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drivers: timer: systick: clear pending events when resetting timeout

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In the unlucky scenario of a SysTick event (wrap) occurring
while we re-program the last_load value, the SysTick ISR
will run immediately after we unlock interrupts. In that
case the timeout we have just configured will expire
instantaneously, leading to operations being executed
much earlier than expected. Avoid this by clearing possibly
pending SysTick exceptions (writing 1 to ICSR.PENDSTCLR).

Signed-off-by: Ioannis Glaropoulos <Ioannis.Glaropoulos@nordicsemi.no>
This commit is contained in:
Ioannis Glaropoulos 2019-04-11 13:01:58 +02:00 • committed by Anas Nashif
commit 2895da02a4
1 changed files with 16 additions and 0 deletions
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16
drivers/timer/cortex_m_systick.c
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@ -147,6 +147,22 @@ void z_clock_set_timeout(s32_t ticks, bool idle)
SysTick->LOAD = last_load - 1;
SysTick->VAL = 0; /* resets timer to last_load */
/*
* In the unlucky scenario of a SysTick event (wrap) occurring
* while we re-program the last_load value, the SysTick ISR
* will run immediately after we unlock interrupts. In that
* case the timeout we have just configured will expire
* instantaneously, leading to operations being executed
* much earlier than expected. Avoid this by clearing possibly
* pending SysTick exceptions (writing 1 to ICSR.PENDSTCLR).
*
* Side effect: any operations that were scheduled to execute
* upon the expiration of the timeout we may be canceling (by
* writing PENDSTCLR to 1) will be postponed, until the new
* timeout (i.e. the one we are programming now) expires.
*/
SCB->ICSR |= SCB_ICSR_PENDSTCLR_Msk;
k_spin_unlock(&lock, key);
#endif
}