SysTick timer on nRF52 fires 1/5 as often as expected

RE: SysTick timer on nRF52 fires 1/5 as often as expected

Susheel Nuguru — Mon, 29 May 2017 19:42:48 GMT

I have found a thread that (possibly) explains why the systick is slowed down in CPU sleep here. Probably someone case test the solution?

RE: SysTick timer on nRF52 fires 1/5 as often as expected

Håkon Alseth — Mon, 16 Nov 2015 07:54:40 GMT

SysTick is based on the CPU frequency (64M), which nordic turns off during sleep to ensure that the power consumption is low. The systick implementation and how it's sourced (ie: during sleep or not) is implementation specific.

RE: SysTick timer on nRF52 fires 1/5 as often as expected

Susheel Nuguru — Mon, 16 Nov 2015 06:28:14 GMT

Apparently, __WFE slows down the system clock which affects the SysTick frequency

Could you please give a reference to this behaviour. Is it documented in ARM infocenter?

RE: SysTick timer on nRF52 fires 1/5 as often as expected

Mike Voytovich — Thu, 05 Nov 2015 22:10:28 GMT

Hi @Håkon Alseth - thanks for your reply and for testing this! I was also able to reproduce your results using blinky.

It turns out my issue was caused by my app calling sd_app_evt_wait() in the main loop, which calls __WFE(). Apparently, __WFE slows down the system clock which affects the SysTick frequency.

If I omit sd_app_evt_wait() from my application, the counter increments at the expected rate.
Thanks again for testing this out, it really helped to figure out the issue!!

RE: SysTick timer on nRF52 fires 1/5 as often as expected

Håkon Alseth — Thu, 05 Nov 2015 14:47:46 GMT

Hi,

Seems to work fine at my end. Here's my code-snippet (based on blinky):

const uint8_t leds_list[LEDS_NUMBER] = LEDS_LIST;

static uint32_t m_systick_cnt;

void SysTick_Handler(void) {
    m_systick_cnt++;
    if ((m_systick_cnt % 100) == 0)
        nrf_gpio_pin_toggle(BSP_LED_0); 
}

int main(void)  
{
    // Configure LED-pins as outputs.
    LEDS_CONFIGURE(LEDS_MASK);
    SysTick_Config(64000);
    NVIC_EnableIRQ(SysTick_IRQn);
    while (true);      
}

And here's the measured interval, based on 100 ticks:

Cheers, Håkon

RE: SysTick timer on nRF52 fires 1/5 as often as expected

Mike Voytovich — Wed, 04 Nov 2015 21:46:57 GMT

Hi @Martijn - I'm using a uint32_t for the counter in the SysTick_Handler interrupt (I expected it to increment by 1000 every second).

RE: SysTick timer on nRF52 fires 1/5 as often as expected

Martijn — Wed, 04 Nov 2015 20:13:15 GMT

Are you using a big enough variable to store your counter value? If you use a uint8_t you will get a value of 232...

RE: SysTick timer on nRF52 fires 1/5 as often as expected

Mike Voytovich — Tue, 03 Nov 2015 17:07:00 GMT

Thanks for the reply @Aryan. Yes, I'm sure app_timer is configured correctly for 1s (it's easy to verify 30 timer events occurring over 30 seconds with a stopwatch).

RE: SysTick timer on nRF52 fires 1/5 as often as expected

Susheel Nuguru — Tue, 03 Nov 2015 08:53:13 GMT

This is ARM-CM4 feature, so we should not have any PAN's on it. Are you sure that you have configured app_timer correctly for 1 second?