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Use of vTaskSuspendAll on hardware access?

Posted by Benjamin Meier on February 9, 2012
Hi all,

I am running FreeRTOS successfully on a STM32F103CBT6 using Keil uVision.

I use the SPI from two different tasks. To ensure only one task is accessing the SPI at a time i use a Mutex.
So far so good.

From the flash.c-file of the demonstration i got the following function.


void vParTestToggleLED( unsigned portBASE_TYPE uxLED )
{
unsigned portSHORT usBit;

vTaskSuspendAll();
{
if( uxLED < partstMAX_OUTPUT_LED )
{
usBit = partstFIRST_LED << uxLED;

if( usOutputValue & usBit )
{
usOutputValue &= ~usBit;
}
else
{
usOutputValue |= usBit;
}

GPIO_Write( GPIOC, usOutputValue );
}
}
xTaskResumeAll();
}


I am wondering about the use of the SuspendAll / ResumeAll calls. What exactly are they for in this application? I see two possible explanations.

1) To ensure no other task is currently accessing the usOutputValue-variable.
2) To ensure no other task is currently calling the GPIO_Write-function.

I think the GPIO_Write-function shouldn't be the reason, there is no read modify-write operation in the function.


void GPIO_Write(GPIO_TypeDef* GPIOx, u16 PortVal)
{
GPIOx->ODR = PortVal;
}


When exactly do I need to use such a "critical section" ?

Thanks for clearing my confusion.

Benjamin

RE: Use of vTaskSuspendAll on hardware access?

Posted by Richard on February 11, 2012
usOutputValue is a file scope variable, so you are correct in your assumption that it protects against multiple access of the usOutputValue variable.

Regards.


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