wfi instruction stops systick timer

[Wojciech Puchar]

I am writing my own code (no libraries etc.) on nRF52832.

For new i still have softdevice and bootloader in flash, and store my program using bootloader protocol, but my program is self contained and doesn't use any of it's code

My interrupt vector table contain pointer to systick handler, and UARTE interrupt handler.

Systick is set to generate interrupt every 1ms.

Both interrupt works properly. This is not the first time i write program to Cortex-M4 based device, i an not a novice. Which does

The problem is when using WFI instruction. 

When executing WFI instruction Systick counter stops and doesn't generate interrupts.

If before WFI instruction UARTE is sending data, UARTE interrupt is generated, handled and WFI properly works for this.

But not with systick alone. Cortex SYST_CSR register is set to 0x3 (normal sleep, no deep).

Cortex SCR register is 0, tried 0x10 - same.

of course i could simply use timer instead of systick but i would like to know why it doesn't work.

[Wojciech Puchar]

Update: Everything works as expected if i use TIMER0 to generate interrupts every 1ms instead of systick. Including WFI.

So for now it's fine me, i will not need more than 4 other timers in my program. But still i would like to know why it doesn't work with systick

[Bas van den Berg]

I have the same thing (post  NRF5340 WFI causes Systick to stop? ). When reading back the SYST_CSR register it always reverts back to 7 (bit 2 is set). Bit 2 is the clock source (1=processor, 0=external).

[Håkon Alseth]

systick behavior is implementation specific, ie. it is up to the vendor how its clocked. Systick only runs when the CPU is running in nRF52/nRF53 series devices.

We strongly recommend that you use the RTC instead, as also recommended in the thread that you linked to, for better energy efficiency (ie. lower power in sleep).

 

Kind regards,

Håkon