Semaphore

Kernel
1

k3nt00 wrote on Thursday, May 21, 2015:

Hello,

I am running FreeRTOS 8.2.1 on an STM32F103 with the newest STM32F1 Hal Driver

I had been having some issues with the interrupt UART Tx beginning a new transmission before the one before it completed. I call the interrupt transmit and then wait on a semaphore from the callback to be given.


if(HAL_UART_Transmit_IT(&huart1, (uint8_t*) outBuffer, dataSize) != HAL_OK)
{
    Error_Handler();
}
   
xSemaphoreTake(waitUntilSendDone, portMAX_DELAY);

The Callback:


void HAL_UART_TxCpltCallback (UART_HandleTypeDef *UartHandle)
{
    xHigherPriorityTaskWoken = pdFALSE;
    xSemaphoreGiveFromISR(waitUntilSendDone, &xHigherPriorityTaskWoken);
}

The problem is, the callback was being called after only 2 bytes had been transmitted, which allowed for the second transmit buffer to be sent prematurely.

The ST Examples use the following:


if(HAL_UART_Transmit_IT(&huart1, (uint8_t*) outBuffer, dataSize) != HAL_OK)
{
    Error_Handler();
}
   
   while (UartReady != SET)
   {
   }

   /* Reset transmission flag */
   UartReady = RESET;

And the Callback:


void HAL_UART_TxCpltCallback (UART_HandleTypeDef *UartHandle)
{
   /* Set transmission flag: transfer complete */
   UartReady = SET;
}

This solution solved the problem. The callback wasn’t called until the first buffer was completely transmitted. I do not understand why this is? Why is the method of waiting effecting the callback?

Any insight would be much appreciated.

Thanks

PS Posted this prematurely without proper title. Apologies.

2

heinbali01 wrote on Friday, May 22, 2015:

Hello,

Your HAL_UART_TxCpltCallback() is calling xSemaphoreGiveFromISR() but what I miss is a call to portYIELD_FROM_ISR().

In other words: the semaphore will be set, the task waiting for the semaphore becomes runnable but it wakes up much later than expected.

It is a pity that the entire ISR handling is done within the STM32 library:

void HAL_DMA_IRQHandler(DMA_HandleTypeDef *hdma)
{
    ...
    /* Transfer complete callback */
    hdma->XferCpltCallback(hdma);
    ....
}

which calls:

static void UART_DMATransmitCplt(DMA_HandleTypeDef *hdma)
{
    HAL_UART_TxCpltCallback(huart);
}

Once I made a minor change in the STM32 library. I had the same problem with the SD-card: once a transfer is ready I wanted to set a semaphore and unblock some process.

/* A global variable that you can set from within a TX call-back */
BaseType_t xDMA_IRQ_HigherPriorityTaskWoken;

void HAL_DMA_IRQHandler(DMA_HandleTypeDef *hdma)
{
    xDMA_IRQ_HigherPriorityTaskWoken = pdFALSE;
    ...
    /* Transfer complete callback */
    hdma->XferCpltCallback(hdma);
    ....
    if( xDMA_IRQ_HigherPriorityTaskWoken != pdFALSE )
    {
        portYIELD_FROM_ISR( xDMA_IRQ_HigherPriorityTaskWoken );
    }
}

PS. here was an earlier post about using ‘portYIELD_FROM_ISR()’ :

https://sourceforge.net/p/freertos/discussion/382005/thread/7217ae20

Regards.