Jitter on Timer1+PPI+GPIOTE?

RE: Jitter on Timer1+PPI+GPIOTE?

Motti Ng — Wed, 22 Aug 2018 03:41:15 GMT

I changed the HFCLKSTART to use sd_clock_hfclk_request() instead of NRF_CLOCK->TASKS_HFCLKSTART.

It works! Now the 4kHz signal has no jittering and the distorted sound disappeared.

I will mark your reply as answer. Thank you!

Below is the modified code.

void buzzer_service(){
	uint32_t is_hfclk_running = 0;
	if (m_main_flag.buzzer_flag){
		sd_clock_hfclk_request();
		do{
			sd_clock_hfclk_is_running(&is_hfclk_running);
		}while(is_hfclk_running != 1);
		
		nrf_drv_gpiote_out_task_enable(BUZZER_PIN_0);
		nrf_drv_timer_enable(&timer1);
	}else{
		sd_clock_hfclk_is_running(&is_hfclk_running);
		if (is_hfclk_running == 1){
			sd_clock_hfclk_release();
		}
		nrf_drv_gpiote_out_task_disable(BUZZER_PIN_0);
		nrf_drv_timer_disable(&timer1);
	}
}

RE: Jitter on Timer1+PPI+GPIOTE?

Motti Ng — Wed, 22 Aug 2018 02:03:19 GMT

I am using SDK v11.0 and softdevice S130 v2.0.0.

I am sorry it is a typo that the crystal used should be 16MHz.

I added HFCLKSTART task as shown below, however I did not see any big different:

void buzzer_service(){
	if (m_main_flag.buzzer_flag){
	    NRF_CLOCK->TASKS_HFCLKSTART = 1;
        /* Wait for the external oscillator to start up */
        while (NRF_CLOCK->EVENTS_HFCLKSTARTED == 0);
        NRF_CLOCK->EVENTS_HFCLKSTARTED = 0;
	
		nrf_drv_gpiote_out_task_enable(BUZZER_PIN_0);
		nrf_drv_timer_enable(&timer1);
	}else{
		nrf_drv_gpiote_out_task_disable(BUZZER_PIN_0);
		nrf_drv_timer_disable(&timer1);
	}
}

I will update this post to include screenshots.

RE: Jitter on Timer1+PPI+GPIOTE?

MartinBL — Tue, 21 Aug 2018 14:08:55 GMT

Hi,

Can you show a screenshot of the jitter?

What SDK are you using?

Are you really using a 12 MHz crystal or is that a typo? nRF51 only supports 16 or 32 MHz crystals.

I tested your code and was caught off guard by this bug/feature. With SDK 12.3.0 you are not allowed to pass NULL as config parameter to nrf_drv_timer_init() (even though it says so in the documentation). The result of passing NULL is a randomly configured timer.

Anyway, after configuring the timer correctly I'm seeing a jitter of max 0.1 us and about 0.35 % error on the frequency. Note that to get an as accurate clock as possible you need to explicitly turn on the HF crystal using the NRF_CLOCK->TASKS_HFCLKSTART task or sd_clock_hfclk_request() if you are using a Softdevice. Otherwise the Timer will use the internal RC Oscillator which has a typical accuracy of only ~5 %.