FreeRTOS Tickless mode
Hi all,
I ran into an issue regarding FreeRTOS tickless idle mode. I wanted to share my findings and solution in case others encounter the same problem and also to ask if that's intended structure (so I am missing something) or if its not generated properly.
Background:
I was using SysTick as the OS kernel timebase.
Tickless idle was enabled (configUSE_TICKLESS_IDLE = 1).
I noticed that the portNVIC_SYSTICK_COUNT_FLAG_BIT was cleared unexpectedly during sleep, which caused the tickless idle implementation (vPortSuppressTicksAndSleep) to miscalculate elapsed ticks.
Debugging revealed that the auto-generated SysTick_Handler in cmsis_os2.c was clearing the COUNTFLAG, even in tickless mode.
The autogenerated handler looked like this:
void SysTick_Handler(void) {
/* Clear overflow flag */
SysTick->CTRL;
if (xTaskGetSchedulerState() != taskSCHEDULER_NOT_STARTED) {
xPortSysTickHandler();
}
}Reading the Cortex-M documentation shows that reading SYST_CSR clears the COUNTFLAG.
This means that the autogenerated handler clears COUNTFLAG before the FreeRTOS tickless idle logic can use it, breaking tickless idle functionality.
Solution:
The fix is to override the SysTick_Handler :
extern void xPortSysTickHandler(void);
void SysTick_Handler(void) {
#if !defined(configUSE_TICKLESS_IDLE) || (configUSE_TICKLESS_IDLE == 0)
/* Clear overflow flag only if not using tickless idle */
SysTick->CTRL;
#endif
if (xTaskGetSchedulerState() != taskSCHEDULER_NOT_STARTED) {
xPortSysTickHandler();
}
}With this override, tickless idle works properly, and COUNTFLAG is preserved for vPortSuppressTicksAndSleep.
Question:
Am i supposed to be doing that overwrite? Shouldn't the tickless auto-generated code be "Ready to use"?
Thank you for your time.