SAADC sample via PPI not working without previous trigger

[Yaxit]

Hello,

I'm trying to achieve a rather simple program with three steps.

1. I initialize the SAADC to sample three channels (currently I only have one setup)
2. I "arm/prepare" the ADC and a timer to control the SAADC samples via PPI
3. SAADC samples n times until buffer is full, driven by the timer.

This setup seems to work if I first trigger a sample manually during the init phase, which I do not want to do.

Here is the initialization code, where adc_end_cb just prints the ADC event.
When I uncomment the nrfx_saadc_mode_trigger(); command, the PPI seems to work and collect samples. If it's commented, the callback is never invoked, as the ADC was disabled or similar.

int hw_adc_init(void (*cb)(void), uint16_t *buffer, size_t buffer_size){
    nrfx_err_t err;

    adc_end_cb_ptr = cb;

    err = nrfx_saadc_init(5);
    if(err != NRFX_SUCCESS){
        LOG_ERR("Failed to initialize SAADC");
        return err;
    }

	IRQ_CONNECT(DT_IRQN(SAADC_NODE), DT_IRQ(SAADC_NODE, priority), nrfx_isr, nrfx_saadc_irq_handler, 0);


    nrfx_saadc_adv_config_t channel_config = NRFX_SAADC_DEFAULT_ADV_CONFIG;
    channel_config.oversampling = NRF_SAADC_OVERSAMPLE_DISABLED;
    channel_config.burst = NRF_SAADC_BURST_DISABLED;
    channel_config.internal_timer_cc = 0;   // if set to 0, TASK_SAMPLE must be triggered manually
    channel_config.start_on_end = true;     // seems to be a "restart" task a the end of sampling
    // todo get pins from dts
    nrfx_saadc_channel_t config = NRFX_SAADC_DEFAULT_CHANNEL_SE(NRF_SAADC_INPUT_AIN0, 0);
    config.channel_config.reference = NRF_SAADC_REFERENCE_VDD4;
    config.channel_config.gain = NRF_SAADC_GAIN1_4;

    err = nrfx_saadc_channel_config(&config);
    if(err != NRFX_SUCCESS){
        LOG_ERR("Failed to configure SAADC channel");
        return err;
    }

    err = nrfx_saadc_advanced_mode_set(0x01,
                                    NRF_SAADC_RESOLUTION_10BIT,
                                    &channel_config,
                                    adc_end_cb);
    if(err != NRFX_SUCCESS){
        LOG_ERR("Failed to set SAADC advanced mode");
        return err;
    }

    err = nrfx_saadc_buffer_set((nrf_saadc_value_t*)buffer, buffer_size);
    if(err != NRFX_SUCCESS){
        LOG_ERR("Failed to set SAADC buffer");
        return err;
    }


    // err = nrfx_saadc_mode_trigger();
    // if(err != NRFX_SUCCESS){
    //     LOG_ERR("Failed to trigger SAADC");
    //     return err;
    // }



    return 0;
}

Then I trigger the START task in software while preparing the timers etc

    nrf_saadc_task_trigger(NRF_SAADC, NRF_SAADC_TASK_START);

The PPI channel is connected with the SAMPLE task

    err = nrfx_ppi_channel_assign(ppi_ch_adc_trigger,
        nrfx_timer_event_address_get(&timer, NRF_TIMER_EVENT_COMPARE4),
        nrf_saadc_task_address_get(NRF_SAADC, NRF_SAADC_TASK_SAMPLE));

Maybe I have some confusion about the TASK and SAMPLE tasks? I thought the START would "prepare" the ADC for sampling, and the SAMPLE actually triggers one sample. Is this assumption wrong? Maybe I'm also mixing APIs a bit but the nrfx does not seem to have a good way of doing this.

Thanks!

[Kenneth]

Hello,

I suggest to check out the hardware description of the SAADC:
https://docs.nordicsemi.com/bundle/ps_nrf52832/page/saadc.html#d944e352 

Do make sure you don't combine scan (more than one channel configured) with oversampling, that is not supported by the hardware. Also do check out the Errata list for the SAADC, maybe one of those impact you:
https://docs.nordicsemi.com/bundle/errata_nRF52832_Rev3/page/ERR/nRF52832/Rev3/latest/err_832_new.html 

Kenneth

[Yaxit]

Hi Kenneth, could it be that after I get EVT_FINISH just triggering START is not sufficient to trigger the ADC?
I noticed this problem does not appear if I use double buffering. The difference with a single buffer is that when the buffer is full I get both a DONE and a FINISHED event.