Detecting open PC COM port from USB Virtual Com Port device

Question

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).

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.

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.

The VCP code has a state variable that takes the following enumerated values:

  UNCONNECTED,
  ATTACHED,
  POWERED,
  SUSPENDED,
  ADDRESSED,
  CONFIGURED

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,.

I need one of two things perhaps:

1. To prevent the USB code from getting stuck in the ISR in the first instance
2. To determine whether the host has the port open from the device end, and only push data for sending when open.

Answer 1

Part (1) - preventing the interrupt lock-up - was facilitated by a USB library bug fix from ST support; it was not correctly clearing the TxEmpty interrupt.

After some research and assistance from ST Support, I have determined a solution to part (2) - detecting whether the host port is open. Conventionally, when a port is opened the DTR modem control line is asserted. This information is passed to a CDC class device, so I can use this to achieve my aim. It is possible for an application to change the behaviour of DTR, but this should not happen in any of the client applications that are likely to connect to this device in this case. However there is a back-up plan that implicitly assumes the port to be open if the line-coding (baud, framing) are set. In this case there is no means of detecting closure but at least it will not prevent an unconventional application from working with my device, even if it then causes it to crash when it disconnects.

Regarding ST's VCP example code specifically I have made the following changes to usb_prop.c:

1) Added the following function:

#include <stdbool.h>
static bool host_port_open = false ;
bool Virtual_Com_Port_IsHostPortOpen()
{
    return bDeviceState == CONFIGURED && host_port_open ;
}

2) Modified Virtual_Com_Port_NoData_Setup() handling of SET_CONTROL_LINE_STATE thus:

else if (RequestNo == SET_CONTROL_LINE_STATE)
{
  // Test DTR state to determine if host port is open
  host_port_open = (pInformation->USBwValues.bw.bb0 & 0x01) != 0 ;
  return USB_SUCCESS;
}

3) To allow use with applications that do not operate DTR conventionally I have also modified Virtual_Com_Port_Data_Setup() handling of SET_LINE_CODING thus:

  else if (RequestNo == SET_LINE_CODING)
  {
    if (Type_Recipient == (CLASS_REQUEST | INTERFACE_RECIPIENT))
    {
      CopyRoutine = Virtual_Com_Port_SetLineCoding;

      // If line coding is set the port is implicitly open 
      // regardless of host's DTR control.  Note: if this is 
      // the only indicator of port open, there will be no indication 
      // of closure, but this will at least allow applications that 
      // do not assert DTR to connect.
      host_port_open = true ;

    }
    Request = SET_LINE_CODING;
  }