HAL_UART_Transmit with Single Wire Half Duplex?

[Andrew Neil]

Using CubeIDE v1.13.0, installed this week.

On STM32F0Discovery (STM32F051):

  /* USER CODE BEGIN WHILE */
  while (1)
  {
	  HAL_UART_Transmit( &huart1, "Hello, UART1 world!\r\n", 21, 10000 );
	  HAL_GPIO_TogglePin( LD3_GPIO_Port, LD3_Pin );
	  HAL_Delay( 500 );

	  HAL_UART_Transmit( &huart2, "Hello, UART2 world!\r\n", 21, 10000 );
	  HAL_GPIO_TogglePin( LD4_GPIO_Port, LD4_Pin );
	  HAL_Delay( 500 );

    /* USER CODE END WHILE */
    /* USER CODE BEGIN 3 */
  }
  /* USER CODE END 3 */

The first HAL_UART_Transmit is to UART1, configured as full-duplex.
This works fine - the full string is transmitted each time:

The second HAL_UART_Transmit is to UART2, configured as Single Wire Half Duplex.
This does not quite work - only the 1st two characters are ever transmitted:

If I single-step the transmit loop in the HAL_UART_Transmit for UART2, it does transmit the entire string.

So what's going on here? Is there some gotcha! with Single Wire Half Duplex?

(Screenshots are from YAT; same results when using TeraTerm)

[Andrew Neil]

I reconfigured the IOC to 'Async' instead of Single Wire Half Duplex & re-generated.

No other changes to code.

Now UART2 is also fine:

So there definitely is something wrong with HAL_UART_Transmit in Single Wire Half Duplex.

[LCE]

Then go through that function and find what's happening when the half-duplex select bit is set.

Anyway, you could still manage the half-duplex by yourself, always turn off the transmitter after each transmission.

[LCE]

... okay, in my H7 uart HAL functions there's not much about half-duplex management, mostly that it is an extra init function, not much else.

And nothing in the HAL_UART_Transmit function.

[Andrew Neil]

"Then go through that function and find what's happening when the half-duplex select bit is set"

Yes - doing that...

Not seen anything in Ref Manual yet that suggests the operation should be any different at all.

"you could still manage the half-duplex by yourself, always turn off the transmitter after each transmission"

Datasheet says that happens in hardware.

[Andrew Neil · Accepted solution]

"Datasheet says that happens in hardware"

Well, sort of ...

What the datasheet is talking about is just the input/output setting of the hardware pin.

The user code does have to manage the enabling & disabling of the UART peripheral's transmitter & receiver.

The problem is that the generated initialisation function leaves both the transmitter and the  receiver enabled!

This is what causes HAL_UART_Transmit to fail.

Two options to fix:

1. Add a HAL_HalfDuplex_EnableTransmitter call (which does disable the receiver) before HAL_UART_Transmit;
   
   
2. Add an LL_USART_DisableDirectionRx call before HAL_UART_Transmit.

[Imen.D]

Hello,

An internal ticket (ID 160120) is submitted in order to work on this issue.
(PS: ID 160120 is only for reference, not available outside of ST)

Imen