Why is system tick frequency so high?

[Tamas Selmeci]

Hello!

We've been doing some serious development on nRF52833 and nRF5340 machines with Zephyr, and on both of them CONFIG_SYS_CLOCK_TICKS_PER_SEC is set to 32768 by default. We've also been using similar category Cortex M4 MCUs with FreeRTOS with satisfaction set to a 1024 systick frequency.

I personally think 32768Hz generates way too much context switch overhead (32768 IRQs in a sec!) and don't really understand why this high rate is required. To be honest, we could do all timings perfectly with FreeRTOS too, which is configured to 1024Hz only, but there's always the possibility to use a MCU timer to get greater granularity if needed.

By the way, I was unable to locate the sources where Zephyr sets up systick, so at the moment I'm not sure whether Zephyr uses the MCU's SysTick peripheral or uses an alternative MCU timer/RTC to set up system tick IRQs. FreeRTOS can be configured to use either SysTick or RTC peripheral according to our taste.

I tried reducing CONFIG_SYS_CLOCK_TICKS_PER_SEC to 1024 and on a short term nothing bad was observed, but I'd need a confirmation from Nordic staff that this setting is also safe. I can keep in hand the granularity of my timings even at this lower systick frequency too.

So, any thoughts on this?

Many thanks,

[Vidar Berg]

Hello,

We use the RTC as the SysTick source to achieve low power, and the 32768 tick rate is chosen because we use the tickless kernel by default. This setting provides better timing granularity.

https://github.com/nrfconnect/sdk-zephyr/blob/b5cf3040298410845311ff7397374540d3451528/kernel/Kconfig#L772 

Best regards,

Vidar

[Tamas Selmeci]

And what if I don't use tickless kernel? How to choose from RTC/SysTick sources? (SysTick is a Cortex M4 peripheral in System Control Block, RTC is a vendor-added peripheral)

[Vidar Berg]

Yes, if you don't use the tickless kernel, you should reduce the tick rate. The limitation with using the SysTick timer is that it stops running when the CPU is idle, see Tick event. 

It does not look like we have added support for using SysTick in Zephyr.

EDIT: SYS_CLOCK_TICKS_PER_SEC defaults to '128' if you don't select the tickless kernel: https://github.com/nrfconnect/sdk-zephyr/blob/3758bcbfa5cd749d977e2600b6687104b8948243/soc/arm/nordic_nrf/Kconfig.defconfig#L22 

[Tamas Selmeci]

So I wasn't wrong when I was unable to find such a SysTick-related code ;)

So Zephyr on Nordic MCUs uses Nordic's RTC which features a low power TICK feature, is it correct? And the CONFIG_SYS_CLOCK_TICKS_PER_SEC option controls the frequency of RTC TICK, is it right? nrf_rtc_timer.c appears to be source file in question...

[Vidar Berg]

Yes, we use the RTC as the system timer, and nrf_rtc_timer.c is the implementation of it.

Tamas Selmeci said:

CONFIG_SYS_CLOCK_TICKS_PER_SEC option controls the frequency of RTC TICK

Yes, if you disable the tickles kernel.