PWM clock on nRF52 does not lock on oscilloscope

RE: PWM clock on nRF52 does not lock on oscilloscope

Øyvind Karlsen — Wed, 13 Jul 2016 10:36:15 GMT

You can use nrw_pwm_tasks_trigger.

RE: PWM clock on nRF52 does not lock on oscilloscope

guraaf — Tue, 12 Jul 2016 22:56:37 GMT

I did want to ask a thing though - I can use functions from nrf_pwm.h for everything other than starting the PWM task itself. For that I have to use the direct write to the register. Is the following correct and intended? Why is there no function to write to TASKS_SEQSTART[0]

nrf_gpio_cfg_output(SAMPLE);
nrf_pwm_enable(NRF_PWM0);
nrf_pwm_pins_set(NRF_PWM0, out_pins);
nrf_pwm_configure(NRF_PWM0, NRF_PWM_CLK_16MHz, NRF_PWM_MODE_UP, 50);
nrf_pwm_decoder_set(NRF_PWM0, NRF_PWM_LOAD_INDIVIDUAL, NRF_PWM_STEP_AUTO);
nrf_pwm_shorts_set(NRF_PWM0, 0);
nrf_pwm_int_set(NRF_PWM0, 0);
nrf_pwm_loop_set(NRF_PWM0, 0);
nrf_pwm_sequence_set(NRF_PWM0, 0, &seq);
NRF_PWM0->TASKS_SEQSTART[0] = 1; // <=== why no function?

RE: PWM clock on nRF52 does not lock on oscilloscope

guraaf — Tue, 12 Jul 2016 20:16:27 GMT

I figured out my mistake and problem. It was the oscilloscope. With the correct settings, the output is stable. Sorry for taking up your time.

RE: PWM clock on nRF52 does not lock on oscilloscope

guraaf — Tue, 12 Jul 2016 17:54:33 GMT

Thanks Øyvind. I tried exactly your code but the waveform still keeps scrolling. The oscilloscope just doesn't lock. It appears that the frequency period isn't correct or maybe there is a phase shift of some sort?

If I do stop the trigger or do a single capture then I do see the waveform period as 1 ms. The low-period is about 600 us and On duration is 400 us. This would be based upon the 10,000 value in pwm_seq[0].

I am using Pin 5 and a few more unconnected pins but they all have scrolling clock. Note that I am using a single channel and not concerned about inverting the signal with Bit 16 (MSB) yet.

RE: PWM clock on nRF52 does not lock on oscilloscope

Øyvind Karlsen — Tue, 12 Jul 2016 13:27:10 GMT

Hi,

Please refer to this answer.

You can try the following code:

int main (void)
{
    // Start 16 MHz crystal oscillator .
    NRF_CLOCK->EVENTS_HFCLKSTARTED = 0;
    NRF_CLOCK->TASKS_HFCLKSTART    = 1;

    // Wait for the external oscillator to start up.
    while (NRF_CLOCK->EVENTS_HFCLKSTARTED == 0) {
    }

    uint16_t pwm_seq[4] = {10000, 11000 | 0x8000, 0, 0};
    NRF_PWM0->PSEL.OUT[0] = (2 << PWM_PSEL_OUT_PIN_Pos) | (PWM_PSEL_OUT_CONNECT_Connected << PWM_PSEL_OUT_CONNECT_Pos);
    NRF_PWM0->ENABLE      = (PWM_ENABLE_ENABLE_Enabled << PWM_ENABLE_ENABLE_Pos);
    NRF_PWM0->MODE        = (PWM_MODE_UPDOWN_Up << PWM_MODE_UPDOWN_Pos);
    NRF_PWM0->PRESCALER   = (PWM_PRESCALER_PRESCALER_DIV_1 << PWM_PRESCALER_PRESCALER_Pos);
    NRF_PWM0->COUNTERTOP  = (16000 << PWM_COUNTERTOP_COUNTERTOP_Pos); //1 msec
    NRF_PWM0->LOOP        = (PWM_LOOP_CNT_Disabled << PWM_LOOP_CNT_Pos);
    NRF_PWM0->DECODER   = (PWM_DECODER_LOAD_Individual << PWM_DECODER_LOAD_Pos) | (PWM_DECODER_MODE_RefreshCount << PWM_DECODER_MODE_Pos);
    NRF_PWM0->SEQ[0].PTR  = ((uint32_t)(pwm_seq) << PWM_SEQ_PTR_PTR_Pos);
    NRF_PWM0->SEQ[0].CNT  = ((sizeof(pwm_seq) / sizeof(uint16_t)) << PWM_SEQ_CNT_CNT_Pos);
    NRF_PWM0->SEQ[0].REFRESH  = 0;
    NRF_PWM0->SEQ[0].ENDDELAY = 0;
    NRF_PWM0->TASKS_SEQSTART[0] = 1;

    while (1) {
    }
}

Make sure that you are not using any pins that conflict with peripherals, for example on the DK you can use pin 0 and 1 as they are used by the HF crystal.

Best regards,

Øyvind