The nrf53 pretick can be used with non-zero
`NRF_RTC_TIMER_USER_CHAN_COUNT` Kconfig option.
The nrf53 pretick requires just one RTC1 CC channel.
The nrf53 pretick handles also RTC1 and RTC0 both CCs and OVERFLOW
events by examination of events scheduled on them. The pretick is set
based on number of ticks to the closest event scheduled that can trigger
an interrupt.
Because the operation in `z_arm_on_enter_cpu_idle` hook would
take too much time with interrupts disabled, the
`z_arm_on_enter_cpu_idle_prepare` hook enabled by Kconfig option
`ARM_ON_ENTER_CPU_IDLE_PREPARE_HOOK` is used. It performs RTC0 and RTC1
examination, and sets pretick without interrupts being blocked.
The LDREX/STREX are leveraged to detect if exception took place
between start of `z_arm_on_enter_cpu_idle_prepare` and
`z_arm_on_enter_cpu_idle`. If exception has not been taken, the pretick
calculation can be trusted because source data could not changed and
too much time could not pass. Otherwise the sleep attempt is disallowed,
the idle will loop again and try later.
Prompt for `SOC_NRF53_RTC_PRETICK` Kconfig option allows to control
this option by an user and turn the feature off if necessary.
Signed-off-by: Andrzej Kuroś <andrzej.kuros@nordicsemi.no>
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