STM32 USB VCP bug

Question

I have been working on a project for the past two weeks now and debugging this one issue has taken up this entire week. Wondering if anyone can help, I will try to be as explicit and clear as possible.

I am trying to implement a USB Virtual Comm Port on a MicroController based on the STM32F302K8 (Cortex M4). I have used STM32CubMX to generate the code needed to set up a USB Full Speed Device implementing a CDC class. My device shows up in both Windows (Device Manager) and Linux. I am able to implement a simple echo function based on the example code but when I now try to use the function USBD_CDC_SetTxBuffer to send data to the PC, this sets off a Hard Fault Handler. I have narrowed this down to the fact that the UsbDeviceFS.pClass (which is needed by USBD_CDC_SetTxBuffer) field is never initialized because USBD_CDC_Init() is never called in the initialization of the USB Device.

I have implemented fixes to several bugs (including changing the heap size, fixing the transmission flag in USBD_CDC_TransmitPacket, and changing the size of CDC_DATA_HS_MAX_PACKET_SIZE to 256 from 512) in the example code as documented on the ST forum but still getting the same errror.

My device setup code is

* USB Device Core handle declaration */
USBD_HandleTypeDef hUsbDeviceFS;

/* init function */                    
void MX_USB_DEVICE_Init(void)
{
  /* Init Device Library,Add Supported Class and Start the library*/
  USBD_Init(&hUsbDeviceFS, &FS_Desc, DEVICE_FS);

  USBD_RegisterClass(&hUsbDeviceFS, &USBD_CDC);

  USBD_CDC_RegisterInterface(&hUsbDeviceFS, &USBD_Interface_fops_FS);

  USBD_Start(&hUsbDeviceFS);

}

Answer 1

To answer my own question, the problem is that my code didn't wait for the USB to finish initialization and immediately began sending data. Inserting an active wait on a boolean or adding a delay (as pointed out by @ramez) solves the problem.

UPDATE This bug has been fixed in subsequent USB CDC driver versions from ST. There is now a HAL_Delay in the setup. Caveat is that if for any reason Sys_Tick does not work/is deactivated/not yet initialised, your code will hang.

Answer 2

So I recently battled with this issue for a REALLY long time.

If your STM32CubeIDE generated code doesn't work from the get-go, then you really want to look into ensuring that your clock is set up properly.

Remember that the USB FS takes a 48 MHz clock, and for me it only worked in the PLL1. I used an STM32H7.

Answer 3

I used CubeMX for generating code for STM32F4 discovery. I used it as virtual COM port as you. I did not use USBD_CDC_SetTxBuffer() function directly. In usbd_cdc_if.c file there is a function named CDC_Transmit_FS(). There was a bug in the generated code, the function took a buffer as parameter and it did nothing with it. The corrected function code is following:

uint8_t CDC_Transmit_FS(uint8_t* Buf, uint16_t Len)
{
  uint8_t result = USBD_OK;
  memcpy(UserTxBufferFS, Buf, sizeof(char) * Len);
  USBD_CDC_SetTxBuffer(hUsbDevice_0, UserTxBufferFS, Len);   
  result = USBD_CDC_TransmitPacket(hUsbDevice_0);
  return result;
}

Actually I had to add the memcpy to the code. After this correction I could send data from the microcotroller to the PC with this transmit function. For example:

int main(void)
{
  HAL_Init();

  SystemClock_Config();

  MX_GPIO_Init();
  MX_USB_DEVICE_Init();
  configureGPIOs();

  uint8_t Buf[] = "Test";

  HAL_Delay(1000);

  while (1)
  {
      CDC_Transmit_FS(Buf, 4);
      HAL_Delay(1000);
  }
}

The initalization in MX_USB_DEVICE_Init() is the same at me as yours.

Answer 4

First, check if hUsbDevice_0 is null (missing element in your solution):

    if (hUsbDevice_0 == NULL)
            return USBD_FAIL;

This will prevent from hanging your uC and does not need busy waiting in delays.

You can place it somewhere in CDC_Transmit_FS:

USBD_StatusTypeDef CDC_Transmit_FS(uint8_t* Buf, uint16_t Len) {

    if (hUsbDevice_0 == NULL)
        return USBD_FAIL;

    USBD_CDC_HandleTypeDef *hcdc = (USBD_CDC_HandleTypeDef*) hUsbDevice_0->pClassData;

    if (hcdc->TxState != 0)
        return USBD_BUSY;

    uint8_t result = USBD_OK;

    USBD_CDC_SetTxBuffer(hUsbDevice_0, Buf, Len);
    result = USBD_CDC_TransmitPacket(hUsbDevice_0);

    return result;
}

Answer 5

This problem is due to the USB host needs time to enum the CDC when the STM32 is plugged in. Once enum is started the function USBD_CDC_Init() in ubd_cdc.c will be called, which in turn calls CDC_Init_FS() in usbd_cdc_if.c. To check whether initialization is done, we could simply check hUsbDeviceFS.pClassData:

In usbd_cdc_if.c, add:

bool CDC_Class_Init_Ok(void)
{
  return hUsbDeviceFS.pClassData;
}

Then in main.c:

int main(void)
{
  HAL_Init();
  SystemClock_Config();
  MX_GPIO_Init();
  MX_USB_DEVICE_Init();
  while (!CDC_Class_Init_Ok());

  while (1) {
  }
}

Answer 6

I had the same problem but it turned out the only thing I need to do was re-plug the USB connection to the computer. Most time you flash the code and reset the microcontroller but on the PC side the enumeration is not updated. USBD_CDC_Init is called when the host starts to probe your device and that's why pClassData is NULL.