nRF52820 PWM Device Tree / API

[CCL-KH]

I am new to Zephyr/nRF environemnt and using the nRF52833-DK to emulate nRF52820 which does not have any dedicated PWM hardware

From my understanding I am limited to the software PWM API from nRF Connect SDK (using v2.3.0) - nordic,nrf-sw-pwm

I am trying to work out the preprocessor macro API available to control the 'sw_pwm' node and its channels

So far I am able to build the following:

printf("DTS sw-pwm driver name: %s\n", (DEVICE_DT_GET(DT_NODELABEL(sw_pwm)))->name);

uint64_t cycles;
pwm_get_cycles_per_sec((DEVICE_DT_GET(DT_NODELABEL(sw_pwm))), 0, &cycles);
printf("pwm cycles per sec: %lld\n", cycles);

Giving:

DTS sw-pwm driver name: sw-pwm
pwm cycles per sec: 16000000       // not validated this value but seems consistent & code compiles so...

Does this API set/get duty/freq/period (whatever it uses)?
Is there any documentation or another project I can reference as I can only find code that uses pwm-leds. Ideally I am looking to bypass modules like pwm-leds and control the sw-pwm channels from application code

nrf52820.dtsi

    sw_pwm: sw-pwm {
        compatible = "nordic,nrf-sw-pwm";
        status = "disabled";
        generator = <&timer2>;
        clock-prescaler = <0>;
        #pwm-cells = <3>;
    };

app device tree (.dts)

&sw_pwm {
	status ="okay";
	channel-gpios = <&gpio0 15 GPIO_ACTIVE_HIGH>,
					<&gpio0 16 GPIO_ACTIVE_LOW>;
	clock-prescaler = <0>;
};

.config (\build Kconfig)

CONFIG_PWM=y
CONFIG_DT_HAS_NORDIC_NRF_SW_PWM_ENABLED=y

snippet from devicetree_generated.h (\build device tree source)

[khelmutlord]

Hello,

I have attached an example project that shows how to use software PWM to blink an LED (directly! not through the LED library). It is set up to be used for an nRF52840-DK because that I don't have a 52833-DK handy but you should be able to just rename the board overlay file that I included.

I included a comment w/ a link to the nRF5 SW-PWM implementation for more information.

https://github.com/zephyrproject-rtos/zephyr/blob/main/drivers/pwm/pwm_nrf5_sw.c

led_pwm.zip

Thanks,

Helmut Lord

[CCL-KH]

Thank you for you help, I have now successfully implemented all 3 pwm channels via the single sw-pwm instance.

I am experimenting adding more sw-pwm instances (I believe each requires 1 generator and X PPI channels). I am hitting compilation errors. In the code below 'sw-pwm' is defined in #include <nordic/nrf52820_qdaa.dtsi> however I am leaving that 'status=disabled' and creating 2 new nodes: 'sw_pwm0: sw-pwm0' & 'sw_pwm1: sw-pwm1' that are compatible with 'nordic,nrf-sw-pwm'

/dts-v1/;
#include <nordic/nrf52820_qdaa.dtsi>

/ {
	sw_pwm1: sw-pwm1 {
		compatible = "nordic,nrf-sw-pwm";
    	status ="okay";
    	channel-gpios = <&gpio0 14 GPIO_ACTIVE_LOW>,
    					<&gpio0 15 GPIO_ACTIVE_LOW>;
    	clock-prescaler = <0>;
    	generator = <&timer2>;
		#pwm-cells = <3>;
	};

	sw_pwm0: sw-pwm0 {
		compatible = "nordic,nrf-sw-pwm";
    	status ="okay";
    	channel-gpios = <&gpio0 16 GPIO_ACTIVE_LOW>;
    	clock-prescaler = <0>;
    	generator = <&timer1>;
		#pwm-cells = <3>;
	};
};

pwm_init()

	const struct device *driver0;
	driver0 = DEVICE_DT_GET(DT_NODELABEL(sw_pwm0));
	pwm_set_cycles(driver0, 0, CONFIG_SYS_CLOCK_TICKS_PER_SEC, CONFIG_SYS_CLOCK_TICKS_PER_SEC / 8, PWM_POLARITY_INVERTED);

	const struct device *driver1;
	driver1 = DEVICE_DT_GET(DT_NODELABEL(sw_pwm1));
    pwm_set_cycles(driver1, 0, CONFIG_SYS_CLOCK_TICKS_PER_SEC, CONFIG_SYS_CLOCK_TICKS_PER_SEC, PWM_POLARITY_INVERTED);

compile error:

[khelmutlord]

Hello,

I was able to get two software PWM instances to compile with the following DeviceTree overlay. Key thing is that the overlay file is named "nrf52833dk_nrf52820.overlay"

You will find this in the attached project.

led_pwm_5_17_2023.zip

Thanks,

Helmut Lord

[CCL-KH]

Hi,

Thanks for your support

I was able to build the project 'as is' however I think you have a typo

line 25: pwm_dev1 = DEVICE_DT_GET(PWM_NODE_LABEL);

should be

line 25: pwm_dev1 = DEVICE_DT_GET(PWM_NODE_LABEL_1);

 which fails build with:

[khelmutlord]

Apologies. Upon further investigation, it seems that only one software PWM instance is supported at a time.

https://github.com/zephyrproject-rtos/zephyr/blob/main/drivers/pwm/pwm_nrf5_sw.c

There is a possible solution that my require modifying some files within the Zephyr SDK. 

https://github.com/zephyrproject-rtos/zephyr/blob/42b121ee9519d5bb96c6e6197733aa204d99486e/dts/bindings/pwm/nordic%2Cnrf-sw-pwm.yaml#L3

In the device tree bindings, it makes clear that the number of PWM channels supported is the number of timer compare registers minus 1. For the nRF52820, that is 3 total for timer[0-2]. However, timer3 has 6 compare registers so would support 5 PWM channels.

...

To make this change, modify the example to use timer3 for the generator, modify the device tree bindings in the Zephyr SDK to support up to 5 PWM cells, and set #pwm-cells to 5.