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Does timeout in blocking system calls effect the idling?

Posted by turboscrew on November 4, 2017

Sorry for the vague title, but it's hard to put the essential in the header... Anyway, we are using STM32L4 family device with M4F-port of FreeRTOS 9.0.0. We use the tickless idle to put the system into STOP2 for most of the time. The use of STOP2 causes some clock setting overhead, especially when we have a bit tricky oscillator for HSE.

A task uses a radio chip for sending a message, and waits for the "message sent" interrupt. The chip driver uses a semaphore for that.

The problem is that with 2 second time-out for xSemaforeTake, the system goes to the tickless idle and sleeps. The interrupt can't take place, because the power is OFF. On the other hand, the clock setting overhead makes it impossible to handle sleep times less than 5 ticks, so the configEXPECTEDIDLETIMEBEFORESLEEP is set to 10.

Is the tickless idle still used, if the timeout is set to, say, 9 ticks? (This is fixing old code to be used with FreeRTOS.) Or is there more recommendable ways of handling this?

Does timeout in blocking system calls effect the idling?

Posted by richard_damon on November 5, 2017

If tickless idle blocks or delays too much an interrupt then you need to prevent the system from going into tickless idle when that interrupt might occur. One way, like you mention, is to keep the timeout below the threshold value, the second is that the tickless idle code has a call back just before entering the idle mode, which allows you to prevent going into tickless idle when you are expecting the interrupt.

Does timeout in blocking system calls effect the idling?

Posted by turboscrew on November 5, 2017

Which callback? Oh, and lots of thanks.

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