PWM Jitter with nRF52DK

[Moe1992]

Hi everyone,

I am getting started with the nRF52832 and am testing it with the nRF52DK. I just completed the nRF Connect SDK Fundamentals course and now wanted to test the PWM Module. I am getting strong jitters on the PWM signal as seen in the attached image (which is 5 periods away from the trigger).

How come the PWM is so jittery? Does it have to do with the used clock? If so, how can I use a more accurate clock?

I have troubles finding information on how to change the clock with the nRF Connect SDK.

This is the code which generates the shown signal:

#include <zephyr/kernel.h>
#include <zephyr/device.h>
#include <zephyr/drivers/pwm.h>

static const struct pwm_dt_spec pwm0 = PWM_DT_SPEC_GET(DT_ALIAS(pwm_led0));

int main(void)
{
	if (!pwm_is_ready_dt(&pwm0)){
        return 0;
    }
	pwm_set_dt(&pwm0, 200000, 5000);

	while(1){}
	return 0;
}

Thank you for your support.

BR Moritz

[Daniel Chisholm]

Try enabling the high frequency crystal oscillator (keyword "HFXO" shoud help you find things).

If HFXO isn't turned on, then the internal 32MHz RC oscillator is used.

[Moe1992]

Thank you :) I did find a forum post which suggests using this pice of code:

NRF_CLOCK->TASKS_HFCLKSTART = 1;
// Wait for clock to start
while(NRF_CLOCK->EVENTS_HFCLKSTARTED == 0);

and I am happy to say that this works.

But since I am still learning, I would like to know, how I could have found this solution if nobody had asked this before here on the forum. What I mean is: How do I find this information using the documentation? Searching "HFXO" in the nRF Connect SDK documentation yields no results: https://docs.nordicsemi.com/search?bundle=ncs-latest&labelkey=nrf52832&labelkey=nrf52-series&q=hfxo

Why does nothing show up in the results?

Thank you

[Susheel Nuguru]

I think you need to know your peripherals to understand which clocks they work with. With PWM, the peripheral itself can work with both internal and external clocks but it is recommended to use XTAL HFCLK. This has been discussed on the forum like here. Unfortunately there is no single source to find such information as this is scattered info.

[Susheel Nuguru]

Also , if you are using a BLE or other protocol stacks, they turn on the hfclk as needed but in your example, since you are not using BLE or any other stack, the responsibility to start the right clocks falls on the applicaiton.