https://devzone.nordicsemi.com/f/nordic-q-a/18611/timeslot-and-freertos-problemI am developing Timeslot program with FreeRTOS. (on nRF51822) timeslot&#39;s signal handler is timeslot_signal_callback() ... xQueueSendFromISR(s_timeslot_queue, &amp;cmd, NULL); .. A few minute age, HardFault was occured. When xQueueSendFromISRen-USTelligent Community 13Tue, 27 Dec 2016 12:54:04 GMThttps://devzone.nordicsemi.com/thread/71850?ContentTypeID=1Tue, 27 Dec 2016 12:54:04 GMT137ad170-7792-4731-bb38-c0d22fbe4515:113c0c71-e7d1-479a-9883-e4ba1ee2f110loquat<p>Thanks. I understand.</p><div style="clear:both;"></div>https://devzone.nordicsemi.com/thread/71849?ContentTypeID=1Tue, 27 Dec 2016 11:04:49 GMT137ad170-7792-4731-bb38-c0d22fbe4515:bd25e2fb-f2ff-4a71-962b-5d56db5f578eSusheel Nuguru<p>The timeslot callback is called with the highest priority 0, and you are not allowed to call FreeRTOS API from this priority.</p> <p>you can only call the FreeRTOS API from priority equal or lower than configLIBRARY_MAX_SYSCALL_INTERRUPT_PRIORITY defined in FreeRTOSConfig.h file.</p> <p>Equal or lower in priority level means equal or higher in number.</p> <p>In short, you cannot call FreeRTOS API from timeslot.</p><div style="clear:both;"></div>