Generating 8MHz clock signal using compatible = "pwm-clock" node on Zephyr

[Fernando Fontes]

Hi guys,

I'm working on a project with a custom board around nrf52832 chip and I'm trying to generate an 8MHz clock signal using the compatible "pwm-clock" on Zephyr.

I found little information on how to get this working but so far I've this on the dt overlay:

tdc_clk_in: tdc_clk_in {

status = "okay";

compatible = "pwm-clock";

#clock-cells = <1>;

pwms = <&pwm0 0 PWM_HZ(8000000) PWM_POLARITY_NORMAL>;

};

&pwm0 {

status = "okay";

pinctrl-0 = <&pwm0_default>;

pinctrl-1 = <&pwm0_sleep>;

pinctrl-names = "default", "sleep";

};

&pinctrl {

pwm0_default: pwm0_default {

group1 {

psels = <NRF_PSEL(PWM_OUT0, 0, 27)>; // TDC_CLK_8MHZ

};

};

pwm0_sleep: pwm0_sleep {

group1 {

psels = <NRF_PSEL(PWM_OUT0, 0, 27)>; // TDC_CLK_8MHZ

low-power-enable;

};

};

};

It's not working since I see static 3v3 on the P0.27.

What should I do to make it work?

Thank you in advance,

Best regards,

Fernando Fontes

[Einar Thorsrud]

Hi,

There should not be any problems using 8 MHz PWM. How are you measuring the output signal? Can you share the plot? Using the PWM is also covered in a DevAcademy course, here.

[Fernando Fontes]

Hi,

Thank you for the reply.

I don't have the oscilloscope with me right now but declaring tdc_clk_in as compatible = "pwm-clock" only allows me to reach frequencies up to 1MHz. 

I've used the function pwm_set_dt(&tdc_clk_in,PWM_HZ(1000000),PWM_HZ(1000000) / 2); and it worked. However with pwm_set_dt(&tdc_clk_in,PWM_HZ(8000000),PWM_HZ(8000000) / 2); it does not.

In the Zephyr pwm-clock documentation they mention a 1MHz internal clock signal, not sure if that is the maximum that I can reach. They also mention that I might need to change prescaler options for different settings.

Thanks in advance,

Fernando

[Einar Thorsrud]

Hi Fernando,

I was incorrect in my previous reply. You cannot generate an 8 MHz output using the PWM peripheral (the maximum is limted to 5.3333 MHz as explaiend in this post). It is possible to generate an 8 MHz putput though, by using a timer and GPIOTE connected via PPI. That can be done as shown in this post.

Einar