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Andrew Neil
Andrew Neil
Super User
August 3, 2023
Solved

HAL_UART_Transmit with Single Wire Half Duplex?

  • August 3, 2023
  • 6 replies
  • 3963 views

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:

AndrewNeil_0-1691052174456.png

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:

AndrewNeil_1-1691052247089.png

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)

 

    This topic has been closed for replies.
    Best answer by Andrew Neil

    "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.

    6 replies

    Andrew Neil
    Andrew NeilAuthor
    Super User
    August 3, 2023

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

    No other changes to code.

    Now UART2 is also fine:

    AndrewNeil_0-1691058338771.png

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

    A complex system that works is invariably found to have evolved from a simple system that worked.A complex system designed from scratch never works and cannot be patched up to make it work.
    LCE
    LCE
    Principal II
    August 3, 2023

    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.

    Andrew Neil
    Andrew NeilAuthor
    Super User
    August 3, 2023

    "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.

    AndrewNeil_0-1691061513408.png

     

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

    Datasheet says that happens in hardware.

    A complex system that works is invariably found to have evolved from a simple system that worked.A complex system designed from scratch never works and cannot be patched up to make it work.
    Andrew Neil
    Andrew NeilAuthorBest answer
    Super User
    August 4, 2023

    "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.
    A complex system that works is invariably found to have evolved from a simple system that worked.A complex system designed from scratch never works and cannot be patched up to make it work.
      LCE
      LCE
      Principal II
      August 3, 2023

      ... 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.

      Imen.D
      ST Technical Moderator
      August 29, 2023

      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

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