Using PWM to dim the LEDs of a NUCLEO-F746ZG

[mdaeron]

I'm experimenting with a NUCLEO-F746ZG board. Using the following code I can control the brightness of LED1 (although the percentage is reversed due to the B0 pin having an inverted PWM/timer/channel).

import machine, pyb
p = machine.Pin('B0')
tim = pyb.Timer(1, freq=1000)
ch = tim.channel(2, pyb.Timer.PWM, pin=p)
ch.pulse_width_percent(50)

Next I wanted to try and implement this in firmware, as the pyboard does for LED(3) and LED(4). In ports/stm32/boards/PYBV11/mpconfigboard.h I find:

#define MICROPY_HW_LED3_PWM         { TIM2, 2, TIM_CHANNEL_1, GPIO_AF1_TIM2 }
#define MICROPY_HW_LED4_PWM         { TIM3, 3, TIM_CHANNEL_1, GPIO_AF2_TIM3 }

And in ports/stm32/boards/NUCLEO_F412ZG/mpconfigboard.h I find:

#define MICROPY_HW_LED1_PWM         { TIM1, 1, TIM_CHANNEL_2, GPIO_AF1_TIM1 }
#define MICROPY_HW_LED2_PWM         { TIM4, 4, TIM_CHANNEL_2, GPIO_AF2_TIM4 }
#define MICROPY_HW_LED3_PWM         { TIM12, 12, TIM_CHANNEL_1, GPIO_AF9_TIM12 }

I tried adding variations of this code to boards/NUCLEO_F746ZG/mpconfigboard.h but the resulting firmware remains unable to smoothly dim LEDs using machine.LED.intensity().

Does anyone have pointers/documentation on what the code above actually do, and how to adapt it so that it controls one or more LEDs on the F746ZG?

[mdaeron]

I've looked into this a bit mode closely, and here is what I found so far:

• LEDs 1,2, and 3 are connectd to CPU pins B0, B7 and B14
• AF1 for B0 and B14 is TM1_CH2N, while AF2 of B7 is TIM4CH2

LED operation is defined in ports/stm32/led.c, with the following comment:

// The following is semi-generic code to control LEDs using PWM.
// It currently supports TIM1, TIM2 and TIM3, channels 1-4.
// Configure by defining the relevant MICROPY_HW_LEDx_PWM macros in mpconfigboard.h.
// If they are not defined then PWM will not be available for that LED.

I assume that my attempt with #define MICROPY_HW_LED2_PWM {TIM4, 4, TIM_CHANNEL_2, GPIO_AF2_TIM4} failed because TIM4 is not supported as per the above comment.

I suspect (but haven't tested yet) that modifying this part of led.c to also include TIM4 might work:

    // TIM configuration
    switch (pwm_cfg->tim_id) {
        case 1:
            __TIM1_CLK_ENABLE();
            break;
        case 2:
            __TIM2_CLK_ENABLE();
            break;
        #if defined(TIM3)
        case 3:
            __TIM3_CLK_ENABLE();
            break;
        #endif
        //////////////////////////// THE FOLLOWING 5 LINES ADD TIM4 SUPPORT (?)
        #if defined(TIM4)
        case 4:
            __TIM4_CLK_ENABLE();
            break;
        #endif
        ////////////////////////////
        default:
            assert(0);
    }

For LEDs 1 and 3, might the problem be that the timer channel for AF1 is inverted, and the code in led.c does not support this?

I'll keep trying and report here. Any hints appreciated.

[mdaeron]

I have now confirmed that adding TIM4 support in led.c solves the issue for LED2. Still not sure what the solution would be for the other LEDs, though.