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STM32L476RG CubeMX generated USB CDC code stops transmitting out of the blue

Question asked by Sven Köhler on Feb 24, 2018
Latest reply on Feb 26, 2018 by Bogdan Golab

I soldered a USB cable to pins PA12 and PA11 on the Nucleo-L476RG board. In CubeMX I enabled USB with its default settings, disabled DMA (it's not supported for FullSpeed USB anyway) and generated the code. I used the latest CubeMX (4.24) as well as the latest CubeL4 (v1.11.0) available.


I adjusted main.c so that it sends data in a loop. This is the code inside the main method:


uint32_t x = 0;
static char buf[1024];

while (1) {
  size_t len = sprintf(buf,
    "0123456789" "0123456789" "0123456789" "0123456789" "0123456789"
   "0123456789" "0123456789" "0123456789" "0123456789" "0123456789"
   "0123456789" "0123456789" "0123456789" "0123456789" "0123456789"
   "0123456789" "0123456789" " %u\r\n", x++);
  uint8_t ret = CDC_Transmit_FS(buf, len);
  HAL_GPIO_WritePin(LD2_GPIO_Port, LD2_Pin,


The CDC_Transmit_FS function is as generated by CubeMX. It checks the TxState and returns USBD_BUSY if the USB is busy sending. If the USB is not busy, it calls USBD_CDC_SetTxBuffer and transmits the packet via USBD_CDC_TransmitPacket.


The CDC_Receive_FS function is also as generated by CubeMX. It simply calls USBD_CDC_SetRxBuffer and USBD_CDC_ReceivePacket. The data written into the buffer is never actually used.


On the PC (Linux in this case), I start a terminal program (picocom) to display all the text that arrives on the virtual COM port /dev/ttyACM1. The lines are displayed in my terminal window just fine - but then stop inexplicably. When it stops, the output usually looks something like this:


01234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789 121520
01234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789 121521
01234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789 121522
4567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789 121523
01234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789 121525


As you can see, line 121523 is shorter than the others and doesn't start with a zero. The last line is complete, but after that line, the transmission has stopped. Sometimes it takes a while to fail. Sometimes it stops working after just a few lines.


I know that CDC_Transmit_FS doesn't wait until the transmission has finished. However, as long as transmission is in progress, it should return USBD_BUSY and it should not disturb any ongoing transmission.


Does somebody have a similar experience? Did somebody get USB to work reliably?

I could very that TxState stays 1 and is not reset to zero.


I have read horrible things here about ST's USB CDC code (race conditions have been found in the past) and it looks like the L4 code still contains such a race condition.