NRFX PWM driver timing

RE: NRFX PWM driver timing

Andy — Mon, 25 Jun 2018 15:09:14 GMT

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

RE: NRFX PWM driver timing

MartinBL — Thu, 21 Jun 2018 10:37:12 GMT

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.

RE: NRFX PWM driver timing

Andy — Tue, 19 Jun 2018 17:10:11 GMT

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

RE: NRFX PWM driver timing

Andy — Tue, 19 Jun 2018 17:09:28 GMT

If I understand correctly I have two options:

Create a sequence with both values, i.e. [100%, 50%] and call the function with a duration of 1 second. This probably wouldn't work because of the size of repeats (uint16), but I could do [100%, 100%, 50%, 50%] and maybe then it'd work. The code then would look like this maybe:

for (uint8_t i = 0; i < 4; i++) {
    // Make the second half at 50%
    uint8_t divide = (i >= 2) ? 2 : 1;
    
	pwm_values_single_color[i].channel_0 = color.r / divide;
	pwm_values_single_color[i].channel_1 = color.g / divide;
	pwm_values_single_color[i].channel_2 = color.b / divide;
	pwm_values_single_color[i].channel_3 = 0;
}

This is not what I'm doing.

What I'm doing is the second choice which would be to create two sequences and then playing them one after the other like this:

nrfx_pwm_complex_playback(&m_pwm0, &pwm_sequence_1, &pwm_sequence_2, 1, NRFX_PWM_FLAG_LOOP);

I am doing this because I'm not using just a static full lit LED, but rather I'm playing back a fade in/out sequence with a sine sequence. I want to use a single color at 0% or 50% as sequence #2 to space the waveform. So I would then call setup_single_color_sequence() to generate this pause with a specific duration.

I hope that makes sense.

RE: NRFX PWM driver timing

MartinBL — Tue, 19 Jun 2018 15:48:35 GMT

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?