I'm using the STM32 VCP firmware and I want to transmit data to my PC from STM32F4 discovery board. The configuration of the virtual COM port is fine, the properties are the following in device manager:

VCP configurations

In english: 9600 bit/s, 8 data bit, no parity, 1 stop bit, no hardware flow control. I'm trying to receive characters in Realterm with these parameters, but I don't get them, it looks like following:

Realterm screenshot

What could I do wrong?


The MCU sends with the following code snippet:

int main(void)
  uint8_t Buf[] = "test";
  while (1)
      CDC_Transmit_FS(Buf, 4);
  • \$\begingroup\$ What data are you sending from the MCU? How have you configured the display of RealTerm? It appears that the PC is receiving 0x00 repeatedly. \$\endgroup\$
    – M D
    Mar 13, 2015 at 20:41
  • \$\begingroup\$ I edited my question. The RealTerm display configuration is in ASCII, turning it in hex format, it gives really 0x00. \$\endgroup\$
    – Milan Tenk
    Mar 13, 2015 at 20:50
  • \$\begingroup\$ Are you giving it some delay between the transmits? \$\endgroup\$
    – Eugene Sh.
    Mar 13, 2015 at 20:55
  • \$\begingroup\$ Yes, there is some delay between the transtmits. \$\endgroup\$
    – Milan Tenk
    Mar 13, 2015 at 21:00
  • \$\begingroup\$ Can you show the enclosing code snippet? \$\endgroup\$
    – Eugene Sh.
    Mar 13, 2015 at 21:01

1 Answer 1


The CDC_Transmit_FS implementation is buggy (at least in the version I am looking at):

uint8_t CDC_Transmit_FS(uint8_t* Buf, uint16_t Len)
  uint8_t result = USBD_OK;
  USBD_CDC_SetTxBuffer(hUsbDevice_0, UserTxBufferFS, Len);  
  result = USBD_CDC_TransmitPacket(hUsbDevice_0);
  /* USER CODE END 8 */
  return result;

As you can see the Buff parameter is never used in the function. You might try modifying the function, by copying the Buff into UserTxBufferFS (using memcpy or whatever).

  • \$\begingroup\$ You were right, using memcpy(UserTxBufferFS, Buf, sizeof(char) * Len); before USBD_CDC_SetTxBuffer(hUsbDevice_0, UserTxBufferFS, Len);, the data is transmitted, and I can see it in RealTerm. :) \$\endgroup\$
    – Milan Tenk
    Mar 13, 2015 at 21:55
  • \$\begingroup\$ Wow, that is exceptionally buggy for vendor-supplied framework sample code. How did that possibly get through QC? \$\endgroup\$
    – RBerteig
    Mar 13, 2015 at 23:26
  • 1
    \$\begingroup\$ @RBerteig: I've seen plenty of vendor-supplied framework code that was sufficiently rotten that I doubt any bugs ever caused any trouble because the code was just plain unworkable. The above function looks like it might have a shot of working if Buf happens to equal UserTxBufferFS. \$\endgroup\$
    – supercat
    Mar 13, 2015 at 23:45
  • \$\begingroup\$ That was probably how it got through. I wish vendors spent a little more effort on code quality and testing for sample code. \$\endgroup\$
    – RBerteig
    Mar 14, 2015 at 2:17
  • 2
    \$\begingroup\$ This question was asked a while ago, but I thought I'd add: In the latest patch of ST's libraries this has been fixed. I thought this was breaking my code but it turns out it's not a problem. The fix was as simple as replacing UserTxBufferFS with Buf, apparently. \$\endgroup\$
    – Jay Greco
    Jan 27, 2016 at 6:42

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