I am using STM32 HAL VCP firmware generated by CubeMX on STM32F4 discovery board. The Virtual COM Port works, I can transmit from the microcontroller to the PC as many messages I want. But in the other direction, from PC to microcontroller I can send only one message. If I try to send a second one, the static int8_t CDC_Receive_FS (char* Buf, uint32_t *Len) callback won't be called, and on the PC RealTerm hangs. My client program hangs too, when it tries to write in the COM port second time.

On microntroller side I removed all of my code in CDC_Receive_FS(), it returns currently only USBD_OK. In the main function there is only a HAL_Delay() periodically called, I tried to remove this too, but it had no effect.

Any ideas, what could be the problem?


1 Answer 1


I found the answer for my own question. There is a well documentated example on the following webpage: http://visualgdb.com/tutorials/arm/stm32/usb/

First of all, I had to complete the generated code in static int8_t CDC_Receive_FS (char* Buf, uint32_t *Len) as it is written in the source above.

After that I had to create and implement the int VCP_read(void *pBuffer, int size) function, as it is written in the above mentioned source.

The key point was, that I had to call periodically the int VCP_read(void *pBuffer, int size) function, to read out from the buffer the received data. If I don't call this function, the PC can't write next time the COM port.

  • 2
    \$\begingroup\$ Thanks for guiding me to the answer. For me, the key to this was adding a call to 'USBD_CDC_ReceivePacket(&USBD_Device);' within the receive callback... 'static int8_t CDC_Itf_Receive(uint8_t* Buf, uint32_t *Len)' It looks to be needed to tell the host PC driver that we have received the packet OK. \$\endgroup\$
    – sergeantKK
    Commented Sep 22, 2016 at 16:35

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