a boosted power generator circuit 11 according to a first embodiment of the present invention will be described with reference to fig2 through 10 . as illustrated in fig2 the boosted power generator circuit 11 includes first and second detector circuits 12 , 13 for detecting a boosted voltage vpp ; an incorrect pulse preventing circuit 14 ; an oscillator circuit 15 ; and a charge pump circuit 16 . the boosted power generator circuit 11 operates at different frequencies in an active mode ( device operative mode ) and a standby mode ( device inoperative mode ). the charge pump circuit 16 of the boosted power generator circuit 11 operates at a first frequency in the active mode to generate the boosted voltage vpp . the charge pump circuit 16 operates at a second frequency in the standby mode to output the boosted voltage vpp . the first frequency is greater than the second frequency , so that the boosted power generator circuit 11 supplies more currents in the active mode than in the standby mode . the first detector circuit 12 is activated by an activation signal φ in the active mode . the first detector circuit 12 is a detector circuit for the active mode which has relatively large current consumption and operates at a relatively high reaction speed . the first detector circuit 12 detects the boosted voltage vpp in the active mode , and generates a detection signal det - a when the boosted voltage vpp decreases to a voltage equal to or smaller than a predetermined voltage . the second detector circuit 13 operates without fail whenever the device is applied with a supply voltage vdd , not only in the standby mode . the second detector circuit 13 is a detector circuit for the standby mode which has relatively small current consumption and operates at a relatively low reaction speed . the second detector circuit 13 detects the boosted voltage vpp , and generates a detection signal det - s when the boosted voltage vpp decreases to a voltage equal to or smaller than a predetermined voltage . as illustrated in fig3 the oscillator circuit 15 includes a periodic circuit 23 ; a switch control circuit 24 ; and an inverter gate 25 . the periodic circuit 23 includes a periodic delay circuit 21 and a switch circuit 22 . an output signal of the periodic delay circuit 21 is supplied to the switch circuit 22 , while an output signal ( oscillator signal osc - s ) of the switch circuit 22 is fed back to the periodic delay circuit 21 as well as supplied to the switch control circuit 24 . the inverter gate 25 receives the detection signal det - s from the second detector circuit 13 , inverts the detection signal det - s , and supplies the inverted detection signal to the switch control circuit 24 . the switch control circuit 24 is activated by the inverted detection signal and the oscillator signal osc - s to supply an activation signal to the switch circuit 22 . the switch circuit 22 is activated by the activation signal , so that the periodic circuit 23 operates as a ring oscillator to generate an oscillator signal osc - s which has a second frequency . the incorrect pulse preventing circuit 14 includes a pulse generator circuit 31 ; a synthesizer 32 ; and an oscillator circuit 33 . the oscillator circuit 33 includes a periodic circuit 43 and a switch control circuit 44 . the periodic circuit 43 includes a periodic delay circuit 41 and a switch circuit 42 . an output signal of the periodic delay circuit 41 is supplied to the switch circuit 42 , while an output signal of the switch circuit 42 ( oscillator signal osc ) is fed back to the periodic delay circuit 41 as well as supplied to the switch control circuit 44 . the pulse generator circuit 31 receives the oscillator signal osc - s from the oscillator circuit 15 and generates a pulse signal p 1 . the synthesizer 32 receives the pulse signal p 1 and the detection signal det - a from the first detector circuit 12 , and supplies a synthesized signal to the switch control circuit 44 . the switch control circuit 44 is activated by the synthesized signal from the synthesizer 32 and the oscillator signal osc to supply an activation signal to the switch circuit 42 . the switch circuit 42 is activated by the activation signal , so that the periodic circuit 43 operates as a ring oscillator to generate the oscillator signal osc . the incorrect pulse preventing circuit 14 receives the detection signal det - a ( first control signal in 1 ) from the first detector circuit 12 , and the oscillator signal osc - s ( second control signal in 2 ) from the oscillator circuit 15 , and generates an oscillator signal osc having a first frequency or a second frequency . the oscillator circuit 33 is activated when the synthesizer 32 is supplied with the detection signal det - a having h - level ( when the control signal in 1 is in a first state ). in this event , the periodic circuit 43 of the oscillator circuit 33 oscillates at the first frequency ( greater than the second frequency ) to output the oscillator signal osc having the first frequency from the periodic circuit 43 . when the synthesizer 32 is supplied with the detection signal det - a having l - level ( when the first control signal in 1 is in a second state ), the oscillator circuit 33 is activated by the pulse signal p 1 based on the oscillator signal osc - s . in this event , the periodic circuit 43 of the oscillator circuit 33 oscillates at the second frequency to output the oscillator signal osc at the second frequency from the periodic circuit 43 . the charge pump circuit 16 performs a charge pump operation in accordance with the oscillator signal osc to generate a boosted voltage vpp which is a boosted one of the supply voltage vdd . as illustrated in fig4 the first detector circuit 12 includes resistors r 1 , r 2 ; a current mirror 51 ; and an inverter 52 . the current mirror 51 includes two pmos transistors tr 1 , tr 2 ; and three nmos transistors tr 3 , tr 4 , tr 5 . the boosted voltage vpp generated by the charge pump circuit 16 is divided by the resistors r 1 , r 2 , and a divided voltage is supplied to a first input terminal ( a gate of the first nmos transistor tr 3 ) of the current mirror 51 . the current mirror 51 is also supplied with a reference voltage vref at its second input terminal ( a gate of the second nmos transistor tr 4 ). the third pmos transistor tr 6 is connected in parallel with the second pmos transistor tr 2 . the activation signal φ indicative of the active mode is supplied to a gate of the third pmos transistor tr 6 and a gate of the nmos transistor tr 5 . output signals generated at drains of the second and third pmos transistors tr 2 , tr 6 , and an output signal generated at a drain of the third nmos transistor tr 4 are supplied to the inverter 52 . the first detector circuit 12 is activated by the activation signal φ indicative of the active mode , and generates the detection signal det - a at h - level for activating the oscillator circuit 33 when the divided voltage of the boosted voltage vpp is equal to or smaller than the reference voltage vref . as illustrated in fig5 the second detector circuit 13 includes resistors r 3 , r 4 ; a current mirror 61 ; and an inverter 62 . the current mirror 61 includes two pmos transistors tr 7 , tr 8 ; and three transistors tr 9 , tr 10 , tr 11 . the boosted voltage vpp generated by the charge pump circuit 16 is divided by the resistors r 3 , r 4 , so that a divided voltage is supplied to a first input terminal of the current mirror 61 ( a gate of the first nmos transistor tr 9 ). a reference voltage vref is supplied to a second input terminal of the current mirror 61 ( a gate of the second nmos transistor tr 10 ). the supply voltage vdd is supplied to a gate of the third nmos transistor tr 11 . the second detector circuit 13 is activated at all times as long as the supply voltage vdd is supplied in the active mode and standby mode . the second detector circuit 13 operates irrespective of the active mode or standby mode , and generates the detection signal det - s having h - level for activating the oscillator circuit 15 when the divided voltage of the boosted voltage vpp is equal to or smaller than the reference voltage vref . as illustrated in fig6 the oscillator circuit 15 includes a periodic circuit 23 ; a switch control circuit 24 ; and an inverter gate 25 . the periodic circuit 23 includes a periodic delay circuit 21 and a switch circuit 22 . the switch circuit 22 includes an inverter 22 a comprised of a pmos transistor tr 12 and an nmos transistor tr 13 ; two pmos transistors tr 14 , tr 15 ; and two nmos transistors tr 16 , tr 17 . the periodic delay circuit 21 includes inverters 21 a at an even number of stages ( for example , four stages ). an output terminal of the periodic delay circuit 21 ( a node n 1 ) is connected to an input terminal of the inverter 22 a ( gates of the transistors tr 12 , tr 13 ) of the switch circuit 22 . an output terminal of the inverter 22 a is connected to an input terminal of the periodic delay circuit 21 ( the inverter at the first stage ). an odd number of stages of inverters ( the four inverters 21 a and inverter 22 a ) are connected to the periodic circuit 23 in a loop configuration to form a ring oscillator . the pmos transistor tr 14 has its source connected to the supply voltage vdd , and its drain connected to a source of the pmos transistor tr 12 of the inverter 22 a . the nmos transistor tr 16 has its source connected to a ground gnd , and its drain connected to a source of the nmos transistor tr 13 of the inverter 22 a . therefore , when the transistors tr 14 , tr 16 are turned on , the periodic circuit 23 is activated to generate the oscillator signal osc - s having the second frequency . the pmos transistor tr 15 has its source connected to the supply voltage vdd , and its gate connected to a gate of the nmos transistor tr 16 . the nmos transistor tr 17 has its source connected to the ground gnd , and its gate connected to a gate of the transistor tr 14 . the pmos transistor tr 15 and nmos transistor tr 17 have their drains connected to each other , and a connection node n 2 is connected to the output terminal of the inverter 22 a ( drains of the transistors tr 12 , tr 13 ). the oscillator signal osc - s is output from the node n 2 . the switch control circuit 24 is comprised of a plurality of logic circuits . for example , the switch control circuit 24 includes an inverter 24 a ; a nand circuit 24 b ; and a nor circuit 24 c . the inverter 24 a has its input terminal connected to an output terminal of the inverter gate 25 ( node n 3 ), which is connected to a first input terminal of the nand circuit 24 b . the nand circuit 24 b is supplied with the oscillator signal osc - s at its second input terminal . the oscillator signal osc - s is supplied to a first input terminal of the nor circuit 24 c which has a second input terminal supplied with an output signal of the inverter 24 a . an output signal of the nand circuit 24 b is supplied to a gate of the nmos transistor tr 16 and to a gate of the pmos transistor tr 15 . an output signal of the nor circuit 24 c is supplied to a gate of the nmos transistor tr 17 and to a gate of the pmos transistor tr 14 . in the foregoing configuration , the switch control circuit 24 controls activation / deactivation of the periodic circuit 23 . in the oscillator circuit 15 , when the inverter gate 25 is supplied with the detection signal det - s having h - level from the second detector circuit 13 , the inverter gate 25 outputs an output signal having l - level . in other words , the voltage at the node n 3 is set to l - level . this causes the nand circuit 24 b to output a signal having h - level , and the nor circuit 24 c to output a signal having l - level , so that the transistors tr 14 , tr 16 are turned on , and the transistors tr 15 , tr 17 are turned off . consequently , the inverter 22 a is activated , and the periodic circuit 23 generates the oscillator signal osc - s having the second frequency . when the inverter gate 25 is supplied with the detection signal having l - level from the second detector 13 , the inverter gate 25 outputs an output signal having h - level . in other words , the voltage at the node n 3 is set to h - level . in this state , when the periodic circuit 23 is outputting the oscillator signal osc - s having l - level , the nand circuit 24 b and nor circuit 24 c both output signals having h - level . this causes the transistors tr 14 , tr 15 to turn off , and the transistors tr 16 , tr 17 to turn on . consequently , the inverter 22 a is deactivated to stop the operation of the periodic circuit 23 . in this event , the oscillator signal osc - s is clamped to l - level ( connected to the ground gnd ) by the transistor tr 17 which has been turned on . also , when the inverter gate 25 is outputting an output signal having h - level and the periodic circuit 23 is outputting the oscillator signal osc - s having h - level , the nand circuit 24 b and nor circuit 24 c both output signals having l - level . this causes the transistors tr 14 , tr 15 to turn on , and the transistors tr 16 , tr 17 to turn off . consequently , the inverter 22 a is deactivated to stop the operation of the periodic circuit 23 . in this event , the oscillator signal osc - s is clamped to h - level ( connected to the supply voltage vdd ) by the transistor tr 15 which has been turned on . as illustrated in fig7 the incorrect pulse preventing circuit 14 includes a pulse generator circuit 31 ; a synthesizer 32 ; and an oscillator circuit 33 . the pulse generator circuit 31 , which is comprised of a plurality of logic circuits , generates a pulse in response to a pulse edge of the oscillator signal osc - s ( second control signal in 2 ) output from the oscillator circuit 15 . in other words , the pulse generator circuit 31 is an edge triggered circuit . the pulse generator circuit 31 includes two delay circuits 71 , 72 each comprised of an odd number ( for example , three stages ) of inverters ; an inverter 73 ; and three nand circuits 74 , 75 , 76 . a first nand circuit 74 is supplied at its first input terminal with the oscillator signal osc - s , and supplied at its second input terminal with the oscillator signal osc - s which is delayed by the first delay circuit 71 and inverted . the oscillator signal osc - s is inverted by the inverter 73 , and the inverted oscillator signal osc - s is supplied to a first input terminal of a second nand circuit 75 . the second nand circuit 75 is supplied at its second input terminal with an output signal of the inverter 73 which is delayed by the second delay circuit 72 and inverted . output signals of the nand circuits 74 , 75 are supplied to a third nand circuit 76 . in response to pulse edges ( a rising edge and a falling edge ) of the oscillator signal osc - s having the second frequency , the third nand circuit 76 outputs a pulse signal p 1 . specifically , the pulse generator circuit 31 generates the pulse signal p 1 having an h - level pulse width corresponding to delay times of the delay circuits 71 , 72 in response to a pulse edge of the oscillator signal osc - s . in other words , the pulse signal p 1 has a frequency twice the second frequency . the periodic circuit 43 of the oscillator circuit 33 operates in a half - period ( or a complete period ) of the oscillator signal osc - s having the second frequency in response to the pulse signal p 1 . specifically , the pulse generator circuit 31 generates the pulse signal p 1 which has a pulse formed for a duration shorter than the time in which the periodic circuit 43 is operated in a half - period ( or a complete period ). the pulse width of the pulse signal p 1 is set according to the delay times of the first and second delay circuits 71 , 72 . the synthesizer 32 is comprised of a nor circuit . the nor circuit receives the detection signal det - a ( first control signal in 1 ) from the first detector circuit 12 , and the pulse signal p 1 from the pulse generator circuit 31 , and generates an output signal at l - level when the nor circuit receives the detection signal det - a at h - level for activating the oscillator circuit 33 or the pulse signal p 1 at h level . the oscillator circuit 33 includes a periodic circuit 43 and a switch control circuit 44 . the periodic circuit 43 includes a periodic delay circuit 41 and a switch circuit 42 . in other words , the oscillator circuit 33 has the same configuration as the oscillator circuit 15 . a transistor comprising each inverter 41 a of the periodic delay circuit 41 has a device parameter ( for example , a channel width ) different from that of the transistor comprising each inverter 21 a of the periodic delay circuit 21 of fig6 . then , the oscillator circuit 33 generates an oscillator signal having an oscillating frequency ( i . e ., the first frequency ) different from the oscillating frequency ( second frequency ) of the oscillator circuit 15 ( first frequency & gt ; second frequency ). alternatively , the number of inverters of the periodic delay circuit 41 may be chosen to be different from the number of inverters in the periodic delay circuit 21 to generate oscillator signals at different oscillating frequencies . the switch control circuit 44 controls activation / deactivation of the periodic circuit 43 , and the periodic circuit 43 outputs the oscillator signal osc from a node n 5 . specifically , when the synthesizer 32 outputs an output signal having l - level ( when the node n 6 falls to l - level ), the inverter 42 a of the switch circuit 42 is causing the periodic circuit 43 to operate . when the synthesizer 32 outputs an output signal having h - level ( when the node n 6 rises to h - level ), the inverter 42 a is deactivated , causing the periodic circuit 43 to stop operating . then , when the periodic circuit 43 is outputting the oscillator signal osc having l - level , the oscillator signal osc is clamped to l - level by the transistor tr 23 which has been turned on . also , when the periodic circuit 43 is outputting the oscillator signal osc having h - level , the oscillator signal osc is clamped to h - level by the transistor tr 21 which has been turned on . in the foregoing manner , the incorrect pulse preventing circuit 14 generates the oscillator signal osc in response to the detection signal det - a or oscillator signal osc - s . specifically , when the synthesizer 32 is supplied with the detection signal det - a having h - level ( active mode ), the transistors tr 20 , tr 22 are turned on , while the transistors tr 21 , tr 23 are turned off . consequently , the oscillator circuit 33 generates the oscillator signal osc having the first frequency . when the synthesizer 32 is supplied with the detection signal det - a having l - level ( in the standby mode ), the oscillator circuit 33 intermittently operates in response to the pulse signal p 1 . the pulse generator circuit 31 is triggered by an edge of the oscillator signal osc - s to generate the pulse signal p 1 which has h - level for a predetermined duration . this causes the synthesizer 32 to output a pulse signal having l - level ( see fig9 ( b )), so that the inverter 42 a of the switch circuit 42 of the oscillator circuit 33 is activated . in this event , as shown in fig9 ( b ), when the voltage of the oscillator signal osc is , for example , at l - level , the voltage at an output terminal ( node n 4 ) of the periodic delay circuit 41 is also at l - level . therefore , when the inverter 42 a is activated , the oscillator circuit 33 immediately outputs the oscillator signal osc having h - level . the pulse signal at l - level from the synthesizer 32 rises to h - level before the inverted oscillator signal osc is transmitted to the node n 4 ( or before the voltage at the node n 4 is changed by the periodic delay circuit 41 ), to deactivate the inverter 42 a . a nand circuit 44 b of the switch control circuit 44 outputs a signal having l - level in response to the oscillator signal osc having h - level . the l - level signal turns on the transistor tr 21 which clamps the oscillator signal osc to h - level . as a result , the periodic circuit 43 of the oscillator circuit 33 operates in a half - period of the oscillator signal osc - s having the second frequency . subsequently , when the synthesizer 32 outputs a pulse signal having l - level in response to the pulse signal p 1 having h - level , the voltage at the output terminal ( node n 4 ) of the periodic delay circuit 41 is at h - level because the oscillator signal osc is clamped to h - level . therefore , when the inverter 42 a is activated , the oscillator circuit 33 immediately outputs the oscillator signal osc having l - level . the l - level pulse signal from the synthesizer 32 rises to h - level before the inverted oscillator signal osc is transmitted to the node n 4 ( or before the voltage at the node n 4 is changed by the periodic delay circuit 41 ), to deactivate the inverter 42 a . then , a nor circuit 44 c outputs a signal having h - level in response to the oscillator signal osc having l - level . the h - level signal turns on the transistor tr 23 which clamps the oscillator signal osc to l - level . as a result , the periodic circuit 43 in the oscillator circuit 33 operates in a half - period . as described above , the oscillator circuit 33 generates the oscillator signal osc having a period synchronized to the period of the second oscillator signal osc - s when the synthesizer 32 is supplied with the detection signal det - a at l - level ( standby mode ). the periodic circuit 43 of the oscillator circuit 33 may operate in a complete period in response to the pulse signal p 1 from the pulse generator circuit 31 , rather than operates in a half - period . as illustrated in fig8 the charge pump circuit 16 includes transistors tr 24 , tr 25 , each functioning as a diode ; and a capacitor c 1 . the capacitor c 1 has an output terminal connected to a node n 7 between the transistors tr 24 and tr 25 . the capacitor c 1 is supplied with the oscillator signal osc at its input terminal . each of the transistors tr 24 , tr 25 is , for example , comprised of an nmos transistor having its gate connected to its drain to form a mos diode . the transistor tr 24 has a drain connected to a high potential power supply vdd , and the boosted voltage vpp is output from a source of the transistor tr 25 . when the oscillator signal osc at l - level is supplied , the capacitor c 1 is charged by the high potential power supply vdd , resulting in the node n 7 set at a voltage smaller by a threshold value of the transistor tr 24 than the voltage of the high potential power supply vdd . subsequently , when the oscillator signal osc at h - level is supplied , the voltage at the node n 7 is boosted by capacitive coupling of the capacitor c 1 . then , when the voltage at the node n 7 becomes greater by a threshold value of the transistor tr 25 than the boosted voltage vpp , a charge from the capacitor c 1 is supplied to boost the boosted voltage vpp . in this manner , the oscillator signal osc at h - level or l - level is repeatedly supplied to generate the boosted voltage vpp . next , the operation of the boosted power generator circuit 11 will be described with reference to fig9 ( a )- 9 ( c ). as shown in fig9 ( a ), when the first detector circuit 12 outputs the detection signal det - a at h - level indicative of the active mode , the voltage at the output terminal ( node n 6 ) of the synthesizer 32 falls to l - level . then , the oscillator circuit 33 generates the oscillator signal osc having the first frequency . subsequently , when the first detector circuit 12 outputs the detection signal det - a at l - level to release the active mode , the voltage at the node n 6 rises to h - level , thereby deactivating the oscillator circuit 33 . at this time , the oscillator signal osc is clamped to h - level . next , as shown in fig9 ( b ), when the first detector circuit 12 outputs the detection signal det - a at l - level , and the oscillator circuit 15 supplies the oscillator signal osc - s , a pulse at l - level is generated at the node n 6 . at this time , the pulse at l - level is generated in response to a pulse edge of the oscillator signal osc - s having the second frequency . therefore , the oscillator circuit 33 oscillates substantially at the same period as the second oscillator signal osc - s . next , description will be given for the case where the oscillator signal osc - s is supplied in a short time period after the first detector circuit 12 outputs the detection signal det - a at l - level ( after releasing the active mode ). as shown in fig9 ( c ), assume that the oscillator circuit 33 is now outputting the oscillator signal osc having the first frequency in response to the detection signal det - a at h - level . next , as the oscillator circuit 33 is supplied with the detection signal det - a at l - level , the voltage at the node n 6 rises to h - level , thereby deactivating the oscillator circuit 33 . in this event , the oscillator signal osc is clamped to l - level . when a pulse ( h - level pulse ) of the oscillator signal osc - s is supplied immediately after switching to the detection signal det - a at l - level , an l - level pulse is generated at the node n 6 , thereby activating the inverter 42 a to release the oscillator signal osc clamped to l - level . however , since the oscillator signal osc at l - level is delayed by the periodic delay circuit 41 at that time , the voltage at the node n 4 is still maintained at h - level . for this reason , the oscillator circuit 33 outputs the oscillator signal osc at l - level for a predetermined time period . further , as shown in fig9 ( c ), since the l - level pulse generated at the node n 6 disappears within a short period , the inverter 42 a of the switch circuit 42 is deactivated before the voltage at the node n 4 falls down . in other words , the periodic circuit 43 stops its operation with the oscillator signal osc maintained at l - level . with the foregoing operation , the oscillator circuit 33 will not generate the oscillator signal osc which has a period shorter than the period of the first frequency . this applies also to the case where the detection signal at h - level is supplied immediately after the oscillator circuit 15 supplies the oscillator signal osc - s . fig1 is a graph showing the relationship between an input frequency and an output frequency of the incorrect pulse preventing circuit 14 . the incorrect pulse preventing circuit 14 receives the oscillator signal osc - s , and stably generates the oscillator signal osc having a frequency smaller than the first frequency . the boosted power generator circuit 11 according to the first embodiment has the following advantages . ( 1 ) the oscillator circuit 33 of the incorrect pulse preventing circuit 14 generates the oscillator signal osc having the first frequency when the synthesizer 32 is supplied with the detection signal det - a in the active mode . the oscillator circuit 33 generates the oscillator signal osc having the same period as the period of the oscillator signal osc - s having the second frequency in response to the oscillator signal osc - s supplied from the oscillator circuit 15 through the pulse generator circuit 31 in the standby mode . therefore , no oscillator signal osc having a period ( pulse width ) shorter than the period of the first frequency will be output from the incorrect pulse preventing circuit 14 . consequently , the charge pump circuit 16 provides a stable charge pump operation , resulting in stabilization of the boosted voltage vpp and efficient supply thereof . ( 2 ) even when the mode is switched , or when a change in the detection signal det - a overlaps a change in the oscillator signal osc - s , the incorrect pulse preventing circuit will not generate the oscillator signal osc having a pulse width shorter than the period of the oscillator signal having the first frequency . ( 3 ) since a single charge pump is connected to the oscillator circuits 15 , 33 , an increase in the semiconductor die size is prevented . in the following , a boosted power generator circuit 81 according to a second embodiment of the present invention will be described with reference to fig1 and 12 . the boosted power generator circuit 81 includes first and second detector circuits 12 , 13 ; an incorrect pulse preventing circuit 82 ; oscillator circuits 15 , 83 ; and a charge pump circuit 16 . in the second embodiment , the oscillator circuit 83 supplies the incorrect pulse preventing circuit 82 with an oscillator signal osc - a ( a first control signal in 1 ) in response to a detection signal det - a of the first detector circuit 12 . the oscillator circuit 83 has the same configuration as the oscillator circuit 15 , so that detailed description thereof is omitted . the oscillator circuit 83 generates an oscillator signal having a third frequency smaller than the first frequency of the oscillator circuit 33 . also , the third frequency is greater than the second frequency of the oscillator circuit 15 . expressed in another way , first frequency & gt ; third frequency & gt ; second frequency is established . as illustrated in fig1 , the incorrect pulse preventing circuit 82 includes a first and a second pulse generator circuit 31 a , 31 b ; a synthesizer 32 ; a periodic circuit 43 ; and a switch control circuit 44 . each of the first and second pulse generator circuits 31 a , 31 b has the same configuration as the pulse generator circuit 31 in fig7 . the synthesizer 32 is supplied with a pulse signal output from the first pulse generator 31 a in response to the oscillator signal osc - a ( the first control signal in 1 ), and with a pulse signal output from the second pulse generator circuit 31 b in response to an oscillator signal osc - s ( a second control signal in 2 ). next , the operation of the boosted power generator circuit 81 will be described . the first detector circuit 12 supplies the third oscillator circuit 83 with the detection signal det - a having h - level when a divided voltage of the boosted voltage vpp is equal to or smaller than a reference voltage vref in the active mode . the third oscillator circuit 83 generates the oscillator signal osc - a having the third frequency in response to the detection signal det - a at h - level . the synthesizer 32 of the incorrect pulse preventing circuit 82 is supplied with the oscillator signal osc - a through the first pulse generator circuit 31 a , and the oscillator circuit 33 oscillates at the same frequency as the third frequency . therefore , the incorrect pulse preventing circuit 82 outputs the oscillator signal having the same frequency as the third frequency . the charge pump circuit 16 performs a boost operation in accordance with the oscillator signal osc to generate a boosted voltage vpp . when a divided voltage of the boosted voltage vpp rises to a voltage equal to or grater than the reference voltage vref , the first detector circuit 12 outputs the detection signal det - a having l - level , thereby deactivating the third oscillator circuit 83 and incorrect pulse preventing circuit 82 . consequently , the charge pump circuit 16 stops its operation . the second detector circuit 13 supplies the oscillator circuit 15 with a detection signal det - s having h - level , when the divided voltage of the boosted voltage vpp is equal to or smaller than the reference voltage vref . the oscillator circuit 15 generates the oscillator signal osc - s having the second frequency in response to the detection signal det - s at h - level . the synthesizer 32 is supplied with the oscillator signal osc - s through the second pulse generator circuit 31 b , and the oscillator circuit 33 oscillates at the same frequency as the second frequency . therefore , the incorrect pulse preventing circuit 82 outputs the oscillator signal having the same frequency as the second frequency . the charge pump circuit 16 performs a boost operation in accordance with the oscillator signal osc to generate the boosted voltage vpp . the second detector circuit 13 outputs the detection signal det - s having l - level when the divided voltage of the boosted voltage vpp rises to a voltage equal to or greater than the reference voltage vref , thereby deactivating the oscillator circuit 15 and incorrect pulse preventing circuit 82 . therefore , the charge pump circuit 16 stops its operation . in the second embodiment , the frequency of the oscillator signal osc of the incorrect pulse preventing circuit 82 is switched each time the first and second control signals in 1 , in 2 ( oscillator signals osc - a , osc - s ) are switched . therefore , the boosted power generator circuit 81 of the second embodiment has the same advantages as the boosted power generator circuit 11 of the first embodiment . it should be apparent to those skilled in the art that the present invention may be embodied in many other specific forms without departing from the spirit or scope of the invention . particularly , it should be understood that the invention may be embodied in the following forms . the periodic delay circuit 41 of the periodic circuit 43 may be comprised of an even number of inverters 41 a such as two , six and the like . oscillator signals having different oscillating frequencies may be generated by providing a number of inverters comprising the periodic delay circuit 41 of the oscillator circuit 33 different from the number of inverters comprising the periodic delay circuit 21 of the oscillator circuit 15 . the present invention may be applied to memory devices such as sdram and the like . in this case , the boosted power generator circuit may be supplied with a system clock as the first control signal in 1 and with a self - refresh request signal as the second control signal in 2 . the present invention may be applied to a negative power generator circuit as well as the boosted power generator circuits 11 , 81 . therefore , the present examples and embodiments are to be considered as illustrative and not restrictive and the invention is not to be limited to the details given herein , but may be modified within the scope and equivalence of the appended claims .