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drivers: timer: systick: fix off-by-one setting

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When the counter reaches zero, it reloads the value in
SYST_RVR on the next clock edge. This means that if the
LOAD value is N, the interrupt ("tick") is triggered
every N+1 cycles. Therefore, when we operate in non-
tickless mode, we need to configure the LOAD value
with CYC_PER_TICK - 1, in order to get an event
every CYC_PER_TICK cycles.

Signed-off-by: Ioannis Glaropoulos <Ioannis.Glaropoulos@nordicsemi.no>
This commit is contained in:
Ioannis Glaropoulos 2019-04-09 22:54:42 +02:00 • committed by Anas Nashif
commit b1b4ec126d
1 changed files with 1 additions and 1 deletions
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2
drivers/timer/cortex_m_systick.c
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@ -102,7 +102,7 @@ void z_clock_isr(void *arg)
int z_clock_driver_init(struct device *device)
{
NVIC_SetPriority(SysTick_IRQn, _IRQ_PRIO_OFFSET);
last_load = CYC_PER_TICK;
last_load = CYC_PER_TICK - 1;
overflow_cyc = 0U;
SysTick->LOAD = last_load;
SysTick->VAL = 0; /* resets timer to last_load */