jj1010 wrote on Tuesday, October 14, 2014:
I use FreeRTOS in Cortex-M3 architecture. My question is:
1 When Systick interrupt and other interrupt arriving at the same time, How FreeRTOS works?
2 If other interrupt preempts Systick interrupt when they arriving at the same time, Systick handler will be deffed. Whether to set the Systick priority highest to ensure the accuracy of the System tick?
rtel wrote on Tuesday, October 14, 2014:
1 When Systick interrupt and other interrupt arriving at the same time,
How FreeRTOS works?
How that works is dependent on the hardware (Cortex-M in this case). FreeRTOS is just software and cannot change how the hardware functions - so the answer is in the hardware reference manuals.
2 If other interrupt preempts Systick interrupt when they arriving at the same
time, Systick handler will be deffed. Whether to set the Systick priority
highest to ensure the accuracy of the System tick?
The SysTick must be the lowest priority interrupt. Again the hardware will take care of it for you - and you will only get into a time slippage issue if you are executing an interrupt handler that takes more than one entire tick interrupt time to execute (that is - more than the time between two tick interrupts).
Regards.
jj1010 wrote on Tuesday, October 14, 2014:
Other tasks want a definite time delay, but the time slippage issue affects accuracy of the delay time. How does such question solve?
About the SysTick priority explanation, has not explained in FreeRTOS tutorial Cortex-M3 edition , where can I get information about the SysTick interrupt explanation?
Regards.
jj1010 wrote on Tuesday, October 14, 2014:
By the way, what does Regards mean? my mother tongue is not English. Thanks!
rtel wrote on Tuesday, October 14, 2014:
Other tasks want a definite time delay,
The accuracy of any delay period can be specified with a granularity of
1 tick period. That is because the next tick interrupt that occurs
after requesting the delay is taken as the first tick of the delay. So,
if you ask for a delay immediately before a tick interrupt occurs then
the first delay period will be a tiny fraction of one tick. Likewise,
if you ask for a delay immediately after a tick interrupt occurs the
first period of the delay will be one complete tick period.
but the time slippage issue affects accuracy of the delay time. How
does such question solve?
As per my previous post - unless you are writing interrupt service
routines that take more than one entire tick period to execute (so
potentially two ticks occur during the execution of a single interrupt
service routine) then there is no time slippage issue. If you interrupt
genuinely do take that long then I would suggest drastically
re-architecting your system as the goal should be for interrupt to be as
short as feasibly possible - the deferred interrupt description in the
book text shows you how to do that.
About the SysTick priority explanation, has not explained in FreeRTOS
tutorial Cortex-M3 edition , where can I get information about the
SysTick interrupt explanation?
I’m not sure exactly what the book says about this, hopefully it is in
the code comments.
Regards.
rtel wrote on Tuesday, October 14, 2014:
As a noun it means “best wishes” (used to express friendliness in
greetings).
In formal letters (before the days of email) a sign off would be either
“yours sincerely”, or “yours truly” (depending on how the letter starts
I think). “best regards” being a slightly less formal version. Its a
bit old fashioned, and I just abbreviated it to “regards”.
richard_damon wrote on Wednesday, October 15, 2014:
The other point about the slippage, unless the task was the highest priority task, it could be delayed by other tasks, and thus not restart at exactly the “right” time.
And, even if it was the highest task, that other interrupt that delayed the sys tick, would have delayed the task too, even if the systick had been highest priority and switched to it.