PWM output with stable frequency

[takashina hiroki]

PWM output with stable frequency

SDK: 15.3.0
Example: low_power_pwm
Device: EYSHJNZWZ (nRF52832)

I want to output PWM with a stable frequency.
In the above sample, PWM output with stable frequency was confirmed.

However, when the following processing was added, the frequency of the PWM output became unstable.
Perhaps the reason is that interrupt processing runs.

static	const	nrf_drv_timer_t	m_timer = NRF_DRV_TIMER_INSTANCE(2U);

static	void timer_init(void)
{
	ret_code_t	err_code;
	uint32_t	ticks;

	nrf_drv_timer_config_t timer_cfg	= NRF_DRV_TIMER_DEFAULT_CONFIG;

	err_code = nrf_drv_timer_init(&m_timer, &timer_cfg, e_timer_short_handler);
	APP_ERROR_CHECK(err_code);

	ticks = nrf_drv_timer_ms_to_ticks(&m_timer, 1U);
	nrf_drv_timer_extended_compare(&m_timer, NRF_TIMER_CC_CHANNEL0, ticks, NRF_TIMER_SHORT_COMPARE0_CLEAR_MASK, true);

	nrf_drv_timer_enable(&m_timer);
}

Is there a way to perform stable PWM output regardless of other factors such as interrupts?
Is it possible to output PWM at the hardware level?

[Sigurd]

Hi,

Is there a way to perform stable PWM output regardless of other factors such as interrupts?

 Yes, you should checkout the pwm_library instead(uses TIMER+GPIOTE+PPI). See this and this link.

Is it possible to output PWM at the hardware level?

Yes, we also have dedicated PWM hardware on the nRF52 series. The pwm_driver uses that. See this link.

[takashina hiroki]

I'm sorry.
I forgot to talk about one important thing.

I want to select LFCLK (XTAL) as the PWM clock source.
Is it possible to select LFCLK (XTAL) with "pwm_library" or "pwm_driver"?

[Sigurd]

takashina hiroki said:

There seems to be no link listed first.

I fixed the links.

takashina hiroki said:

Is it possible to select LFCLK (XTAL) with "pwm_library" or "pwm_driver"?

For PWM in pwm_library / pwm_driver, it's the HFCLK that decides how stable the frequency is. Use nrf_drv_clock_hfclk_request() to request high-accuracy(HFXO) source HFCLK.

[takashina hiroki]

>> For PWM in pwm_library / pwm_driver, it's the HFCLK that decides how stable the frequency is.

I want to select LFCLK (XTAL) as the PWM clock source.
Therefore, these cannot be used.
Is there a way to perform stable PWM output with a PWM whose clock source is LFCLK (XTAL)?

[Sigurd]

The low_power_pwm library uses the app_timer (which in turn use RTC1(uses LFCLK)). But the toggling is done in interrupt context, so other interrupts could affect the PWM timing. If you want to avoid that, use e.g. pwm_library instead.

 

[takashina hiroki]

>> For PWM in pwm_library / pwm_driver, it's the HFCLK that decides how stable the frequency is.
>> The low_power_pwm library uses the app_timer (which in turn use RTC1 (uses LFCLK)). But the toggling is done in interrupt context, so other interrupts could affect the PWM timing.

From the above, I recognize that there is no way to achieve stable PWM output with PWM with LFCLK (XTAL) as the clock source.
Is my perception correct?