NRFX PWM driver timing

[Andy]

Hello,

I'm working on some LED sequences using the NRFX PWM driver and I can't get the durations I want right.

Let's say that I want a semi-blinking LED (100% - 50% - 100%...) with a period of 500 ms on each state and I initialize the PWM driver as follows:

nrfx_pwm_config_t const pwm_config =
{
	.output_pins =
	{
		led.red | led.polarity,     // Channel 0
		led.green | led.polarity,   // Channel 1
		led.blue | led.polarity,    // Channel 2
		NRFX_PWM_PIN_NOT_USED       // Channel 3
	},
	.irq_priority = APP_IRQ_PRIORITY_LOW,
	.base_clock   = NRF_PWM_CLK_16MHz,						// NRF_PWM_CLK_125kHz,
	.count_mode   = NRF_PWM_MODE_UP_AND_DOWN,				// For symmetric color mixing
	.top_value    = 255,
	.load_mode    = NRF_PWM_LOAD_INDIVIDUAL,
	.step_mode    = NRF_PWM_STEP_AUTO
};
err_code = nrfx_pwm_init(&m_pwm0, &pwm_config, pwm_event_handler);
APP_ERROR_CHECK(err_code);

As far as I understand I could do this to get my timing right:

void setup_single_color_sequence(nrf_pwm_sequence_t * sequence, color_t color, uint32_t duration_in_us) {
	uint32_t top = 255;
	uint32_t pwm_period = top / 16; 						// In µs (top / clock_frequency in MHz)
	uint16_t pwm_pulses_for_duration = duration_in_us / (pwm_period * NRF_PWM_VALUES_LENGTH(pwm_values_single_color));

	for (uint8_t i = 0; i < 2; i++) {
		pwm_values_single_color[i].channel_0 = color.r;
		pwm_values_single_color[i].channel_1 = color.g;
		pwm_values_single_color[i].channel_2 = color.b;
		pwm_values_single_color[i].channel_3 = 0;
	}

	sequence->values.p_individual		= pwm_values_single_color;
	sequence->length					= NRF_PWM_VALUES_LENGTH(pwm_values_single_color);
	sequence->repeats					= pwm_pulses_for_duration;
	sequence->end_delay					= 0;
}

That is using the .repeats field to make each duty cycle repeat in order to get my desired timing. That however doesn't really work, I get shorter periods.

My logic is: The PWM counts up to 255. If I'm running the PWM at 16 MHz, then each duty cycle has a duration of 255/16 MHz = ~16 us. If I want the PWM to stay at that level for 500 ms, then I divide that duration (in us) by the duration of the duty cycle and I get the number of times that I need to repeat each duty cycle in order to get my target duration.

Is that logic wrong?

[MartinBL]

I'm not sure if I follow your logic. Can you show me how  you would use this function to generate a signal of "(100% - 50% - 100%...) with a period of 500 ms on each state "?

How much shorter than expected are the periods?

[Andy]

And the periods are maybe half as long as they should be maybe. So it's noticeable.

[MartinBL]

Hi,

Maybe you can try something like this:

nrfx_pwm_t m_pwm0 = NRFX_PWM_INSTANCE(0);

#define PWM_TOP_VALUE 100

static void pwm_init(void)
{
    nrfx_pwm_config_t const pwm_config =
    {
	    .output_pins =
	    {
		    LED_1,     // Channel 0
		    NRFX_PWM_PIN_NOT_USED,   // Channel 1
		    NRFX_PWM_PIN_NOT_USED,    // Channel 2
		    NRFX_PWM_PIN_NOT_USED       // Channel 3
	    },
	    .irq_priority = APP_IRQ_PRIORITY_LOW,
	    .base_clock   = NRF_PWM_CLK_16MHz,						// NRF_PWM_CLK_125kHz,
	    .count_mode   = NRF_PWM_MODE_UP_AND_DOWN,				// For symmetric color mixing
	    .top_value    = PWM_TOP_VALUE,
	    .load_mode    = NRF_PWM_LOAD_INDIVIDUAL,
	    .step_mode    = NRF_PWM_STEP_AUTO
    };
    uint32_t err_code = nrfx_pwm_init(&m_pwm0, &pwm_config, NULL);
    APP_ERROR_CHECK(err_code);
}


static nrf_pwm_values_individual_t pwm_values[2]; 

nrf_pwm_sequence_t static pwm_sequence =
{
    .values.p_individual = pwm_values,
    .length          = NRF_PWM_VALUES_LENGTH(pwm_values),
    .repeats         = 1,
    .end_delay       = 0
};


void make_sequence(uint32_t duration_us)
{
    uint32_t pwm_period = 2 * PWM_TOP_VALUE / 16; // In µs (top / clock_frequency in MHz). Multiply by 2 if using NRF_PWM_MODE_UP_AND_DOWN
    uint32_t repeats = duration_us / pwm_period;
    
    pwm_sequence.repeats = repeats;
    
    pwm_values[0].channel_0 = 100;
    pwm_values[1].channel_0 = 5;
}


int main(void)
{    
    NRF_CLOCK->TASKS_HFCLKSTART = 1;
    
    LEDS_CONFIGURE(LEDS_MASK);
    LEDS_OFF(LEDS_MASK);

    pwm_init();
    
    make_sequence(500000);
    nrfx_pwm_simple_playback(&m_pwm0, &pwm_sequence, 1, NRFX_PWM_FLAG_LOOP);

    for (;;)
    {

    }
}

This code will blink LED_1 with a frequency of 1 Hz.

 

[Andy]

Hi Martin, just to let you know that I still haven't been able to make it work, but it looks like there's something else wrong with my implementation. I'll update here when I figure it out