title: Detecting open PC COM port from USB Virtual Com Port device
body: <p>I am using an STM32F105 microcontroller with the STM32_USB-FS-Device_Lib_V3.2.1 USB library and have adapted the VCP example for our purposes (integration with RTOS and serial API).</p>
  
  <p>The problem is that if the USB cable is attached, but the port is not open on the Windows host, after a few minutes the device ends up permanently re-entering the USB ISR until the port is opened and then it all starts working normally.</p>
  
  <p>I have instrumented interrupt handler and can see that when the fault occurs, the ISR handler exits and then immediately re-enters.  This occurs because on exit from the interrupt the IEPINT flag in OTG_FS_GINTSTS is not clear. The OTG_FS_DAINT at this time contains 0x00000002 (IEPINT1 set), while DIEPINT1 has 0x00000080 (TXFE).  The line in OTGD_FS_Handle_InEP_ISR() that clears TXFE is called, but the bit either does not clear or becomes immediately reasserted. When the COM port on the host is reopened, the state of OTG_FS_GINTSTS and OTG_FS_DAINT at the end of the interrupt is always zero, and further interrupts occur at the normal rate.  Note that the problem only occurs if data is being output but the host has no port open.  If either the port is open or no data is output, the system runs indefinitely.  I believe that the more data that is output the sooner the problem occurs, but that is anecdotal at present.</p>
  
  <p>The VCP code has a state variable that takes the following enumerated values:</p>
  
  <pre><code>  UNCONNECTED,
    ATTACHED,
    POWERED,
    SUSPENDED,
    ADDRESSED,
    CONFIGURED
  </code></pre>
  
  <p>and we use the CONFIGURED state to determine whether to put data into the driver buffer for sending. However the CONFIGURED state is set when the cable is attached not when the host has the port open and an application connected.  I see that when Windows does open the port, there is a burst of interrupts so it seems that some communication occurs on this event; I wonder if it is possible therefore to detect whether the host has the port open,.</p>
  
  <p>I need one of two things perhaps:</p>
  
  <ol>
  <li>To prevent the USB code from getting stuck in the ISR in the first instance </li>
  <li>To determine whether the host has the port open from the device end, and only push data for sending when open. </li>
  </ol>