SAADC Limit Monitoring

[wael]

Hello,

I'm writing an application that uses the SAADC and would like to set up a high limit to trigger on.

If I use the following code, I'm able to trigger 1 ADC sample successfully:

errCode = nrf_drv_saadc_buffer_convert( &adcValue, 1 );
APP_ERROR_CHECK( errCode );

errCode = nrf_drv_saadc_sample();
APP_ERROR_CHECK( errCode );

And the registered callback when init'ing SAADC is:

static void _TriggeredEventCallback( nrf_drv_saadc_evt_tconst * inEvent )
{
    if ( inEvent->type == NRF_DRV_SAADC_EVT_DONE )
    {
        // Get the ADC conversion result
        nrf_saadc_value_t adcValue = inEvent->data.done.p_buffer[0];
        NRF_LOG_INFO( "Read ADC %d\n", adcValue );
    }
    else if ( inEvent->type == NRF_DRV_SAADC_EVT_LIMIT )
    {
        NRF_LOG_INFO( "Limit exceeded!\n" );
    }
}

Then I went further and tried to set the high limit by adding a call to nrf_drv_saadc_limits_set() right before the call to nrf_drv_saadc_buffer_convert()

// Set the high limit
nrf_drv_saadc_limits_set( kCurrentMonitorADCChannel, NRF_DRV_SAADC_LIMITL_DISABLED, 200 );

But I didn't see the behavior I was expecting when I increase the voltage that is connected to the pin. However if I boot up with an input voltage that exceeds the limit, I see the printout indicating the limit is exceeded.

My questions are:

1. If I set up a limit and start a single conversion, would ADC conversions be triggered automatically internally until the limit is exceeded or some other part of the code stops the ADC?
2. Does the event NRF_DRV_SAADC_EVT_DONE trigger for all ADC conversions when I have a limit set or am I only expecting the event NRF_DRV_SAADC_EVT_LIMIT to trigger?

Thanks!

[Carsten Wulff]

1. You have to trigger a sample. The intention was that one can setup a timer and via ppi trigger sample, but only interrupt the MCU if the value was outside a limit.
2. Done will always trigger

[wael]

Thanks for your answer Carsten.

Just to clarify, your answers for (1) and (2) sound contradicting to me. If the MCU is interrupted only when the value is outside the limit, then how can it be that DONE will always trigger? The code in SAADC_IRQHandler() suggests that the NRF_DRV_SAADC_EVT_DONE will always get triggered (which is inline with your answer to (2)). What do you mean exactly by "only interrupt the MCU if the value was outside a limit"?