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PWM driver accuracy

MaximeMylle

Hello everyone

first if all, im not an expert in PWM, but doing my best to understand everything

I have successfully created and tested a project using the pwm_driver example.

 nrf_drv_pwm_config_t const config0 =
    {
        .output_pins =
        {
            OUTPUT_PIN_1 | NRF_DRV_PWM_PIN_INVERTED,             // channel 0
            OUTPUT_PIN_2 | NRF_DRV_PWM_PIN_INVERTED,             // channel 1
            OUTPUT_PIN_3 | NRF_DRV_PWM_PIN_INVERTED,             // channel 2
            NRF_DRV_PWM_PIN_NOT_USED                             // channel 3
        },
        .irq_priority = APP_IRQ_PRIORITY_LOWEST,
        .base_clock   = NRF_PWM_CLK_1MHz,
        .count_mode   = NRF_PWM_MODE_UP,
        .top_value    = 100,
        .load_mode    = NRF_PWM_LOAD_INDIVIDUAL,
        .step_mode    = NRF_PWM_STEP_AUTO
    };

in this piece of code, the top value is set to 100. 
I understand that I have an accuracy of 0 to 100, so its actually just a percentage

Now i would like to have an accuracy of 0 to 65535 ( actually just a 16-bit integer )

I have already tried playing around with the top value but it seems to only go around 30 000.
I have been looking around and my guess is that I have to change the base_clock, but i don't really know what I'm doing at ths point.

Is this even possible to achieve? If not, whats the maximum top value then?

Thanks in advance

Maxime Mylle

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  • +1

    Hi Maxime

    The PWM duty cycle is configured by a 16-bit register, where 15-bits are used for the value and the last bit is used to set the polarity. 

    This limits the top value of the PWM module to 32767   (or 2^15 - 1)

    Keep in mind that increasing the top value of the PWM will reduce the PWM frequency, which can be a problem depending on what you are planning to use it for. 

    If you are using it to control LED's you might get away with a PWM frequency as low as 60Hz, but if you are controlling something else (like a motor) you might want to have a significantly higher PWM frequency. 

    Best regards
    Torbjørn

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  • +1

    Hi Maxime

    The PWM duty cycle is configured by a 16-bit register, where 15-bits are used for the value and the last bit is used to set the polarity. 

    This limits the top value of the PWM module to 32767   (or 2^15 - 1)

    Keep in mind that increasing the top value of the PWM will reduce the PWM frequency, which can be a problem depending on what you are planning to use it for. 

    If you are using it to control LED's you might get away with a PWM frequency as low as 60Hz, but if you are controlling something else (like a motor) you might want to have a significantly higher PWM frequency. 

    Best regards
    Torbjørn

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