UART Busy before Start.

[federico.massimi]

Hi, I have a problem with a STM32L476RG.

The problem is UART interface in DMA receive mode, at the beginning of my main function, after having initialized some variable, I call the function:

 __HAL_UART_ENABLE_IT(&huart3, UART_IT_IDLE);
HAL_UART_Receive_DMA(&huart3,TMBUS_RxBuffer, TM_RX_BUFFER_SIZE)

very often the HAL_UART_Receive_DMA function returns HAL_BUSY. What I don't understand is how does the UART get busy if it hasn't started yet?

If I disconnect the RX pin of the UART, the error does not occur, and the HAL_UART_Receive_DMA function returns correctly HAL_OK.

How can I solve?

[Uwe Bonnes]

Missing pull on the RX line or other end yet not powered and pulling the line to invalid state?

[Tesla DeLorean]

Double check how you've initialized structures, and then make sure you clear any pending errors/status, ie Framing, Noise or Overrun, etc.

[federico.massimi]

@Uwe Bonnes​  The UART is connected to a 422 driver which has its own internal pullup. In any case I added a delay before starting the UART to give the driver time to turn on.

I tried to simply move the line

__HAL_UART_ENABLE_IT (......)

after the

HAL_UART_Receive_DMA (.....)

and it seems to work, is it a coincidence or could this be the solution?