SAADC Measured sampling acquisition time is longer than expected

RE: SAADC Measured sampling acquisition time is longer than expected

sammy91 — Mon, 24 Jun 2019 04:09:50 GMT

Ok I see, as I increase the oversampling, I get closer to what is expected

Over 4x = 24us = 6us/sample
Over 8x = 41us = 5.1us/sample
Over 16x = 74us = 4.6us/sample

Thanks for you help

RE: SAADC Measured sampling acquisition time is longer than expected

haakonsh — Fri, 21 Jun 2019 13:13:22 GMT

but you're still toggling a pin via CPU with significant overhead due to the driver processing the END event.

If you want to measure the sample time then you need to use PPI and GPIOTE to toggle the pin.

RE: SAADC Measured sampling acquisition time is longer than expected

sammy91 — Fri, 21 Jun 2019 11:17:51 GMT

Ok I've done it your way, and still measure about 11us when the time should be <(t_ACQ+t_CONV)×2^OVERSAMPLE
Im not using oversample, so it should be <5us

Here's my new code:

Event Handler:

bool ADC_IN_PROGRESS = false;
nrf_saadc_value_t adc_buf;
void saadc_event_handler(nrf_drv_saadc_evt_t const * p_event)
{
    if (p_event->type == NRF_DRV_SAADC_EVT_DONE)
    {
        uint32_t err_code;

        adc_buf = p_event->data.done.p_buffer[0];

        nrf_gpio_pin_clear(P1_PIN);     //TOGGLE PIN TO MEASURE TIME

        //set up conversion again
        err_code = nrfx_saadc_buffer_convert(p_event->data.done.p_buffer, 1);
        if (err_code != NRF_SUCCESS){
            NRF_LOG_DEBUG("ADC error %d", err_code);
        }        
        APP_ERROR_CHECK(err_code);

        ADC_IN_PROGRESS = false;
    }
}

Take sample with nfrx_saadc_sample:

int32_t get_adc_value()
{
    ret_code_t err_code;

    nrf_gpio_cfg_output(P1_PIN);
    nrf_gpio_pin_set(P1_PIN);       //TOGGLE PIN TO MEASURE TIME

    //perform conversion
    ADC_IN_PROGRESS = true;
    err_code = nrfx_saadc_sample();
    if (err_code != NRF_SUCCESS){
        NRF_LOG_DEBUG("ADC error %d", err_code);
    }
    APP_ERROR_CHECK(err_code);

    //wait for adc to finish
    while(ADC_IN_PROGRESS);
    ADC_IN_PROGRESS = false;

    // return avg_adc;
    return adc_buf;
}

RE: SAADC Measured sampling acquisition time is longer than expected

haakonsh — Wed, 19 Jun 2019 13:48:02 GMT

Short answer, you're doing it wrong.

I suggest you increase the buffer size and find an alternative trigger source for the SAMPLE task, like an RTC or TIMER.
Right now you're taking individual samples via CPU, with substantial overhead in the driver, and then averaging them.

You can also enable oversample and average over x number of samples with just one trigger of the SAMPLE task.

See SAADC — Successive approximation analog-to-digital converter, SAADC driver documentation, SAADC driver API documentation, SAADC HAL API documentation.

See also PPI Example, PPI — Programmable peripheral interconnect, PPI and HAL API documentation.

RE: SAADC Measured sampling acquisition time is longer than expected

sammy91 — Wed, 19 Jun 2019 07:45:42 GMT

Also, I am using nRF52840 with SDK v15.2.0