How to change pwm period on runtime in ncs?

[ilockit_CFR]

Hi, 

I'm trying to play some sounds on a piezo with the help of the pwm. I tried to set the period with pwm_set_dt but only the first call is successful and every subsequent call raised an error. 

Here is my test code:

    for(uint16_t i = 400; i < 800; i++)
    {
        int ret = pwm_set_dt(&piezo_pwm_pos, PWM_HZ(i),
						   PWM_HZ(i) / 2);
        LOG_DBG("pwm_set_dt = %d", ret);

        ret = pwm_set_dt(&piezo_pwm_neg, PWM_HZ(i),
						   PWM_HZ(i) / 2);
        LOG_DBG("pwm_set_dt = %d", ret);
        k_msleep(10);
    }

The results looked like this:

00> D: pwm_set_dt = 0
00> D: pwm_set_dt = 0
00> E: Incompatible period.
00> D: pwm_set_dt = -22
00> E: Incompatible period.
00> D: pwm_set_dt = -22
00> E: Incompatible period.
00> D: pwm_set_dt = -22
00> E: Incompatible period.
00> D: pwm_set_dt = -22
00> E: Incompatible period.
00> D: pwm_set_dt = -22

I didn't find anything in the documentation of pwm_set for the error 22.

How can I change period and pulse while the pwm is running?

Best regards,

Christian

[Sigurd]

Hi!

I see you are getting this in the log: "Incompatible period". Looking at the source code where this is printed, https://github.com/nrfconnect/sdk-zephyr/blob/v3.7.99-ncs1/drivers/pwm/pwm_nrfx.c#L94, there is a comment that explains this error:

 /* If any other channel is driven by the PWM peripheral, the period
* that is currently set cannot be changed, as this would influence
* the output for that channel.
*/

[ilockit_CFR]

Hi,

at the moment I'm using SDK5 and can easily change the frequency within an event handler, which is called when the PWM is finished. How is this possible in Nordic Connect SDK?

Best regards,

Christian

[ilockit_CFR]

Hi, 

in your case you just changed the period of one pin. If I do it also for only one channel everything works. I want to set two pins exactly inverted with changing frequency over time as seen in the image below.

But I only can change the frequency for one channel or set it once for two channels. It's not possible to change the frequency while the pwm is active. I could change the frequency if I disable the pwm for both channels before setting the new frequency, but this results in an interrupted output sound on the piezo. 

How can I invert channel 1 in relation to channel 0? 

Best regards,

Christian

[Sigurd]

Try using the  pwm_set_cycles() function. And use PWM_POLARITY_NORMAL on one channel, and  

PWM_POLARITY_INVERTED on the other. Snippet:

ret = pwm_set_cycles(pwm_led0->dev, pwm_led0->channel, period_cycles, period_cycles / 2,
	PWM_POLARITY_INVERTED);

ret = pwm_set_cycles(pwm_led1->dev, pwm_led1->channel, period_cycles, period_cycles / 2,
		PWM_POLARITY_NORMAL);

[ilockit_CFR]

Hi, 

you already suggested this, but it also results in "Incompatible period." and the functions return -EINVAL after the first time. Is it a problem, that I call these functions from a timeout handler?

In the nrf5 SDK there was NRF_DRV_PWM_EVT_FINISHED. In this event handler I could change the frequency of the pwm and it results in a nice alarm sound. Why is this not possible with the nrf connect SDK. 

[Sigurd]

Ah, right!

How do you define these channels?

If you do it like this in the dts:

https://github.com/nrfconnect/sdk-zephyr/blob/v3.7.99-ncs2/boards/nordic/nrf52dk/nrf52dk_nrf52832.dts#L48-L53

you can have e.g. pwm_led_0 with PWM_POLARITY_INVERTED,

and define a pwm_led_1 with PWM_POLARITY_NORMAL

[ilockit_CFR]

It's defined like this:

pwmpiezo0 {
		compatible = "pwm-leds";
		status = "okay";
	
		piezo_pwm_pos: piezo_pwm_pos {
				status = "okay";
				pwms = <&pwm1 0 PWM_HZ(7500) PWM_POLARITY_NORMAL>;
				label = "Piezo positive";
		};
	
		piezo_pwm_neg: piezo_pwm_neg {
				status = "okay";
				pwms = <&pwm1 1 PWM_HZ(7500) PWM_POLARITY_INVERTED>;
				label = "Piezo negative";
		};
	};

One of the pins is already inverted. I then call 

ret = pwm_set_dt(&piezo_pwm_pos, PWM_HZ(m_piezo_params.frequency),
					PWM_HZ(m_piezo_params.frequency) / 2);

ret = pwm_set_dt(&piezo_pwm_neg, PWM_HZ(m_piezo_params.frequency),
				   PWM_HZ(m_piezo_params.frequency) / 2);

within a timeout handler every x ms. If m_piezo_params.frequency stays the same, the function calls return 0. But if I change the frequency it breaks after the first timeout.