# About configMAX\_SYSCALL\_INTERRUPT\_PRIORITY and  xQueueSendFromISR

**URL:** <https://forums.freertos.org/t/about-configmax-syscall-interrupt-priority-and-xqueuesendfromisr/4386>\
**Category:** Kernel\
**Created:** [January 26, 2016, 7:53am UTC](https://forums.freertos.org/t/about-configmax-syscall-interrupt-priority-and-xqueuesendfromisr/4386 "2016-01-26T07:53:21Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![system](https://d2v6vdsk2p900z.cloudfront.net/original/1X/7fc8fdb729c645aab5b20282bbe34e57cb407be6.jpeg) [@system](https://forums.freertos.org/u/system)\
**Post date:** [January 26, 2016, 7:53am UTC](https://forums.freertos.org/t/about-configmax-syscall-interrupt-priority-and-xqueuesendfromisr/4386/1 "2016-01-26T07:53:21Z")

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**[joe\_her](https://sourceforge.net/u/joe_her/)** wrote on [Tuesday, January 26, 2016](https://sourceforge.net/p/freertos/discussion/382005/thread/c0d2d63b/#9e50):

I have a STM32F4 UART interrupt which sends a queue msg as soon as data is ready.  
When UART interrupt priority was 0, I had assertion fail.  
The text describes the failure (thanks for the comment near the assertion, very helpful) , and changing the priority to 15 ( was 0) solves the problem.  
However I am not sure I fully understand what I did.  
The configMAX\_SYSCALL\_INTERRUPT\_PRIORITY equals 0x50 ( which I don’t get as the interrupt priority can be 0 - 15).  
Why when the UART priority is 0 (highest) it does not work?  
What should be the correct value of the UART interrupt priority for safe operation?

Thanks  
Johanan

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<div class="post-metadata">

**Author:** ![system](https://d2v6vdsk2p900z.cloudfront.net/original/1X/7fc8fdb729c645aab5b20282bbe34e57cb407be6.jpeg) [@system](https://forums.freertos.org/u/system)\
**Post date:** [January 26, 2016, 9:02am UTC](https://forums.freertos.org/t/about-configmax-syscall-interrupt-priority-and-xqueuesendfromisr/4386/2 "2016-01-26T09:02:25Z")

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**[davedoors](https://sourceforge.net/u/davedoors/)** wrote on [Tuesday, January 26, 2016](https://sourceforge.net/p/freertos/discussion/382005/thread/c0d2d63b/#3e61):

did you see this page already [http://www.freertos.org/RTOS-Cortex-M3-M4.html](http://www.freertos.org/RTOS-Cortex-M3-M4.html) ?

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**Author:** ![system](https://d2v6vdsk2p900z.cloudfront.net/original/1X/7fc8fdb729c645aab5b20282bbe34e57cb407be6.jpeg) [@system](https://forums.freertos.org/u/system)\
**Post date:** [January 26, 2016, 10:05am UTC](https://forums.freertos.org/t/about-configmax-syscall-interrupt-priority-and-xqueuesendfromisr/4386/3 "2016-01-26T10:05:37Z")

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**[joe\_her](https://sourceforge.net/u/joe_her/)** wrote on [Tuesday, January 26, 2016](https://sourceforge.net/p/freertos/discussion/382005/thread/c0d2d63b/#da2a):

Now I did 🙂  
Thanks