SAADC V2 sampling using the RTC

Question

Hi there,

I'm having some trouble piecing together an app to use V2 of the NRFX SAADC API, compounded with this being my first time using the SAADC functionality on an nRF board.

In summary, my program receives an "ERROR 8 [NRF_ERROR_INVALID_STATE]" when calling the nrfx_saadc_mode_trigger function.

My understanding is that we first initialise SAADC:

static void saadc_init()
{
    ret_code_t err_code;

    nrfx_saadc_adv_config_t config = {
        .oversampling = NRF_SAADC_OVERSAMPLE_DISABLED,
        .burst = false,
        .internal_timer_cc = 0,
        .start_on_end = false,
    };

    err_code = nrfx_saadc_init(NRFX_SAADC_CONFIG_IRQ_PRIORITY);
    APP_ERROR_CHECK(err_code);
    err_code = nrfx_saadc_channels_config(m_accel, 3);
    APP_ERROR_CHECK(err_code);

    err_code = nrfx_saadc_advanced_mode_set((1 << 0) | (1 << 1) | (1 << 2),
                                            NRF_SAADC_RESOLUTION_8BIT,
                                            &config,
                                            on_saadc_in_evt);
    APP_ERROR_CHECK(err_code);

    err_code = nrfx_saadc_buffer_set(m_accel_sample_bufs[m_accel_current_buffer], ACCEL_SAMPLES_PER_BUFFER);
    APP_ERROR_CHECK(err_code);
}

My intention is to use the RTC to sample three analogue pins previously defined as:

nrfx_saadc_channel_t m_accel[] = {
    NRFX_SAADC_DEFAULT_CHANNEL_SE(NRF_SAADC_INPUT_AIN5, 0),
    NRFX_SAADC_DEFAULT_CHANNEL_SE(NRF_SAADC_INPUT_AIN6, 1),
    NRFX_SAADC_DEFAULT_CHANNEL_SE(NRF_SAADC_INPUT_AIN7, 2)};

A timer is created and, when it fires, it then triggers the sampling:

static void accel_sampler_timer(void *p_context)
{
    ret_code_t err_code = nrfx_saadc_mode_trigger();
    APP_ERROR_CHECK(err_code);
}

...and that's when the invalid state is reported.

The SAADC event handler is as follows:

static void on_saadc_in_evt(nrfx_saadc_evt_t const *p_event)
{
    ret_code_t err_code;

    switch (p_event->type)
    {
    case NRFX_SAADC_EVT_DONE:
        // TODO: Use p_event->data.done.p_buffer[0];
        break;
    case NRFX_SAADC_EVT_BUF_REQ:
        m_accel_current_buffer = 1 - m_accel_current_buffer;
        err_code = nrfx_saadc_buffer_set(m_accel_sample_bufs[m_accel_current_buffer], ACCEL_SAMPLES_PER_BUFFER);
        APP_ERROR_CHECK(err_code);
        break;
    default:
        break;
    }
}

Am I under the false impression that the timer should trigger the sampling?

Thanks for your guidance.

Christopher Hunt · Accepted Answer

Christopher Hunt said: Thanks for everything Karl. I'm still having difficulty understanding why my SAADC sampling fires just once. Any pointers there would be great. I feel like I'm very close, but struggling with this last aspect. Here's my current code again: 
 
 I think I've now got to the bottom of the problem I had in understanding SAADC. My assumption was that I could drive the nrfx_saadc V2 state machine by firing SAADC tasks. I paired back everything to the Nordic Playground SAADC simple low power app timer example and replaced the app timer with RTC and PPI behaviour. My conclusion is that the NRF_SAADC_TASK_SAMPLE task has no effect when fired from PPI as the nrfx_saadc state machine is not in a "sampling" state. To attain a sampling state, nrfx_saadc_mode_trigger() must be called i.e. it isn't enough to issue a NRF_SAADC_TASK_SAMPLE task, even when nrfx_saadc_mode_trigger() has been called once to kick everything off. 
 I've put my fork of the ADC playground low power app with RTC here for reference: https://github.com/huntc/nRF52-ADC-examples/tree/rtc-ppi/nrfx_saadc_simple_low_power_app_timer . To see my specific changes, please see this commit: https://github.com/huntc/nRF52-ADC-examples/commit/d4075a7ce59f840819b642f93c80af308a1e4f12 . 
 I think I shall now abandon the RTC/PPI driven approach here as it doesn't seem possible. The earlier suggestion of using LPCOMP seems more useful for my particular use-case anyhow. Thanks, and I hope this dialogue has been useful.

SAADC V2 sampling using the RTC

Top Replies