers  806  as mentioned above. The Linking Circuit is further coupled to instruction output bus  840  of Instruction Circuit  810  to input instruction control, and to the Tap Domains  510  of Tap Region  522 , via signals TDI 1 - 4   132  output, TDO 1 - 4   140  input, TMST- 4   134  output, and RCK 1 - 4   308  input signals. 
       FIG. 9  illustrates an example of how the Address Circuit  808  may be designed. The address circuit consists of an address shift register  902 , an address compare circuit  904 , and a local and global address circuit  906 . The shift register  902  responds to the address clock signal  834  to shift in an address from the TDI  824 . The compare circuit  904  operates to compare the address shifted into the shift register  902  to the local and global addresses output from local and global address circuit  906 . The compare circuit outputs the result of the compare on the address match signal  838 . Since the global address will be the same for all Selection circuits  514 , it will be fixed by design. The unique local address may be provided by the blowing of electronic fuses, an address programmed into a programmable memory, an address functionally written into a memory, an address shifted into a shift register, an address established on externally accessible device (IC/core) terminals, or by any other suitable address supplying means. A local address may not share the same address as the global address. The compare circuit is capable of comparing the data shifted into the address register  902  against both the local address and the global address output from address circuit  906 . If a match occurs between the data in the address register  902  and the local or global address, the address match signal  838  will be set high. If desired, two address match outputs, one for indicating a local address match and another for indicating a global address match, could be used instead of the single address match signal  838 . 
       FIG. 10  illustrates an example of how the instruction circuit  810  may be designed. The instruction circuit consist of an instruction shift register  1002 , instruction decode logic  1004 , and an instruction update register  1006 . The shift register  1002  responds to an instruction clock (I-Clock) signal from IC bus  832  to shift in an instruction from the TDI  824  input. The decode logic  1004  operates to decode the instruction shifted into the shift register  1002  and to output the decode to the update register  1006 . The update register  1006  stores the instruction decode in response to an instruction update (I-Update) signal from IC bus  832 . The stored instruction decode is output from the update register  1006  on instruction output bus  840 . The hard reset (HR) signal  836  is input to both the shift register  1002  and update register  1006  to reset the registers to known states when the hard reset signal from Controller  806  is active low. 
       FIG. 11  illustrates an example of how the Linking Circuit  812  is interfaced to the Tap Domains  510  of Tap Region  522 . The Linking Circuit  812  comprises TDI multiplexer circuitry  1102 , TDO multiplexer  1104 , TMS gating circuit  1110 , RCK selection circuit  1106 , and a Tap Tracker circuit  1114 . 
     The TDI multiplexer circuitry  1102  comprises four individual multiplexers for TDI 1 , TDI 2 , TDI 3 , and TDI 4  as shown in the dotted line box. Each individual multiplexer is coupled to TDISEL signals from instruction output bus  840 . The TDI output of each multiplexers (TDI 1 -TDI 4 ) is coupled to a respective TDI input of Tap domains  1 - 4   510 . In response to the TDISEL input, the TDI multiplexers allow any of the Tap domains to be coupled to the TDI signal  824 , or to the TDO outputs ( 1 - 4 ) of any other Tap Domain  1 - 4   510 . The TDO multiplexer  1104  is a single multiplexer that can select any of the TDO outputs (TDO 1 - 4 ) from a Tap Domains  1 - 4   510  to be coupled to the TDO signal  820  in response to TDOSEL signals from the instruction output bus  840 . As can be seen, using the above described TDI and TDO multiplexer circuits, the Tap Domains  1 - 4   510  may be individually selected between TDI  824  and TDO  820 , or selectively linked serially together between TDI  824  and TDO  820 . 
     TMS gating circuit  1110  receives TMSSEL 1 - 4  signals from instruction output bus  840  to allow any of the TMS 1 - 4  inputs of Tap Domain  1 - 4   510  to be coupled to the TMS signal  830 . A high on a TMSSEL signal will couple TMS  830  to a respective TMS input of a Tap Domain  510 . A low on a TMSSEL signal will force a respective TMS input of a Tap Domain  510  low. 
     The TCK signal  136  is coupled to all TCK inputs of Tap Domains  510 . When the Enable signal  842  from Reset, Address, and Instruction Controllers  806  is high, TCK  136  is coupled to the TCK terminal  516  via And gate  850  of  FIG. 8 . 
     The HR input  836  from Reset, Address, and Instruction Controllers  806  is input to the TRST input of the Tap Domains  510  of Tap Region  522 . 
     RCK selection circuit  1106  receives RCKSEL signals from instruction output bus  840  to allow any one or a combination of RCK  1 - 4  outputs of Tap Domains  1 - 4   510  to be coupled to the RCK signal  826 . In response to the RCKSEL signals, an RCK  1 - 4  from any Tap Domain  1 - 4   510  may be coupled to RCK  826 , a combination of RCK signals may be coupled to RCK  826  from voting circuit  1116 , or the RCK signal  826  may be coupled to a static logic level (a HI in this example) when no RCK is used by a Tap Domain  510 . The absence of an RCK signal from a Tap Domain is indicated by dotted line. The voting circuit  1116  is used whenever two or more Tap Domains each having an RCK are linked together for serial access. In this example, the AND gate of the voting circuit  1116  detects the condition where both RCKs are high and the OR gate of the voting circuit  1116  detects the condition where both RCKs are low. As mentioned previously, RCKs are handshaking signals fed back to the external controller to indicate when a Tap Domain of a core have synchronized the TCK signal level input from the external controller with a functional clock of the core. 
     The Tap Tracker circuit  1114  is an IEEE 1149.1 Tap state machine that is used in Linking Circuit  812  to track the states of the Tap Domain(s) being accessed in the Tap Region  522 . The main function of the Tap Tracker  1114  is to control the output enable  1  (OE 1 ) signal to the output buffer  814  of I/O circuit  802 . The Tap Tracker will output a signal on OE 1  to enable the output buffer to output onto terminal TDI/TDO  520  whenever the Tap Tracker (and selected Tap Domain(s)) are in the Shift-DR or Shift-IR states (see Tap Diagram of  FIG. 16 ). In these states, the selected Tap Domains will be shifting data from TDI  824  to TDO  820  and the I/O circuit  802  will be in its mode of simultaneously inputting and outputting this shift data on TDI/TDO terminal  520 . When not in the Shift-DR or Shift-IR states, the Tap Domains will not be shifting data and the OE 1  signal will be set to disable output buffer  814  of I/O circuit  802  from operating in the simultaneous input and output mode on TDI/TDO terminal  520 . While output buffer  814  is disabled, I/O circuit  802  operates in an input only mode to input data appearing of the TDI/TDO terminal  520 . As seen in  FIG. 11 , the Tap Tracker inputs the TCK signal  136 , the HR signal  836  (as its TRST input), and TMST- 4  signals via OR gate  1112 . 
       FIG. 12  illustrates a block diagram of the Hard and Soft Reset controller  1202  and the Address and Instruction Controller  1204  within the Reset, Address, and Instruction Controllers Circuit  806 . The Hard and Soft Reset controller  1202  inputs the TCK signal  516 , the TMS signal  830 , and the functional reset and/or power up clear signal  844 , and outputs the Hard Reset (HR)  836  signal and a Soft Reset signal  1206 . The Hard Reset (HR)  836  signal is input to the Instruction Circuit  810  of  FIG. 8  and the Tap Domains  510  of Tap Region  522 . The Address and Instruction Controller  1204  inputs the TCK signal  516 , the TMS signal  830 , the Address Match (AM) signal  838 , and the Soft Reset signal  1206  from controller  1202 , and outputs the instruction control (IC) signals  832  to instruction circuits  810 , address clock (AC) signal  834  to address circuit  808 , and the Enable signal  842  to And gates  848  and  850 . As indicated by dotted line, the Hard and Soft Reset controllers  1202  respond to the TCK  516  and TMS  830  inputs according to the previously mentioned first protocol, and the Address and Instruction controller  1204  responds to the TCK  516  and TMS  830  inputs according to the previously mentioned second protocol. 
       FIG. 13  illustrates an example of how the Hard and Soft Reset controller  1202  may be designed. The Hard and Soft Reset controller  1202  consists of two separate controllers, a hard reset controller  1302  and a soft reset controller  1304 . The hard reset controller  1302  consists of inverters  1306  and  1308 , Or gate  1310 , and flip flop pairs  1312  and  1314  connected as shown. Flip flop pairs  1312  and  1314  each include a rising edge clock flip flop feeding data to a falling edge flip flop, so it takes both a rising and falling clock edge to propagate an input to the output of the pair. The soft reset controller  1304  consists of inverters  1316  and  1318 , flip flop pairs  1320  and  1322 , and And gate  1324  connected as shown. Again the flip flop pairs  1320  and  1322  include a rising edge clock flip flop feeding data to a falling edge clock flip flop. In response to a low input on the function reset and/or power up clear input  844 , flip flop pairs  1312  and  1314  are reset, which sets the Hard Reset output  836  low and the Soft Reset output  1206  low, via And gate  1324 . In response to the function reset/power up clear  844  returning high, the Hard Reset controller  1302  will remain in the reset state (Hard Reset  836  output low) if the TCK  516  input is high and the TMS  830  input is in a stable low or high state. The Soft Reset controller flip flop pairs  1320  and  1322  are set while the TCK  516  input is high. 
     During the operation of a second or third protocol, the TCK  516  input is active, forcing the flip flop pairs of the Hard and Soft Reset controllers to be continuously forced to their set state due to the TCK  516  signal being coupled to the set (S) input of the pair&#39;s flip flops. In the set state, the Hard and Soft Reset controllers output highs on the Hard  836  and Soft  1206  Reset outputs, respectively. At the end of a second or third protocol operation, the Hard and Soft Reset controllers may be reset by a first protocol sequence applied on the TCK  516  and TMS  830  inputs. The Soft Reset controller  1304  is always reset following a second or third protocol operation so that a new second protocol operation may be initiated. The Soft Reset output  1206  of the Soft Reset controller  1206  is used to force the Address and Instruction controller  1204  to a Home state (see  FIG. 14 ). From the Home state, another address and instruction input operation can be performed using the second protocol. The Hard Reset controller  1302  is reset (Hard Reset output  836  goes low) using the first protocol whenever all required second and third protocol operations have been performed. A low on the Hard Reset output  836  resets the instruction circuit  810  to a known state, forces the Address and Instruction Controller  1204  to the Home state, and resets the Tap Domains  510  via their TRST input. 
     Timing diagram  1326  of  FIG. 13  illustrates a first protocol sequence on TCK and TMS that will reset the Hard Reset controller  1302  and output a low on the Hard Reset signal  836  and Soft Reset signal  1206 . The sequence includes the steps of holding the TCK signal  516  high while inputting a clock pulse or pulses on the TMS signal  830 . This Hard Reset controller design example requires two clock pulses on the TMS signal due to the choice of using two serially connected flip flop pairs  1312  and  1314 . With the TCK signal high, the rising and falling edges of the first TMS clock pulse sets the output of flip flop pair  1312  low and the rising and falling edges of the second TMS clock pulse sets the output of flip flop pair  1314  low, which forces the Hard Reset and Soft Reset outputs low. The low on the Hard and Soft Reset outputs will be maintained until the TCK signal goes low, which will set the outputs of flip flop pairs  1312  and  1314  high and the Hard and Soft Reset outputs  836  and  1206  high. As indicated in dotted line, if desired, additional TMS clock signals can occur after the Hard Reset controller  1302  has received the two TMS clock pulses required to set the Hard Reset output  836  low. 
     Timing diagram  1326  of  FIG. 13  illustrates a first protocol sequence on TCK and TMS that will reset the Soft Reset controller  1304  and output a low on the Soft Reset output  1206 . The sequence includes the steps of holding the TCK signal low and inputting two clock pulses on the TMS signal. Like the Hard Reset controller  1302  design example above, the Soft Reset controller  1304  design example uses two serially connected flip flop pairs  1320  and  1322  for use with two TMS clock pulses. With TCK low, the rising and falling edges of the first TMS clock pulse sets the output of flip flop pair  1320  low and the rising and falling edges of the second TMS clock pulse sets the output of flip flop pair  1322  low, which forces the Soft Reset output  1206  low. The low on the Soft Reset output  1206  will be maintained until the TCK signal goes high, which sets the outputs flip flop pairs  1320  and  1322  high and the Soft Reset output  1206  high. As indicated in dotted line, if desired, additional TMS clock signals can occur after the Soft Reset controller  1304  has received the two TMS clock pulses required to set the Soft Reset output low. 
     While two TMS clock pulses were used in the Hard and Soft Reset controller design examples, a lesser or greater number of TMS clock pulses, and corresponding number flip flop pairs, may be used as well. Two TMS clock pulses were used in these examples because it reduces the probability that noise or signal skew problems might accidentally produce the hard and soft first protocol sequences on TCK and TMS, causing the Hard and Soft controllers to inadvertently enter their reset states. The first protocol sequence of TCK and TMS shown in the timing diagrams  1326 - 1328  are TCK and TMS sequences that are never produced during second and third protocol operations. The first protocol sequences are only detectable by the Hard and Soft Reset controllers. 
       FIG. 14  illustrates the state diagram of the Address and Instruction Controller  1204 . In response to a Soft Reset output  1206  from the Hard and Soft Reset controller  1202  the Address and Instruction controller  1204  will enter the Home state  1402 . The Home state is maintained while TMS is high. The controller transitions to the Input Address state  1404  when TMS goes low and remains there while TMS is low. During the Input Address state, the A-Clock  834  is active to shift in an address from TDI into the Address circuit  808 . When TMS goes high, the controller  1204  transitions to the Address match state  1406  to test for a match between the address shifted in and the local or global address. If the address does not match the local or global address, the controller will transition into the Idle state  1414  and remain there until a hard or soft first protocol sequence sets the Soft Reset output  1206  low, forcing the controller to return to the Home state. If the address matches the local or global address, the controller  1204  transitions into the Input Instruction state  1408  and remains there while TMS is low. In the Input Instruction state, the I-Clock signal on IC bus  832  will become active to shift in an instruction from TDI to the Instruction Circuit  810 . When TMS goes high, the controller will transition to the Update Instruction state  1410  an output the I-Update signal on IC bus  832  to update and output the instruction from the Instruction Circuit. When TMS goes low, the controller transitions to the Enable state  1412 . The Enable output  842  is set high during the Enable state to enable TCKs to be applied to the selected Tap Domains  510 . The controller will remain in the Enable state independent of logic levels on TMS. The TMS sequences shown in  FIG. 14  that move the controller through its states define the second protocol. While the controller  1204  is in the Enable state  1412 , the TMS signal is operable to perform the third protocol operations to access the Tap Domains  510  without effecting the Enable state  1412  of controller  1204 . The controller returns to the Home state  1402  only when the Soft Reset signal  1206  goes low. 
       FIG. 15A  illustrates an example of how the Address and Instruction controller  1204  of  FIG. 12  may be designed. The controller  1204  consists of; (1) a state machine  1502  having inputs for TCK  516 , TMS  830 , Address Match  838 , and Soft Reset  1206 , and outputs for indicating when the state machine is in the input address state  1404 , input instruction state  1408 , update instruction state  1410 , and Enable state  1412 , and (2) flip flops  1504 - 1510 , and And gates  1512 - 1516 . The state machine  1502  responds to the TMS and Address Match inputs on the rising edge of TCK  516  to move though its states. The flip flops  1512 - 1516  respond to the falling edge of TCK  516  to gate the A-Clock, I-Clock, I-Update output signals on an off, and to set the Enable output signal. 
     In response to a low on the Soft Reset input  1206 , the state machine is forced to the Home state  1402 . While the state machine is in the Input Address state  1404 , the A-Clock signal  834  will be gated on to clock an address into the Address Circuit  808 . While the state machine is in the Input Instruction state  1408 , the I-Clock signal  832  will be gated on to clock an instruction into the Instruction Circuit  810 . While the state machine is in the Update Instruction state  1410 , the I-Update signal  832  will be gated on to update the instruction from the Instruction Circuit&#39;s output bus  840 . While the state machine is in the Enable state  1412 , the Enable output will be set high to enable Tap Domain access. 
       FIG. 15B  illustrates an example of how the state machine  1502  may be designed. The state machine consists of next state decode logic  1518 , state flip flops A, B, C, and output state decode logic  1520 . The ABC state assignments are shown in the  FIG. 14  state diagram. If the Soft Reset  1206  input is low, the state machine  1502  is reset to the Home state (ABC=000). If the Soft Reset  1206  input is high, the state machine responds to the rising edge of TCK to transition through its states according to the state diagram of  FIG. 14 . The output state decode logic  1520  indicates when the state machine is in the input address state  1404  (ABC=001), the input instruction state  1408  (ABC=011), the update instruction state  1410  (ABC=100), and Enable state  1412  (ABC=101). 
       FIG. 16  illustrates the state diagram of the standard IEEE 1149.1 Tap controller. This state diagram and the design of the controller that uses it is well known and documented in IEEE Std 1149.1 and therefore does not require further teaching. Each Tap Domain  510  in Tap Region  522  will have a Tap controller that operates according to this standard state diagram. The TCK and TMS operation of the standard Tap controller shown in  FIG. 16  defines the third protocol of the present disclosure. 
       FIG. 17  illustrates a group of target devices  1702 - 1706  on a board or other substrate  1700 , each target device including the Addressable Tap Domain Selection Circuit  514  and its associated 3 pin TCK, TDI/TDO, and TMS/RCK interface, as well as Tap Domain Region  522 . The target devices could be packaged ICs or unpacked IC die. The 3 pin interface of each target device is coupled to an external controller  1708  via cable connector  1710  to provide access for test, debug, emulation, and trace operations. Each target device  1702 - 1706  may contain embedded core target circuits  1712 - 1716  which also are interfaced to the external controller  1708  via the 3 pin interface. Further, each core  1712 - 1716  may contain embedded core targets circuits  1718 - 1722  also interfaced to the external controller  1708  via the 3 pin interface. As indicated, the external controller  1708  may be realized by using an interface card  1724  in a personal computer  1726  to control the 3 pin interface communication with the targets  1702 - 1706 ,  1712 - 1716 ,  1718 - 1722  via a cable connection  1728 . The 3 pin interface communicates to target circuits using the previously mentioned first, second, and third protocols. 
     Each target  1702 - 1706 ,  1712 - 1716 ,  1718 - 1722  of  FIG. 17  has the previously mentioned local address to allow it to be individually addressed and instructed by the controller  1708  using the second protocol. Following the individual addressing and instructing of a target using the second protocol, the Tap Domains  510  within the target may be access by the controller  1708  using the third protocol to perform test, debug, emulation, and/or trace operations. Additionally, each target has the previously mentioned global address to allow all targets to be simultaneously addressed and instructed using the second protocol. The purpose of the global addressing is to allow all target devices to receive a global instruction. The global instruction may be an instruction that; (1) causes all targets to enter into a particular mode suitable for a test, emulation, debug, and/or trace operation, (2) causes all targets to enter into a mode to perform a global self test operation, (3) causes all targets to suspend functional operation, or (4) causes all targets to resume functional operation. Other types of global instructions may be conceived as well. 
       FIG. 17A  illustrates an alternate configuration of  FIG. 17  whereby a group of stacked die targets devices  1732 - 1736  exist on a board or other substrate  1730 . Each die in the stacks  1732 - 1736  includes the Addressable Tap Domain Selection Circuit  514  and its associated 3 terminal TCK, TDI/TDO, and TMS/RCK interface, as well as Tap Domain Region  522 . The TCK, TDI/TDO, and TCM/RCK terminals of each die in a stack are commonly connected to the TCK  1738 , TMS/RCK  1740 , and TDI/TDO  1742  signal interface to the external controller  1708 , via cable connector  1710  to provide access for test, debug, emulation, and trace operations. Each die in the stacks may contain embedded core target circuits  1712 - 1716  and  1718 - 1722  as described in  FIG. 17 . The controller  1708  communicates to the stacked die targets using the previously mentioned first, second, and third protocols. 
     Each die in a stack  1732 - 1736  has the previously mentioned local address to allow it to be individually addressed and instructed by the controller  1708  using the second protocol. Following the individual die addressing and instructing, the Tap Domain  510  within the selected die may be accessed by the controller  1708  using the third protocol to perform test, debug, emulation, and/or trace operations. Additionally, each die in stacks  1732 - 1736  has the previously mentioned global address to allow all die in stacks  1732 - 1736  to be simultaneously addressed and instructed using the second protocol, for the reasons mentioned in regard to  FIG. 17 . 
       FIG. 18  illustrates a group of legacy target devices  1802 - 1806 , each including the standard IEEE 1149.1 5 signal interface comprising TRST, TCK, TMS, TDI, and TDO terminals, but not the Addressable Tap Domain Selection Circuit  514 . The term legacy means that the devices are pre-existing devices whose design is fixed and cannot be altered. As shown, each legacy target device may also include the RCK terminal. The legacy target devices could be ICs  1802 - 1806  on a board or other substrate  1800 , embedded core circuits  1802 - 1806  within an IC  1800 , or embedded core circuits  1802 - 1806  within a core circuit  1800 . 
     As seen, a separate device  1808  exists between the legacy target devices  1802 - 1806  and the external controller  1708 . This separate device  1808  implements the Addressable Tap Domain Selection Circuit  514  as shown and described in regard to  FIG. 8  and operates according the previously described first, second, and third protocols. It also includes the previously described local and global addressing modes. The local address  1810  is shown, in this example, as being input to the separate device  1808  on externally accessible terminals of device  1808 , which is one of the previously mentioned means for supplying the local address. The separate device  1808  serves to provide the interface between the 5 signal IEEE 1149.1 terminals, and optional RCK terminal, of each legacy target device and the 3 pin interface to the external controller  1708 . The operation of the separate device  1808  in accessing the legacy device Tap Domains is the same as described in  FIG. 8  where the Addressable Tap Domain Selection Circuit  514  was described accessing the Tap Domains  510  of Tap Region  522 . 
     The arrangement shown in  FIG. 18  could represent the legacy target devices  1802 - 1806  and separate device  1808  as being; (1) ICs/die on a board or substrate  1800 , embedded core circuits within an IC  1800 , or (3) embedded core circuits within a core circuit  1800 .  FIG. 18  advantageously illustrates how legacy devices designed using the IEEE 1149.1 interface, and optional RCK, can be interfaced to the 3 pin controller  1708  by providing the Addressable Tap Selection Circuit  514  as a separate circuit to serve as the interface between the legacy devices  1802 - 1806  and external controller  1708 . The separate circuit  1808  could contain only the Addressable Tap Domain Selection Circuit  514  or it could contain the Addressable Tap Domain Selection Circuit  514  along with other circuits. Indeed, the separate circuit  1808  could be a larger functional IC/die or embeddable core circuit that includes the Addressable Tap Domain Selection Circuit  514  and its external terminal interfaces as a sub-circuit within the larger functional circuit. 
       FIG. 19  illustrates a group  1902  of IEEE 1149.1 legacy target devices  1802 - 1806  as described in  FIG. 18 , and a group  1904  of target devices  1702 - 1706  as described in  FIG. 17 . Each legacy target device  1803 - 1806  of group  1902  is interfaced to the external controller  1708  via the separate device  1808  as described in  FIG. 18  whereas each target device  1702 - 1706  of group  1904  is interfaced to the external controller directly. This example is provided to illustrate how legacy devices  1802 - 1806  that are not designed according to the present disclosure and other devices  1702 - 1704  that are designed according to the present disclosure can both be accessed by an external controller  1708  by using separate device  1808  as the interface between the legacy devices and external controller. 
       FIG. 20  illustrates the TDI/TDO signal wire connection  2002  between the TDI/TDO terminal of an I/O circuit  802  of a controller  1708  and a TDI/TDO terminal of the I/O circuits  802  of the Addressable Tap Domain Selection Circuits  514  of target circuits  1 -N. The controller will have to have the I/O circuit  802  in order to interface to and communicate with I/O circuits  802  of the target circuits  1 -N via the TDI/TDO signal wire. Preferably, the output buffer  814  of the controller  1708  and the output buffers  814  of the target circuits will have approximately the same current sink/source drive strength. Also preferably the resistors  816  of the controller  1708  and target circuit I/O circuits  802  will have approximately the same resistance. 
     As seen in this example, the output buffer  814  of the controller&#39;s I/O circuit  802  is always enabled to output TDO data to the target circuits, while the output buffers  814  of the target circuit I/O circuits  802  are selectively enabled to and disabled from outputting TDO data to the controller  1708  by the output enable  1  (OE 1 ) signal  822  from Tap Linking Circuit  812 . As previously described, the TDI  824  signal of the target I/O circuit  802  is coupled to the Address Circuit  808 , the Instruction Circuit  810 , and the Tap Linking Circuit  812  of Addressable Tap Domain Selection Circuit  514 , and the TDO  820  signal of the target I/O circuit  802  is coupled to the Tap Linking Circuit  812  of Addressable Tap Domain Selection Circuit  514 . The TDI  824  signal of the controller&#39;s I/O circuit  802  is coupled to a circuit within the controller designed to receive serial data input signals from TDI/TDO signal wire  2002 , and the TDO  820  signal of the controller&#39;s I/O circuit  802  is coupled to a circuit within the controller designed to transmit serial data output signals to TDI/TDI signal wire  2002 . 
     During first protocol operations the TDI/TDO signal wire is not used and the output buffers of the target circuits are disabled by the OE 1  signals  822 . 
     During second protocol operations when the controller  1708  is inputting address and instruction signals to the target circuits  1 -N, the output buffers  814  of the target circuits  1 -N are disabled by OE 1   822 , allowing the output buffer  814  of the controller to be the sole driver of the TDI/TDO signal wire  2002 . Thus during second protocols the I/O circuit  802  of target circuits  1 -N operates as an input buffer on the TDI/TDO signal wire  2002 . 
     During third protocol operations when the controller  1708  is not inputting and outputting data to a selected one or more Tap Domain in the Shift-DR or Shift-IR states, the output buffer  814  of the addressed and all other target circuits will be disabled by the OE 1  signal  822 . In this mode, the output buffer  814  of the controller is the sole driver of the TDI/TDO signal wire  2002 . 
     During third protocol operations when the controller  1708  is inputting and outputting data to a selected one or more Tap Domain in the Shift-DR or Shift-IR states, the output buffer  814  of the addressed target circuit will be enabled by the OE 1  signal  822 . In this mode, both the output buffers  814  of the controller and addressed target circuit will be driving the TDI/TDO signal wire  2002 . This mode of operation allows data to flow simultaneously between the controller  1708  and the addressed target circuit via the TDI/TDO signal wire during each TCK period. 
     If, during this simultaneous data flow mode, the output buffer  814  of the controller  1708  and the output buffer  814  of the addressed target circuit are both outputting the same logic level, the voltage on the TDI/TDO signal wire  2002  will driven to that full logic level. The data input circuits  818  of the controller  1708  and addressed target circuit will detect that full logic level and input that logic level to the controller  1708  and to the addressed target circuit via their respective TDI signals  824 . 
     If, during this simultaneous data flow mode, the output buffer  814  of the controller  1708  and the output buffer  814  of the addressed target circuit are outputting opposite logic levels, the TDI/TDO signal wire  2002  will be driven to a mid point voltage level between the two opposite logic levels. The data input circuits  818  of the controller  1708  and addressed target circuit will detect that mid level voltage and, based on the logic level each was attempting to output, will input a logic level to the controller  1708  and to the addressed target circuit on their respective TDI signal  824  that is the opposite of logic level each was outputting. 
     When the output buffers  814  of the controller and addressed target circuit are driving opposite logic levels on TDI/TDO wire  2002 , the resistors  816  serve to limit the current flow between the two output buffers  814  and to serve as voltage droppers to allow the mid point voltage level on TDI/TDO signal wire  2002  to be more easily detected by the data input circuit  818  as a voltage level that is distinctly different from the normal full high or low logic level voltages output from the output buffers  816 . The operation of data input circuit  818  will be described later in regard to  FIG. 22 . 
       FIG. 21  illustrates the TMS/RCK signal wire connection  2102  between the TMS/RCK terminal of an I/O circuit  804  of a controller  1708  and the TMS/RCK terminal of the I/O circuits  804  of the Addressable Tap Domain Selection Circuits  514  of target circuits  1 -N. When target circuits use Tap domains with RCKs, the controller will have to have the I/O circuit  804  in order to interface to and communicate with I/O circuits  804  of the target circuits  1 -N via the TMS/RCK signal wire. As with the TDI/TDO I/O circuits  802  above, the output buffers  814  of the controller and target circuits will preferably have approximately the same current sink/source drive strength and the resistors  816  will have approximately the same resistance. 
     As seen in this example, the output buffer  814  of the controller is always enabled to output TMS signals to the target circuits, while the output buffers  814  of the target circuits are selectively enabled to and disabled from outputting RCK signals  826  to controller  1708  by the output enable  2  (OE 2 ) signal  828 . As previously described, the TMS  830  signal of the target I/O circuit  804  is coupled to the Tap Linking Circuit  812  and to the Reset, Address, &amp; Instruction Controllers  806 , and the RCK  826  signal of the target I/O circuit  804  is coupled to the Tap Linking Circuit  812  of Addressable Tap Domain Selection Circuit  514 . The RCK  826  signal of the controller&#39;s I/O circuit  804  is coupled to a circuit within the controller designed to receive RCK input signals from the TMS/RCK signal wire  2102 , and the TMS  830  signal of the controller&#39;s I/O circuit  804  is coupled to a circuit within the controller designed to transmit TMS output signals to the TMS/RCK signal wire  2102 . 
     During first protocol operations when the controller  1708  is inputting soft or hard reset sequences to Hard and Soft Controller  1202 , the TMS/RCK signal wire will be driven by the output buffer  814  of controller  1708  and may or may not be driven by the output buffer  814  of a target circuit  1 -N. If the first protocol is performed following a power up or function reset of target circuits  1 -N, the output buffers  814  of the target circuits will not be enabled by OE 2  and therefore only output buffer  814  of controller  1708  drives the TMS/RCK signal wire  2102 . Also, if a first protocol is performed following a second or third protocol where the OE 2  signal is set low by instruction control signal  846 , only the output buffer  814  of controller  1708  will be driving the TMS/RCK signal wire  2102 . However, if a first protocol is performed following a second or third protocol where the OE 2  signal is set high by an instruction, via instruction control signal  846 , both the output buffer  814  of controller  1708  and the output buffer of the address target circuit will be driving the TMS/RCK signal wire  2102 . 
     Following the input of a soft reset first protocol sequence, the OE 2  will be forced low by the Soft Reset signal  1206  from the Hard and Soft Reset Controller  1202  going low. As previously mentioned, the Soft Reset signal  1206 , when low, forces the Address and Instruction controller  1204  into the Home state  1402 . In the Home state  1402 , the Enable signal output  842  of the Address and Instruction controller  1204  is low, which forces the OE 2  signal  828  low via And gate  848 . Thus if the output buffer  814  of a target circuit was enabled prior to the input of a soft reset first protocol sequence, it will be disabled at the end of the soft reset protocol sequence. 
     Following the input of a hard reset first protocol sequence, the OE 2  will be forced low by the Hard Reset signal  836  from the Hard and Soft Reset Controller  1202  going low. When Hard Reset signal  836  goes low, the instruction circuit  810  is reset to an instruction that sets the instruction control output signal  846  low which forces the OE 2  output  828  of And gate  848  low. Also the Hard Reset signal going low will set the Soft Reset signal  1206  low, via And gate  1324  of  FIG. 13 , which sets the Enable signal  842  low and the OE 2  output of And gate  848  low. Thus if the output buffer  814  of a target circuit was enabled prior to the input of a hard reset first protocol sequence, it will be disabled at the end of the soft reset protocol sequence. 
     During second protocol operations when the controller  1708  is inputting address and instruction signals to the target circuits  1 -N, the output buffers  814  of the target circuits  1 -N are disabled by OE 2   828  being low, allowing the output buffer  814  of the controller to be the sole driver of the TMS/RCK signal wire  2102 . Thus during second protocols the I/O circuits  804  of target circuits  1 -N operate as an input buffers on the TMS/RCK signal wire  2102 . 
     During third protocol operations when the controller  1708  is communicating to a selected one of more Tap Domains of target circuits that do not use RCKs, the output buffer  814  of the addressed and all other target circuits will be disabled by the OE 2  signal  828  being low. In this mode, the output buffer  814  of the controller is the sole driver of the TMS/RCK signal wire  2102 . 
     During third protocol operations when the controller  1708  is communicating to a selected one of more Tap Domains of target circuits that use RCKs, the output buffer  814  of the addressed target circuit will be enabled by its OE 2  signal  828  being high and the output buffer  814  of all other target circuits will be disabled by their OE 2  signals  828  being low. In this mode, the output buffer  814  of the controller and the output buffer  814  of the addressed target circuit will both be driving the TMS/RCK signal wire  2102 . In this mode of operation, a TMS signal can flow from the controller  1708  to the addressed target circuit and an RCK signal can flow from the addressed target circuit to the controller  1708  simultaneously via TMS/RCK signal wire  2102  during each TCK period. 
     If, during this simultaneous TMS and RCK signal flow mode, the output buffer  814  of the controller  1708  and the output buffer  814  of the addressed target circuit are both outputting the same logic level, the voltage on the TMS/RCK signal wire  2102  will driven to that full logic level. The data input circuits  818  of the controller  1708  and addressed target circuit will detect that full logic level and input that logic level to the controller  1708  via its RCK  826  and to the addressed target circuit via its TMS signal  830 . If, during this simultaneous data flow mode, the output buffer  814  of the controller  1708  and the output buffer  814  of the addressed target circuit are outputting opposite logic levels, the TMS/RCK signal wire  2102  will be driven to a mid point voltage level between the two opposite logic levels. The data input circuits  818  of the controller  1708  and addressed target circuit will detect that mid level voltage and, based on the logic level each was attempting to output, will input a logic level to the controller  1708  on its RCK  826  and to the addressed target circuit on its TMS  830  that is the opposite of logic level each was outputting. 
     When the output buffers  814  of the controller and addressed target circuit are driving opposite logic levels on TMS/RCK wire  2102 , the resistors  816  serve to limit the current flow between the two output buffers  814  and to serve as voltage droppers to allow the mid point voltage level on TMS/RCK signal wire  2102  to be more easily detected by the data input circuit  818  as a voltage level that is distinctly different from the normal full high or low logic level voltages output from the output buffers  814 . 
       FIG. 22  illustrates one example of how to design the data input circuit  818  of the I/O circuit  802  and  804 . The data input circuit  818  includes a voltage comparator circuit  2202 , a multiplexers  2204 , an inverter  2206 , and a buffer  2208 . The voltage comparator circuit  2202  inputs voltages from its wire input  2210  and outputs digital control signals S 0  and S 1  to multiplexer  2204 . The wire input  2210  for I/O circuit  802  is coupled to the TDI/TDO signal wire  2002  of  FIG. 20  via TDI/TDO terminals of the controller  1708  and target circuits  1 -N. The wire input  2210  for I/O circuit  804  is coupled to the TMS/RCK signal wire  2102  of  FIG. 21  via TMS/RCK terminals of controller  1708  and target circuits  1 -N. 
     As seen, the first voltage (V) to ground (G) leg  2218  of voltage comparator circuit  2202  comprises a series P-channel transistor and current source and the second voltage to ground leg  2220  comprises a series N-channel transistor and current source. As seen, S 1  is connected at a point between the P-channel transistor and current source of the first leg  2218  and S 0  is connected at a point between the N-channel transistor and current source of the second leg  2220 . The gates of the transistors are connected to wire input  2210  to allow voltages on the wire signal  2210  to turn the transistors on and off. 
     The operation of the voltage comparator circuit  2202  and multiplexer  2204  is shown in table  2222  and described herein. If the voltage on wire input  2210  is at a low level (logic zero), the S 0  and S 1  outputs are set high, which causes the multiplexer  2204  to select its low input  2224  and output the low input to In signal  2212  via buffer  2208 . If the voltage on wire input  2210  is at a mid level (mid point voltage), the S 0  is set low and the S 1  is set high, which causes the multiplexer  2204  to select its Out* input  2226  (inverted Out signal  2214 ) and output the Out* input to In  2212  via and buffer  2208 . If the voltage on wire connection  2210  is high (logic one), the S 0  and S 1  outputs are set low, which causes the multiplexer  2204  to select its high input  2228  and output the high input to In  2212  via and buffer  2208 . 
     For I/O circuits  802 , the In signal  2212  is connected to the TDI signal  824  of the controller  1708  and Addressable Tap Domain Selection Circuits  514  of target circuits  1 -N of  FIG. 20 , and the Out signal  2214  is connected to the TDO signal  820  of the controller  1708  and Addressable Tap Domain Selection Circuits  514  of target circuits  1 -N of  FIG. 20 . 
     For I/O circuits  804 , the In signal  2212  is connected to the RCK signal  826  of the controller  1708  and to the TMS signal  830  of the Addressable Tap Domain Selection Circuits  514  of target circuits  1 -N of  FIG. 21 . The Out signal  2214  is connected to the TMS signal  830  of the controller  1708  and to the RCK signal  826  of the Addressable Tap Domain Selection Circuits  514  of target circuits  1 -N of  FIG. 21 . 
       FIG. 23A  illustrates the case where the output buffers  814  of the controller  1708  and an addressed target circuit are both outputting logic lows on TDI/TDO  2002  or TMS/RCK  2102  signal wires. In this case the signal wire  2002 / 2102  is low and the wire input  2210  to the data input circuits  818  is low. This causes the data input circuit  818  of the controller  1708  to input a low to the controller on In signal  2212  and the data input circuit  818  of the addressed target circuit to input a low to the target circuit on In signal  2212 . 
       FIG. 23B  illustrates the case where the output buffer  814  of the controller  1708  is outputting a low on signal wire  2002 / 2102  and the output buffer  814  of an addressed target circuit is outputting a high on signal wire  2002 / 2102 . In this case a current path exists from the high voltage output (V) from the target circuit to the low voltage output (G) from the controller. The resistors  816  limit the current flow and the voltage drops across them produce a distinctly detectable mid point voltage level on the signal wire  2002 / 2102 . The mid point voltage level on the signal wire  2002 / 2102  is input to the data input circuits  818  of the controller and target circuit via wire inputs  2210 . 
     Since the data input circuit  818  of the controller  1708  knows the controller was outputting a logic low, it responds to the mid point voltage by inputting a logic high to the controller on In signal  2212 , which is the only logic level that can be output from the target circuit to cause the mid point voltage on signal wire  2002 / 2102 . Also since the data input circuit  818  of the target circuit knows the target circuit was outputting a logic high, it responds to the mid point voltage by inputting a logic low to the target circuit on In signal  2212 , which is the only logic level that can be output from the controller to cause the mid point voltage on signal wire  2002 / 2102 . 
       FIG. 23C  illustrates the case where the output buffer  814  of the controller  1708  is outputting a high on signal wire  2002 / 2102  and the output buffer  814  of an addressed target circuit is outputting a low on signal wire  2002 / 2102 . In this case a current path exists from the high voltage output (V) from the controller to the low voltage output (G) from the addressed target circuit. Again the resistors  816  limit the current flow and the voltage drops across them produce a distinctly detectable mid point voltage level on the signal wire  2002 / 2102 . The mid point voltage level on the signal wire  2002 / 2102  is input to the data input circuits  818  of the controller and target circuit via wire inputs  2210 . 
     Since the data input circuit  818  of the controller  1708  knows the controller was outputting a logic high, it responds to the mid point voltage by inputting a logic low to the controller on In signal  2212 , which is the only logic level that can be output from the target circuit to cause the mid point voltage on signal wire  2002 / 2102 . Also since the data input circuit  818  of the target circuit knows the target circuit was outputting a logic low, it responds to the mid point voltage by inputting a logic high to the target circuit on In signal  2212 , which is the only logic level that can be output from the controller to cause the mid point voltage on signal wire  2002 / 2102 . 
       FIG. 23D  illustrates the case where the output buffers  814  of the controller  1708  and an addressed target circuit are both outputting logic high on signal wire  2002 / 2102 . In this case the signal wire  2002 / 2102  is high and the wire input  2210  to the data input circuits  818  is high. This causes the data input circuit  818  of the controller  1708  to input a high to the controller on In signal  2212  and the data input circuit  818  of the addressed target circuit to input a high to the target circuit on In signal  2212 . 
       FIG. 24  illustrates timing waveforms  2402  for the four cases (A,B,C,D) in which simultaneous data communication occurs between the I/O circuit  802 / 804  of controller  1708  and the I/O circuit  802 / 804  of an Addressable Tap Domain Selection Circuit  514  of an addressed target circuit via a TDI/TDO or TMS/RCK signal wire  2002 / 2102 . In this example, the output enable  1  or  2  (OE 1 /OE 2 ) signal  822 / 828  of the target circuit is set to enable output buffer  814 . Each case A-D is indicated in the timing diagram by vertical dotted line boxes. 
     Case A shows the controller and the target circuit outputting lows from their buffers  814 . In response, the wire  2002 / 2102  is low and both the controller and target circuit input lows via the In signal  2212  from their data input circuits  818 . 
     Case B shows the controller outputting a low from its buffer  814  and the target circuit outputting a high from its buffer  814 . In response, the wire  2002 / 2102  is at a mid voltage level causing the controller to input a high from the In signal  2212  of its data input circuit  818 , while the target circuit inputs a low from the In signal  2212  of its data input circuit  818 . 
     Case C shows the controller outputting a high from its buffer  814  and the target circuit outputting a low from its buffer  814 . In response, the wire  2002 / 2102  is at a mid voltage level causing the controller to input a low from the In signal  2212  of its data input circuit  818 , while the target circuit inputs a high from the In signal  2212  of its data input circuit  818 . 
     Case D shows the controller and the target circuit outputting high from their buffers  814 . In response, the wire  2002 / 2102  is high and both the controller and target circuit input highs via the In signal  2212  from their data input circuits  818 . 
       FIG. 25  illustrates a timing diagram of the operation of the present disclosure performing a first protocol Soft Reset Sequence  1328  followed by a second protocol showing entry into the Home state  1402  followed by entry into the Input Address state  1404 . 
       FIG. 26  illustrates a timing diagram of the operation of the present disclosure performing a first protocol Soft Reset Sequence  1328  followed by a second protocol that immediately enters the Input Address state  1404 . 
       FIG. 27  illustrates a timing diagram of the operation of the present disclosure performing a first protocol Hard Reset Sequence  1326  followed by a second protocol showing entry into the Home state  1402  followed by entry into the Input Address state  1404 . 
       FIG. 26  illustrates a timing diagram of the operation of the present disclosure performing a first protocol Hard Reset Sequence  1326  followed by a second protocol that immediately enters the Input Address state  1404 . 
       FIG. 29  illustrates a timing diagram of the operation of the present disclosure performing a full second protocol sequence  2902  of inputting an address  1404 , matching the address  1406 , inputting an instruction  1408 , updating the instruction  1410 , and entering the enable state  1412 , followed by performing a third protocol sequence  2904  to access the Tap domain(s)  510  selected by the instruction using the standard IEEE 1149.1 TMS protocol, followed by performing a first protocol sequence  2906  to input either a Soft Reset sequence  1328  or a Hard reset sequence  1326  to terminate the operation. 
     As seen, the second protocol  2902  uses the TCK  516 , TMS  830 , and TDI  824  signals, but not the TDO  820  signal. The third protocol  2904  uses the TCK  516 , TMS  830 , TDI  824 , and TDO  820  signals according to the Tap protocol defined in standard IEEE 1149.1. The first protocols  2906  ( 1328  and  1326 ) use only the TCK  516  and TMS  830  signals. The timing diagram of  FIG. 29  illustrates in detail the present disclosure performing the previously described protocols A-D  712 - 718  sequences discussed early in regard to  FIG. 7B . 
       FIG. 30  illustrates a timing diagram of the operation of the present disclosure performing a full second protocol sequence  2902  of inputting an address  1404 , matching the address  1406 , inputting an instruction  1408 , updating the instruction  1410 , and entering the enable state  1412 , followed by performing a first protocol sequence  2906  to input either a Soft Reset sequence  1328  or a Hard reset sequence  1326  to terminate the operation. 
     As seen, the second protocol  2902  uses the TCK  516 , TMS  830 , and TDI  824  signals, but not the TDO  820  signal. The first protocols  2906  ( 1328  and  1326 ) use only the TCK  516  and TMS  830  signals. The timing diagram of  FIG. 30  illustrates in detail the present disclosure performing the previously described protocols E-H  720 - 726  sequences discussed early in regard to  FIG. 7B . 
     While the description of the disclosure to this point has shown that the disclosure includes an Addressable Tap Domain Selection Circuit  514  capable of selecting one or more of a plurality of Tap Domains  510  within a Tap Region  522  ( FIGS. 6 and 8 ) using a reduced number of interface signals, it is possible to simplify the disclosure when access to only one JTAG circuit Tap Domain is required. A reduction of interface signals is achieved in the simplified version of the disclosure. 
       FIG. 31  illustrates a connected controller  3102  accessing the conventional JTAG circuit  104  of  FIG. 1  using the 5 IEEE 1149.1 standard signals TDI, TDO, TMS, TCK, and TRST. The JTAG circuit  104  could be used in an IC or core for controlling test, debug, emulation, trace, boundary scan, or other operations of the IC or core. 
       FIG. 32  illustrates I/O circuits  802  of the present disclosure being used to reduce the signal interface between the connected controller  3102  and JTAG circuit  104  from 5 to 4 signals. One I/O circuit  802  is connected to the controller&#39;s TDO output via Out signal  2214 , to the controllers TDI input via In signal  2212 , and to the TDI/TDO signal wire  3202  via Wire signal  2210 . The other I/O circuit  802  is connected to the JTAG circuit&#39;s TDO output via Out signal  2214 , to the JTAG circuit&#39;s TDI input via In signal  2212 , and to the TDI/TDO signal wire  3202  via Wire signal  2210 . 
     As seen in  FIG. 32 , the I/O circuit  802  associated with the controller can exist as a separate circuit from the controller  3102  or the I/O circuit  802  may be integrated with the controller  3102  to form a new controller  3204 . Preferably, but not necessarily, the output buffer  814  of the I/O buffer associated with the controller  3102  will be enabled all the time by setting its output enable signal  822  high, which allows the TDI/TDO wire  3202  to a always be driven to a valid signal level. 
     Also as seen in  FIG. 32 , the I/O circuit  802  associated with the JTAG circuit  104  can exist as a separate circuit from the JTAG circuit  104  or the I/O circuit  802  may be integrated with the JTAG circuit  104  to form a new JTAG circuit  3206 . If the I/O circuit  802  associated with the JTAG circuit is a separate circuit, its output buffer  814  will be enabled, via output enable signal  822 , all the time since their is no signal available from the JTAG circuit  104  to act as an enable or disable signal to the output buffer  814 . If the I/O circuit  802  associated with the JTAG circuit  104  is integrated with the JTAG circuit  104  to form new JTAG circuit  3206 , the output enable  822  of the I/O circuit  802  will be connected to the JTAG&#39;s Enable signal  126  so that the output buffer  814  can be enabled during TDI and TDO shift operations and disabled during non shift operations. 
     The Enable signal  126  is a standard signal output from Tap controller  120  during data and instruction shift operations. The Enable signal  126  controls the enable and disable state of the JTAG circuit&#39;s TDO tristate output buffer  128 . If the I/O circuit  802  is integrated with JTAG circuit  104  to form new JTAG circuit  3206  it is preferred that the TDO tristate buffer  128  be removed, as indicated by crossed dashed lines, so that the TDO signal path formed between flip flop  124  and Out signal  2214  of I/O circuit  802  does not enter into a tristate (floating) state when shift operations are not being performed. 
       FIG. 33  illustrates a connected controller  3302  accessing the JTAG circuit  302  of  FIG. 3  using the 5 IEEE 1149.1 standard signals TDI, TDO, TMS, TCK, and TRST plus the non-standard RCK signal. The JTAG circuit  302  could be used in an IC or core for controlling test, debug, emulation, trace, boundary scan, or other operations of the IC or core. 
       FIG. 34  illustrates I/O circuits  802  and  804  of the present disclosure being used to reduce the signal interface between the connected controller  3302  and JTAG circuit  302  from 6 to 4 signals. The connection and operation of I/O circuits  802  associated with controller  3302  and JTAG circuit  302  are the same as described previously in  FIG. 32  in the following separate and integrated implementation descriptions of I/O circuit  804 . One I/O circuit  804  is connected to the controller&#39;s TMS output via Out signal  2214 , to the controllers RCK input via In signal  2212 , and to the TMS/RCK signal wire  3402  via Wire signal  2210 . The other I/O circuit  804  is connected to the JTAG circuit&#39;s RCK output via Out signal  2214 , to the JTAG circuit&#39;s TMS input via In signal  2212 , and to the TMS/RCK signal wire  3402  via Wire signal  2210 . 
     As seen in  FIG. 34 , the I/O circuit  804  associated with the controller can exist as a separate circuit from the controller  3302  or the I/O circuit  804  may be integrated with the controller  3302  to form a new controller  3404 . Preferably, but not necessarily, the output buffer  814  of the I/O buffer associated with the controller  3302  will be enabled all the time by setting its output enable signal  822  high, which allows the TMS/RCK wire  3402  to a always be driven to a valid signal level. 
     Also as seen in  FIG. 34 , the I/O circuit  804  associated with the JTAG circuit  302  can exist as a separate circuit from the JTAG circuit  302  or the I/O circuit  804  may be integrated with the JTAG circuit  302  to form a new JTAG circuit  3406 . Regardless of whether I/O circuit  804  is a separate circuit or integrated with JTAG circuit  302 , its output buffer  814  will be enabled, by setting its output enable signal  822  high, all the time since the RCK signal of JTAG circuit  302  must always be output to the controller  3302  during test, debug, emulation, trace, and/or other operations. 
     From the above examples shown in  FIG. 31-34 , it is clear that the I/O circuits  802 - 804  of the present disclosure can be used to provide a method of reducing the interface signals between a controller  3102 ,  3204 ,  3302 , and  3404  and a JTAG circuit  104 ,  3206 ,  302 , and  3406 . While the access approach described in  FIGS. 31-34  is a point-to-point access between a controller and a connected JTAG circuit, i.e. it does not provide the multiple JTAG circuit Tap Domain selecting features as described earlier in the present disclosure, it does offer a reduced signal interfacing approach which is simple and can be realized with a minimum of additional circuitry. 
       FIG. 35  illustrates an IC or core  3504  containing the emulation, trace, and/or debug circuit  106  of  FIG. 1  coupled internally to a functional circuit  102  of the IC or core via bus  112  and externally to an emulation, trace, and/or debug interface  3506  of a controller  3502  via bus  110 . The bus  110  consists of input and output connections for allowing signals to flow between circuit  3506  and  106  during an emulation, trace, and/or debug operation. In this example, 8 connections are used on bus  110 . 
     The signals could be control signals, data signals, triggering signals, protocol signals used in message communications, and/or other signals used during an I/O operation of an emulation, trace, and/or debug operation. To increase the bandwidth of signal flow between the IC/core  3504  and controller  3502  it is advantageous to have as many input and output signals on bus  110  as possible. However, only so many IC terminals may be used on bus  110 , since the IC&#39;s functional input and output terminals  103  take priority and therefore will consume most of the available IC input and output terminals. 
       FIG. 36  illustrates how the controller  3502  and IC/core  3504  of  FIG. 35  can be adapted with I/O circuits  802  of the present disclosure to reduce the number of signal connections between the controller and IC/core by one half without reducing the signaling bandwidth. 
     As seen in  FIG. 36 , controller circuit  3602  differs from controller circuit  3502  of  FIG. 35  in that the input and output signals of bus  110  to emulation, trace, and debug circuit  3506  are interfaced to I/O circuits  802 , via the I/O circuit&#39;s input  2214  and output  2212 . If desired, circuit  3506  may optionally be modified, as seen in dotted line, to allow inputting control to the  802  I/O circuit&#39;s output enable signal  822 , otherwise the output enable  822  input of I/O circuit  802  will be fixed to always enable the output buffer  814  of I/O circuit  802 . 
     Similarly, the IC/core circuit  3604  differs from IC/core circuit  3504  in that the input and output signals of bus  110  to emulation, trace, and debug circuit  106  are interfaced to I/O circuits  802 , via the I/O circuit&#39;s input  2214  and output  2212 . If desired, circuit  106  may optionally be modified, as seen in dotted line, to allow inputting control to the  802  I/O circuit&#39;s output enable signal  822 , otherwise the output enable  822  input of I/O circuit  802  will be fixed to always enable the output buffer  814  of I/O circuit  802 . 
     As seen in  FIG. 36 , the number of bus  3606  connections, via wire terminals  2210  of the I/O circuits  802  of circuits  3602  and  3604 , is reduced by one half of that shown in bus  110  of  FIG. 35 . Thus, the present disclosure provides a way of reducing the number of required emulation, debug, and/or trace signal connections between circuits  3602  and circuits  3604  of  FIG. 36  on bus  3606  by one half that used in the prior art of  FIG. 35 . 
     The following  FIGS. 37-40  are provided to illustrate how the I/O circuits  802  (or  804 ) can be used to reduce the functional signal connections between functional circuits of an IC or core circuit. 
       FIG. 37  illustrates ICs or cores  3702  and  3704  each containing the functional circuit  102  of  FIG. 1 . At least some of the functional circuits  102  inputs and outputs are coupled to each other via functional bus  103  of  FIG. 1 . The bus  103  consists of input and output connections for allowing signals to flow between functional circuits  102  during functional operation. In this example, 8 connections are used on bus  103 . The signals could be data bus signals, address bus signals, or control bus signals used during functional communicating between functional circuits  102 . 
       FIG. 38  illustrates how the functional circuits  102  of ICs or cores  3702  and  3704  can be adapted with I/O circuits  802  of the present disclosure to reduce the number of signal connections on functional bus  103  between the functional circuits  102 . As seen, the functional bus  3806  between the adapted ICs or cores  3802  and  3804  require only one half the connections required by functional bus  103  of  FIG. 37 . Also functional bus  3806  maintains the signaling bandwidth of functional bus  103  of  FIG. 37 . 
     As seen in  FIG. 38 , IC or core circuits  3802  and  3804  differ from IC or core circuits  3702  and  3704  of  FIG. 37  in that the input and output signals of bus  103  to functional circuits  102  are interfaced to I/O circuits  802 , via the I/O circuit&#39;s input  2214  and output  2212 . Also as seen, functional circuits  102  in IC or core circuits  3802  and  3804  may optionally be modified, as seen in dotted line, to allow inputting control to the  802  I/O circuit&#39;s output enable signal  822 , otherwise the output enable  822  input of I/O circuit  802  will be fixed to always enable the output buffer  814  of I/O circuit  802 . 
     As seen in  FIG. 38 , the number of bus  3806  connections, via wire terminals  2210  of the I/O circuits  802  of circuits  3802  and  3804 , is reduced by one half of that shown in bus  103  of  FIG. 37 . Thus, the present disclosure provides a way of reducing the number of required functional signal connections between IC or core circuits  3802  and  3804  of  FIG. 38  on bus  3806  by one half that used in the prior art functional bus  103  of  FIG. 37 . 
       FIG. 39  illustrates conventional ICs  3902 ,  3908 ,  3912  on a board/substrate or core circuits  3902 ,  3908 ,  3912  within an IC being connected functionally together via functional bus  103  and select and control bus  3906 . IC/core  3902  contains a master functional circuit  3904 , such as a processor or DSP, that controls communication to slave functional circuits  3910  and  3914 , such as memories or other types of input and output circuits, in IC/cores  3908  and  3912  via buses  103  and  3906 . In this example, the select and control bus  3906  from the master functional circuit functions as a bus that selects a functional slave circuit  3910  or  3914  then inputs control to cause the selected slave circuit to input data from the master circuit or to output data to the master circuit via bus  103 . The functional bus  103  in this example is 8 signals wide. 
       FIG. 40  illustrates how the functional circuits  3904 ,  3910 ,  3914  can be adapted with I/O circuits  802  of the present disclosure to reduce the number of signal connections on functional bus  103  between the functional circuits. As seen, the functional bus  4008  between the adapted ICs or cores  4002 ,  4004 ,  4006  require only one half the connections required by functional bus  103  of  FIG. 39 . Also functional bus  4008  maintains the signaling bandwidth of functional bus  103  of  FIG. 39 . 
     As seen in  FIG. 40 , IC or core circuits  4002 - 4006  differ from IC or core circuits  3902 ,  3908 , and  3912  of  FIG. 39  in that the input and output signals of bus  103  to functional circuits  3904 ,  3910 ,  3914  are interfaced to I/O circuits  802 , via the I/O circuit&#39;s input  2214  and output  2212 . Also as seen, the master functional circuit  3904  of IC/core circuit  4002  may optionally be modified, as seen in dotted line, to allow inputting control to the  802  I/O circuit&#39;s output enable signal  822 , otherwise the output enable  822  input of I/O circuit  802  will be fixed to always enable the output buffer  814  of I/O circuit  802 . Providing the ability to disable the output buffer  814  of I/O circuits  802  connected to master functional circuit  2904  in IC/core circuit  4002  allows for the output buffers  814  of a selected slave functional circuit&#39;s I/O circuits  802 , say slave circuit  3910 , to be enabled to drive the bus  4008  to communicate data to another one or more of the slave functional circuits, say slave circuit  3914 . 
     As seen in  FIG. 40 , the number of bus  4008  connections, via wire terminals  2210  of the I/O circuits  802  of circuits  4002 ,  4004 ,  4008 , is reduced by one half of that shown in bus  103  of  FIG. 39 . Thus, the present disclosure provides a way of reducing the number of required functional signal connections between IC or core circuits  4004 - 4006  of  FIG. 40  on bus  4008  by one half that used in the prior art functional bus  103  of  FIG. 39 . 
     While the preceding description has shown and described the Addressable Tap Domain Selection Circuit  514  as having a 3 pin interface consisting of TCK, TMS/RCK, and TDI/TDO signals, the Addressable Tap Domain Selection Circuit  514  may be designed to use the standard IEEE 1149.1 TDI, TDO, TMS, and TCK signals, plus the non-standard RCK signal. 
       FIG. 41  illustrates an Addressable Tap Domain Selection circuit  4102  that has been designed in an IC or core to use the standard IEEE 1149.1 interface signals TDI, TDO, TMS, TCK, and non-standard RCK signal. The only difference between the Addressable Tap Domain Selection circuit  4102  and the Addressable Tap Domain Selection circuit  514  of  FIG. 8  is that the I/O circuits  802  and  804  have been removed and the TDI  824 , TDO  820 , TMS  830 , and RCK  826  signals have been coupled, via buffers  4104 - 4110 , to externally accessible signal terminals TDI  4112 , TDO  4114 , TMS  4116 , and RCK  4118 , respectively. 
     The operation of Addressable Tap Domain Selection circuit  4102 , in response to the first, second, and third protocols, is identical to that previously described with Addressable Tap Domain Selection circuit  514 . For example, the first protocol uses TCK and TMS  830  as previously described for Hard and Soft resets, the second protocol uses TCK, TMS  830 , and TDI  824  as previously described for loading address and instruction, and the third protocol uses TCK, TMS  830 , TDI  824 , and TDO  820  as previously described to access Tap domains. The only difference is that is that TDI  824  is coupled to a TDI input terminal  4112  instead of I/O circuit  802 , TDO  820  is coupled to a TDO output terminal  4114  instead of I/O circuit  802 , and TMS  830  is coupled to a TMS input terminal  4116  instead of I/O circuit  804 . Also the RCK  826  is coupled to RCK output terminal  4118  instead of I/O circuit  804 . 
     As seen, the control input of output buffer  4106  is coupled to OE 1   822  to enable TDO output during shift operations and to disable TDO output during non-shift operations during third protocol (JTAG) operations. Also, the control input of output buffer  4110  is coupled to OE 2  enable or disable RCK outputs during third protocol (JTAG) operations. 
       FIG. 42  illustrates a group of target devices  4202 - 4206  on a board or other substrate  4200 , each target device including the Addressable Tap Domain Selection Circuit  4102  and its associated 5 pin TCK, TDI, TDO, TMS, and RCK interface, as well as Tap Domain Region  522 . The target devices could be packaged ICs or unpackaged IC die. The 5 pin interface of each target device is coupled to an external controller  4208  via cable connector  4210  to provide access for test, debug, emulation, and trace operations. Each target device  4202 - 4206  may contain embedded core target circuits as described in  FIG. 17 , which also are interfaced to the external controller  4208  via the 5 pin interface. As indicated, the external controller  4208  may be realized by using an interface card  4212  in a personal computer  1726  to control the 5 pin interface communication with the targets  4202 - 4206  via a cable connection  4214 . The 5 pin interface communicates to target circuits using the previously mentioned first, second, and third protocols. 
     Each target  4202 - 4206  has the previously mentioned local address to allow it to be individually addressed and instructed by the controller  4208  using the second protocol. Following the individual addressing and instructing of a target using the second protocol, the Tap Domains  510  within the target may be accessed by the controller  4208  using the third protocol to perform test, debug, emulation, and/or trace operations. Additionally, each target has the previously mentioned global address to allow all targets to be simultaneously addressed and instructed using the second protocol for the purposes previously mentioned in regard to  FIG. 17 . 
     When a target is selected for a third protocol (JTAG) communication to one or more of its Tap Domains, its TDO  4114  terminal will be enabled by OE 1  to output TDO  820  data from the Tap Domain(s) during the 1149.1 Shift-IR and Shift-DR states as previously mentioned. During third protocol (JTAG) operations to Tap Domains with RCK signals, the RCK  4118  terminal will be enabled by OE 2  to output RCK  826  signals to the controller  4208 . During third protocol (JTAG) operations to Tap Domains without RCK signals, the RCK  4118  terminal will be disabled by OE 2  to not output RCK  826  signals to the controller  4208 . 
     Only the addressed target circuit will be enabled to output on its TDO  4114  and RCK  4118  terminals. The TDO  4114  and RCK  4118  terminals of non-addressed target circuits will be disabled, via control signals OE 1  and OE 2  to buffers  4106  and  4110 , so that only the addressed target device is enabled to drive the TDO and RCK signal connections to the controller  4208 . 
       FIG. 43  illustrates the legacy target devices  1802 - 1806  of  FIG. 18 , each including the standard IEEE 1149.1 TRST, TCK, TMS, TDI, and TDO terminals, and optionally the non-standard RCK terminal. The legacy target devices could be ICs  1802 - 1806  on a board or other substrate  4300 , embedded core circuits  1802 - 1806  within an IC  4300 , or embedded core circuits  1802 - 1806  within a core circuit  4300 . 
     As seen, a separate device  4302  exists between the legacy target devices  1802 - 1806  and the external controller  4208 . This separate device  4302  implements the Addressable Tap Domain Selection Circuit  4102  of  FIG. 41  and operates according the previously described first, second, and third protocols. It also includes the previously described local and global addressing modes. The local address is shown, in this example, as being input to the separate device  4302  on externally accessible terminals of device  4302 . The separate device  4302  serves to provide the interface between the JTAG plus RCK interface of each legacy target device and the 5 signal interface to the external controller  4208 . The operation of the separate device  4302  in accessing the legacy device Tap Domains is the same as described in  FIG. 41  where the Addressable Tap Domain Selection Circuit  4102  was described accessing the Tap Domains  510  of Tap Region  522 . 
     The arrangement shown in  FIG. 43  could represent the legacy target devices  1802 - 1806  and separate device  4302  as being; (1) ICs/die on a board or substrate  4300 , embedded core circuits within an IC  4300 , or (3) embedded core circuits within a core circuit  4300 .  FIG. 43  advantageously illustrates how legacy devices designed using the conventional IEEE 1149.1 interface and optional RCK can be interfaced to the 5 signal controller  4208  by providing the Addressable Tap Selection Circuit  4102  as a separate circuit to serve as the interface between the legacy devices  1802 - 1806  and external controller  4208 . As described for separate circuit  1808  of  FIG. 18 , the separate circuit  4302  could contain only the Addressable Tap Domain Selection Circuit  4102  or it could contain the Addressable Tap Domain Selection Circuit  4102  along with other circuits. 
     There may be instances where it may be desirable to select between using a 5 signal interface to an Addressable Tap Domain Selection circuit as shown  FIG. 41  and a 3 signal interface to an Addressable Tap Domain Selection circuit as shown in  FIG. 8 . For example, for test operations it may be advantageous to use the 5 signal interface to enable standard JTAG communication from pre-existing JTAG controllers and testers designed to operate according to the standard JTAG interface and optional non-standard RCK signal. On the other hand, it may be advantageous to use the 3 signal interface during debug, emulation, and trace operations so that unused signals of the 5 signal interface may be used for other purposes. 
       FIG. 44  illustrates an Addressable Tap Domain Selection circuit  4402  that has been designed in an IC or core to selectively use either the 5 signal interface of  FIG. 41  or the 3 signal interface of  FIG. 8 . The Addressable Tap Domain Selection circuit  4402  is the same as that described in  FIGS. 8 and 41  with the exception that an Interface Select Circuit  4404  has been substituted for the I/O circuits  802  and  804  of  FIG. 8  and the input and output buffers  4104 - 4110  of  FIG. 41 . The Interface Select Circuit  4404  has input terminals for receiving control  4416  from the instruction output bus, TDO signal  820 , OE 1  signal  822 , RCK signal  826 , and OE 2  signal  828 . The Interface Select Circuit  4404  has output terminals for outputting TDI signal  824  and TMS signal  830 . The Interface Select Circuit  4404  has input and output terminals for an auxiliary I/O bus (AUXI/O)  4406 . The interface Select circuit  4404  has I/O terminals for an “AUXI/O 1  or TDI” signal  4408 , “TDI/TDO or TDO” signal  4410 , “AUXI/O 2  or TMS” signal  4412 , and “TMS/RCK or RCK” signal  4414 . 
     The function of the Interface Select Circuit  4402  is to respond to control inputs  4416  from the instruction register output bus to operate as either a 5 signal interface or as a 3 signal interface to the Addressable Tap Domain Selection Circuit  4402 . 
     If 5 signal interface operation is selected, the “AUXI/O 1  or TDI” signal  4408  will operate as TDI  4112  of  FIG. 41 , the “TDI/TDO or TDO” signal  4410  will operate as TDO signal  4114  of  FIG. 41 , the “AUXI/O 2  or TMS” signal  4412  will operate as TMS signal  4116  of  FIG. 41 , and the “TMS/RCK or RCK” signal  4414  will operate as RCK signal  4118  of  FIG. 41 . 
     If 3 signal interface operation is selected, the “AUXI/O 1  or TDI” signal  4408  will operate as an auxiliary input or output signal, the “TDI/TDO or TDO” signal  4410  will operate as TDI/TDO signal  520  of  FIG. 8 , the “AUXI/O 2  or TMS” signal  4412  will operate as an auxiliary input or output signal, and the “TMS/RCK or RCK” signal  4414  will operate as TMS/RCK signal  518  of  FIG. 8 . 
     When the instruction register  810  is reset, at power up or following a hard reset first protocol, it will output control  4416  to select either the 3 or 5 signal interface to the Addressable Tap Domain Selection circuit  4402 . Since the IEEE 1149.1 standard requires that its interface be enabled to operate following a reset or power up event, the 5 signal interface is preferably the interface selected by the instruction register  810  following reset or power up. However, while the 5 signal interface is preferred for consistency to the IEEE 1149.1 standard, users of the present disclosure may select the 3 signal interface at reset/power up as well. 
       FIG. 45  illustrates an example of how the Interface Select Circuit  4404  may be designed. The Interface Select Circuit includes I/O circuits  802  and  804  of  FIG. 8 , TDO output buffer  4106  and TMS output buffer  4110  of  FIG. 41 , multiplexers  4516 - 4526 , I/O buffers  4528  and  4104 , and I/O buffers  4532  and  4108 . 
     TDO  820  is coupled to an input of I/O circuit  802  and to the input of buffer  4106 . “TDI/TDO or TDO”  4410  is coupled to an output of I/O circuit  802  and to the output of buffer  4106 . TDI  824  is coupled, via multiplexer  4516 , to either an output of I/O circuit  802  or to the output of buffer  4104 . “AUXI/O 1  or TDI”  4408  is coupled to the input of buffer  4104  and to the output of buffer  4528 . OE 1   822  is coupled to the control input of buffer  814  of I/O circuit  802  via multiplexer  4518  and to the control input of buffer  4106  via multiplexer  4520 . 
     RCK  826  is coupled to an input of I/O circuit  804  and to the input of buffer  4110 . “TMS/RCK or RCK”  4414  is coupled to an output of I/O circuit  804  and to the output of buffer  4110 . TMS  830  is coupled, via multiplexer  4522 , to either an output of I/O circuit  804  or buffer  4108 . “AUXI/O 2  or TMS”  4412  is coupled to the input of buffer  4108  and to the output of buffer  4532 . OE 2   828  is coupled to the control input of buffer  814  of I/O circuit  804  via multiplexer  4524  and to the control input of buffer  4110  via multiplexer  4526 . 
     The output of buffer  4104  is coupled to an auxiliary input  1  (AUXIN 1 ) signal  4504 . The input of buffer  4528  is coupled to an auxiliary output  1  (AUXOUT 1 ) signal  4502 . The output of buffer  4528  is coupled to “AUXI/O 1  or TDI”  4408 . The control input of buffer  4528  is coupled to control signal  4512  from instruction control bus  4416 . 
     The output of buffer  4532  is coupled to an auxiliary input  2  (AUXIN 2 ) signal  4508 . The input of buffer  4532  is coupled to an auxiliary output  2  (AUXOUT 2 ) signal  4506 . The output of buffer  4532  is coupled to “AUXI/O 2  or TMS”  4412 . The control input of buffer  4532  is coupled to control signal  4514  from instruction control bus  4416 . 
     The select input of multiplexers  4516 - 4526  is coupled to control signal  4510  from instruction control bus  4416 . A low on control signal  4510  enables the 3 signal interface mode of Interface Select Circuit  4404 , and a high on control signal  4510  enables the 5 signal interface mode of Interface Select Circuit  4404 . 
     While control signal  4510  is high, multiplexer  4516  couples TDO  824  to the output of buffer  4104 , multiplexer  4518  couples OE 1   822  to the control input of buffer  4106 , multiplexer  4520  couples a low (disable) signal to the control input of I/O circuit  802 , multiplexer  4522  couples TMS  830  to the output of buffer  4108 , multiplexer  4524  couples OE 2   828  to the control input of buffer  4110 , and multiplexer  4526  couples a low (disable) signal to the control input of I/O circuit  804 . In this mode, the Interface Select Circuit  4404  enables the 5 signal interface to operate the Addressable Tap Domain Selection Circuit  4402  as described in  FIG. 41 . That is to say the “AUXI/O 1  or TDI”  4408  signal operates as the TDI signal  4112  of  FIG. 41 , the “TDI/TDO or TDO”  4410  signal operates as the TDO  4114  signal of  FIG. 41 , the “AUXI/O 2  or TMS” signal  4412  operates as the TMS signal  4116  of  FIG. 41 , and the “TMS/RCK or RCK”  4414  signal operates as the RCK  4118  signal of  FIG. 41 . 
     While control signal  4510  is low, multiplexer  4516  couples TDO  824  to the output of I/O circuit  802 , multiplexer  4518  couples OE 1   822  to the control input of I/O circuit  802 , multiplexer  4520  couples a low (disable) signal to the control input of buffer  4106 , multiplexer  4522  couples TMS  830  to the output of I/O circuit  804 , multiplexer  4524  couples OE 2   828  to the control input of I/O circuit  804 , and multiplexer  4526  couples a low (disable) signal to the control input of buffer  4110 . In this mode, the Interface Select Circuit  4404  enables the 3 signal interface to operate the Addressable Tap Domain Selection Circuit  4402  as described in  FIG. 8 . That is to say the “TDI/TDO or TDO”  4410  signal operates as the TDI/TDO  520  signal of  FIG. 8  and the “TMS/RCK or RCK”  4414  signal operates as the TMS/RCK  518  signal of  FIG. 8 . 
     While control signal  4510  is low, selecting the 3 signal interface mode of operation, the “AUXI/O 1  or TDI” signal  4408  can be used as an input, if control signal  4512  is set to disable buffer  4528 , to transmit a signal to the AUXIN 1  signal  4504  output. Alternately, if control signal  4512  is set to enable buffer  4528 , the “AUXI/O 1  or TDI” signal  4408  can be used as an output to transmit a signal from the AUXOUT 1  signal  4502  input. 
     While control signal  4510  is low, selecting the 3 signal interface mode of operation, the “AUXI/O 2  or TMS” signal  4412  can be used as an input, if control signal  4514  is set to disable buffer  4532 , to transmit a signal to the AUXIN 2  signal  4508  output. Alternately, if control signal  4514  is set to enable buffer  4532 , the “AUXI/O 2  or TMS” signal  4412  can be used as an output to transmit a signal from the AUXOUT 2  signal  4506  input. 
       FIG. 46  illustrates an example of the configuration of the Interface Select Circuit  4404  when it is in the 3 signal interface mode. In this mode, the Interface Select Circuit  4404  is configured to access the Addressable Tap Domain Selection Circuit  4402  using the 3 signals “TDI/TDO or TDO”  4410  (named simply TDI/TDO), “TMS/RCK or RCK”  4414  (named simply TMS/RCK), and TCK  516  as described in  FIG. 8 . Since “AUXI/O 1  or TDI”  4408  (named simply AUXI/O 1 ) and “AUXI/O 2  or TMS”  4412  (named simply AUXI/O 2 ) are not used as interface signals, they are shown being used for auxiliary input or output signals to AUXI/O bus  4406 . As seen in  FIGS. 45 and 46 , AUXI/O 1   4408  can be programmed by instruction control input  4512  of control bus  4416  to be an output for AUXOUT 1   4502  or an input for AUXIN 1   4504 . Likewise, AUXI/O 2   4412  can be programmed by instruction control input  4514  of control bus  4416  to be an output for AUXOUT 2   4506  or an input for AUXIN 2   4508 . The AUXIN 1   4504 , AUXOUT 1   4502 , AUXIN 2   4508 , and AUXOUT 2   4506  signals may be data signals, control signals, interrupt signals, triggering signals, or other types of signals that may be necessary to during test, debug, emulation, and/or trace operations using the 3 signal interface. 
       FIG. 47  illustrates an example of the configuration of the Interface Select Circuit  4404  when it is in the 5 signal interface mode. In this mode, the Interface Select Circuit  4404  is configured to access the Addressable Tap Domain Selection Circuit  4402  using the 5 signals “AUXI/O 1  or TDI”  4408  (named simply TDI), “TDI/TDO or TDO”  4410  (named simply TDO), “AUXI/O 2  or TMS”  4412  (named simply TMS), “TMS/RCK or RCK”  4414  (named simply RCK), and TCK as described in  FIG. 41 . As seen, since “AUXI/O 1  or TDI”  4408  (named simply TDI) and “AUXI/O 2  or TMS”  4412  (named simply TMS) are used as interface signals, they cannot be used for auxiliary I/O signals as they were in  FIG. 45 . 
       FIG. 48  illustrates a group of target devices  4204 - 4208  on a board or other substrate  4800 , each target device including the Addressable Tap Domain Selection Circuit  4402  of  FIG. 44  and its selectable 3 or 5 pin interface, as well as Tap Domain Region  522 . The selectable 3/5 pin interface of each target device is coupled to an external controller  4808  via cable connector  4810  to provide access for test, debug, emulation, trace, and/or auxiliary I/O operations. Each target device  4802 - 4806  may contain embedded core target circuits as described in  FIG. 17 , which also are interfaced to the external controller  4808  via the selectable 3/5 pin interface. As indicated, the external controller  4808  may be realized by using an interface card  4812  in a personal computer  1726  to control the selectable 3/5 pin interface communication with the targets  4802 - 4806  via a cable connection  4814 . The interface card  4812  is designed to communicate to the targets using either the 3 or 5 pin interface. The PC  1726  contains software for controlling the card  4812  to access the targets using either the 3 and 5 pin interface, and to switch the targets from operating in the 3 pin interface mode to the 5 pin interface mode, and from operating in the 5 pin interface mode to the 3 pin interface mode. The card  4814  is designed to interface with the auxiliary I/O signals of target circuits when the 3 pin interface mode is selected, and the PC  1726  contains software for comprehending auxiliary I/O signaling from the targets. 
     Each target  4802 - 4806  has the previously mentioned local address to allow it to be individually addressed and instructed by the controller  4808  using the second protocol in either the 3 or 5 pin interface modes. Following the individual addressing and instructing of a target using the second protocol, the Tap Domains  510  within the target may be accessed by the controller  4808  using the third protocol in either the 3 or 5 pin interface mode to perform test, debug, emulation, trace, and/or auxiliary I/O operations. Additionally, each target has the previously mentioned global address to allow all targets to be simultaneously addressed and instructed using the second protocol in either the 3 or 5 pin interface mode for the purposes previously mentioned in regard to  FIG. 17 . 
     The controller  4808  may selectively switch the target circuits  4802 - 4806  from operating in either the 3 or 5 pin interface mode by issuing a global address to all targets, then loading an instruction into all targets that cause all targets to switch from their current interface mode to the other interface mode. For example, if all targets are operating in the 5 pin interface mode they can be switched to the 3 pin interface mode by the controller issuing a global address followed by an instruction that selects the 3 pin interface mode of the targets. Likewise, if all targets are operating in the 3 pin interface mode they can be switched to the 5 pin interface mode by the controller issuing a global address followed by an instruction that selects the 5 pin interface mode of the targets. 
       FIG. 49  illustrates the legacy target devices  1802 - 1806  of  FIG. 18 , each including the standard IEEE 1149.1 TRST, TCK, TMS, TDI, and TDO terminals, and optionally the non-standard RCK terminal. The legacy target devices could be ICs  1802 - 1806  on a board or other substrate  4900 , embedded core circuits  1802 - 1806  within an IC  4900 , or embedded core circuits  1802 - 1806  within a core circuit  4900 . 
     As seen, a separate device  4902  exists between the legacy target devices  1802 - 1806  and the external controller  4808 . This separate device  4902  implements the Addressable Tap Domain Selection Circuit  4402  of  FIG. 44  and operates in either the 3 or 5 signal interface mode according the previously described first, second, and third protocols. It also includes the previously described local and global addressing modes. The local address is shown, in this example, as being input to the separate device  4902  on externally accessible terminals of device  4902 . The separate device  4902  serves to provide the interface between the standard IEEE 1149.1 (plus optional RCK) interface of each legacy target device and the selectable 3 or 5 signal interface to the external controller  4808 . The operation of the separate device  4902  in accessing the legacy device Tap Domains is the same as described in  FIGS. 44-47  where the Addressable Tap Domain Selection Circuit  4402  was described accessing the Tap Domains  510  of Tap Region  522 . 
     The arrangement shown in  FIG. 49  could represent the legacy target devices  1802 - 1806  and separate device  4902  as being; (1) ICs/die on a board or substrate  4900 , embedded core circuits within an IC  4900 , or (3) embedded core circuits within a core circuit  4900 .  FIG. 49  advantageously illustrates how legacy devices designed using the conventional IEEE 1149.1 interface and optional RCK can be interfaced to the selectable 3 or 5 signal interface controller  4808  by providing the Addressable Tap Selection Circuit  4402  as a separate circuit to serve as the interface between the legacy devices  1802 - 1806  and external controller  4808 . As described for separate circuits  1808  and  4302  of  FIGS. 18 and 432 , the separate circuit  4902  could contain only the Addressable Tap Domain Selection Circuit  4402  or it could contain the Addressable Tap Domain Selection Circuit  4402  along with other circuits. 
     As seen in  FIG. 49 , the Addressable Tap Selection Circuit  4402  of separate circuit  4902  has terminals for connecting to auxiliary I/O signals  4502 - 4508 . Thus when separate circuit  4902  is set to operate in the 3 signal interface mode, the “AUXI/O 1  or TDI” signal  4408  and the “AUXI/O 2  or TMS” signal  4412  can be used for communicating auxiliary I/O signals between a further circuit on assembly  4900  and the controller  4808 . The further circuit could be one or more of the target circuits  1802 - 1806 , or a circuit separate from target circuits  1802 - 1806 . The further circuit could also be a circuit contained within separate circuit  4902 . 
     Although the present disclosure has been described in detail, it should be understood that various changes, substitutions and alterations may be made without departing from the spirit and scope of the disclosure as defined by the appended claims.
This disclosure describes a reduced pin bus that can be used on integrated circuits or embedded cores within integrated circuits. The bus may be used for serial access to circuits where the availability of pins on ICs or terminals on cores is limited. The bus may be used for a variety of serial communication operations such as, but not limited to, serial communication related test, emulation, debug, and/or trace operations of an IC or core design. Other aspects of the disclosure include the use of reduced pin buses for emulation, debug, and trace operations and for functional operations. In a fifth aspect of the present disclosure, an interface select circuit, FIGS.  41 - 49 , provides for selectively using either the 5 signal interface of FIG.  41  or the 3 signal interface of FIG.  8.