nucleo_f767zi PWM on built-in LEDs - help

[mrbya]

Help needed with PWM on nucleo_f676zi

Hi, I've been trying to get PWM working with the built-in LEDs of the nucleo_f767zi board, however no matter what period/pulse width I set the pin output stays @ 50% duty cycle.

There's a high possibility that I've misconfigured the timer or pwm device, though I've followed the docs and everything I've found online.

Here's the overlay I'm using for the f767 nucleo board:

&pinctrl {
    pwmled_ctrl: pwmled_ctrl {
        pinmux = <STM32_PINMUX('B', 7, AF2)>;
        bias-pull-up;
        drive-push-pull;
    };
};

&timers4 {
    status = "okay";
    pwm4: pwm {
        status = "okay";
        pinctrl-0 = <&pwmled_ctrl>;
        pinctrl-names = "default";
    };
};

/ {
    pwmleds: pwmleds {
        compatible = "pwm-leds";
        status = "okay";
        
        pwm4_led: pwm4_led {
            pwms = <&pwm4 1 200000 0 0>;
        };
    };

    aliases {
        pwmtest = &pwm4_led;
    };
};

And the snippet I'm trying the PWM with:

#include <cstdio>
#include <iostream>
#include <zephyr/kernel.h>
#include <zephyr/drivers/pwm.h>

constexpr uint32_t period = PWM_NSEC(200000);
constexpr struct pwm_dt_spec led = PWM_DT_SPEC_GET(DT_ALIAS(pwmtest));

int main(void) {
    if (pwm_is_ready_dt(&led)) {
        std::cout << pwm_set_dt(&led, period, 0) << std::endl;
    } else {
        std::cout << led.dev -> name << " not ready!" << std::endl; 
    }

    k_msleep(1000);

    if (pwm_is_ready_dt(&led)) {
        std::cout << pwm_set_dt(&led, period, period) << std::endl;
    } else {
        std::cout << led.dev -> name << " not ready!" << std::endl; 
    }

    k_msleep(1000);

    if (pwm_is_ready_dt(&led)) {
        std::cout << pwm_set_dt(&led, period, period / 2u) << std::endl;
    } else {
        std::cout << led.dev -> name << " not ready!" << std::endl; 
    }

    return 0;
}

pwm_is_ready is returning true, pwm_set_dt are returning 0, but still, the LED stays @ 50% duty cycle.

Tried lowering the frequency, changing pull-up/pull-down config - frequency changes had no effect, if configured as pull-down then the output was 0 throughout the run, same when using the tim4_ch2_pb7 pre-defined pinctrl node.

Any help would be kindly appreciated! ^^

Environment

• Board: nucleo_f676zi
• Zephyr: v4.1.99

[ldomaigne]

You first need to lookup your ST datasheet for the alternate function mapping (table 13 on p89)
For PB7, you can use AF2 / TIM4_CH2

We could do the pin muxing ourself, our simply use the node &tim4_ch2_pb7, which is defined in the ST/HAL pinctrl dtsi:
modules/hal/stm32/dts/st/f7/stm32f767zitx-pinctrl.dtsi

Since we're using timer4, we need to enable that node, as well configure the corresponding pwm4 function:

&timers4 {
    status = "okay";
    pwm4: pwm {
        status = "okay";
        pinctrl-0 = <&tim4_ch2_pb7>;
        pinctrl-names = "default";
    };
};

The last bit consists of defining the pwmleds. For the pwms phandle array, the definition for the items to pass is given by the ST pwm yaml file (see #pwm-cells): zephyr/dts/bindings/pwm/st,stm32-pwm.yaml

/ {
    pwmleds: pwmleds {
        compatible = "pwm-leds";
        status = "okay";

        pwm4_led: pwm4_led {
            pwms = <&pwm4 2 20000 PWM_POLARITY_NORMAL>;
        };
    };

    aliases {
        pwmtest = &pwm4_led;
    };
};

Hope this helps!