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Long latency times (ARM Cortex-M3/STM32)

Posted by Johan on May 19, 2009
I have a semaphore being released from an external interrupt using

void isr(void)
xSemaphoreGiveFromISR( m_handle, &xHigherPriorityTaskWoken );
if( xHigherPriorityTaskWoken == pdTRUE)
portYIELD( );

The thread is waiting for the semaphore:
if( xSemaphoreTake( m_handle, ticksToWait ) == pdTRUE )
// Do something

However, it takes approx. 400 us before the sleeping thread, which also has the highest priority, is started. How do I reduce this time? 400 us is far too much time.


RE: Long latency times (ARM Cortex-M3/STM32)

Posted by MEdwards on May 19, 2009
Make sure xHigherPriorityTaskWoken is initialized to 0 before it is used.

RE: Long latency times (ARM Cortex-M3/STM32)

Posted by Johan on May 19, 2009
I added that now, but it makes no difference.

RE: Long latency times (ARM Cortex-M3/STM32)

Posted by Richard on May 19, 2009
If this is the highest priority interrupt AND it is waking the highest priority task AND your application does not have huge sections where the scheduler is suspended then 400us seems improbably long.

I measure 120ns typical interrupt entry, where interrupts are above the max syscall interrupt priority and therefore not effected by critical sections.

How are you taking the timing?


RE: Long latency times (ARM Cortex-M3/STM32)

Posted by Johan on May 19, 2009
I measured with scope.

This was however not an RTOS problem. The thing was that I called uxTaskGetStackHighWaterMark before I started doing anything.

Placing this somewhere else solved the problem.

Thanks for all the good work you are doing!

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