Unintended pulse at start up of a PWM output

RE: Unintended pulse at start up of a PWM output

keithauhk — Tue, 08 Jul 2025 03:53:28 GMT

Hi Benjamin,

Thanks! I just able to solve the issue.
It is caused by the "nordic,invert;" line in overlay.
As I am using PWM_POLARITY_NORMAL, that line should not be added.

RE: Unintended pulse at start up of a PWM output

Benjamin — Mon, 07 Jul 2025 14:04:27 GMT

Hi again Keith,

The LED pins have pull-up resistors, so after a reset you will see a high pulse regardless of whether the PWM polarity is set to normal or inverted. This happens because, during the initial startup phase, the PWM signal has not started yet.

RE: Unintended pulse at start up of a PWM output

keithauhk — Fri, 04 Jul 2025 09:57:39 GMT

Hello Benjamin,

1. I thought this is the a way that I can get the PWM output pin as low level after power on. I am fine with any settings in the overlay as I can just change the output frequency by using pwm_set_dt() in my codes.

BTW, I just changed this period value as PWM_MSEC(20) in the overlay file. In this case I find a much longer pulse high level pulse after power on while running the pwm example. I tried to comment the do while loop in the example code, so it becomes:

int main(void)
{
	const struct device *led_pwm;
	uint8_t led;

	led_pwm = DEVICE_DT_GET(LED_PWM_NODE_ID);
	if (!device_is_ready(led_pwm)) {
		LOG_ERR("Device %s is not ready", led_pwm->name);
		return 0;
	}

	if (!num_leds) {
		LOG_ERR("No LEDs found for %s", led_pwm->name);
		return 0;
	}

	// do {
	// 	for (led = 0; led < num_leds; led++) {
	// 		run_led_test(led_pwm, led);
	// 	}
	// } while (true);
	return 0;
}

In this case, the voltage level appears to be kept as high. Is this an expected behavior even with the setting of PWM_POLARITY_NORMAL?

2. Please find a capture of the pulse.

RE: Unintended pulse at start up of a PWM output

Benjamin — Fri, 04 Jul 2025 09:18:40 GMT

Alright, I have tested with the setup you described and I cannot immediately reproduce the issue.

1. What are you trying to achieve when setting PWM period to 0 in the overlay file?

2. Could you zoom in on the pulse on the capture?

RE: Unintended pulse at start up of a PWM output

keithauhk — Fri, 04 Jul 2025 07:22:16 GMT

Hello Benjamin,

Yes, this is the sample code that I tested.

RE: Unintended pulse at start up of a PWM output

Benjamin — Fri, 04 Jul 2025 07:19:30 GMT

Thank you! Just to confirm, is this the code sample that you are using? zephyr/samples/drivers/led/pwm/

RE: Unintended pulse at start up of a PWM output

keithauhk — Fri, 04 Jul 2025 02:06:38 GMT

Hello,

Attached please find the capture of the waveform. A pulse is observed at ~1s.
The waveform was measured at nRF52dk PCA10040, P0.17, i.e. control of LED1. This confirms the issue is not related to P0.07.

The waveform is captured by running the pwm example code, with the following added in DT overlay. No modification was done in the example code.

/ {
	zephyr,user {
		io-channels = <&adc 0>;
	};
    pwmleds {
		compatible = "pwm-leds";
		pwm_led0: pwm_led_0 {
			pwms = <&pwm0 0 PWM_MSEC(0) PWM_POLARITY_NORMAL >;
		};
	};
};

RE: Unintended pulse at start up of a PWM output

Benjamin — Thu, 03 Jul 2025 14:35:26 GMT

Hi Keith,

Do you have a logic analyze trace of this?

Best regards,
Benjamin