nRF52840 saadc results are in the wrong order

[Philip]

Hi. In our application we are reading the voltage on 3 Analog input pins via saadc. System is also a BLE central and UBD CDC device so lots going on.

Quite often the 3 analog results are shifted so if they were in the order A,B,C normally they could randomly become B,A,C or C,B,A etc.

I have seen a few threads on the forum about this but no clear answers.

I am trying to avoid adding a separate ADC to SPI IC on the board, but the SAADC on the nRF52840 is not up to the job which is a shame since.

Here are some code snippets. Maybe I could re-initialise the saadc after every sample? I only need to read them every second or so.

nrf_drv_saadc_sample(); Is triggered every few seconds from timer interrupt.

Thanks

void saadc_init(void)
{
    ret_code_t err_code;
    nrf_saadc_channel_config_t channel_config6 
      = NRF_DRV_SAADC_DEFAULT_CHANNEL_CONFIG_SE(NRF_SAADC_INPUT_AIN6); //P0.30 = AN6 = Regulated voltage
    nrf_saadc_channel_config_t channel_config4 
      = NRF_DRV_SAADC_DEFAULT_CHANNEL_CONFIG_SE(NRF_SAADC_INPUT_AIN4); //P0.28 = AN4 = Current sense input
    nrf_saadc_channel_config_t channel_config5 
      = NRF_DRV_SAADC_DEFAULT_CHANNEL_CONFIG_SE(NRF_SAADC_INPUT_AIN5); //P0.29 = AN5 = Voltage In

    err_code = nrf_drv_saadc_init(NULL, saadc_callback);
    APP_ERROR_CHECK(err_code);

    err_code = nrf_drv_saadc_channel_init(0, &channel_config6);
    APP_ERROR_CHECK(err_code);
    err_code = nrf_drv_saadc_channel_init(1, &channel_config4);
    APP_ERROR_CHECK(err_code);
    err_code = nrf_drv_saadc_channel_init(2, &channel_config5);
    APP_ERROR_CHECK(err_code);

    err_code = nrf_drv_saadc_buffer_convert(m_buffer, SAMPLES_IN_BUFFER);
    APP_ERROR_CHECK(err_code);
}

void saadc_callback(nrf_drv_saadc_evt_t const * p_event)
{
    if (p_event->type == NRF_DRV_SAADC_EVT_DONE)
    {
        ret_code_t err_code;

        err_code = nrf_drv_saadc_buffer_convert(p_event->data.done.p_buffer, SAMPLES_IN_BUFFER);
        APP_ERROR_CHECK(err_code);

        output[0] = p_event->data.done.p_buffer[0];
        output[1] = p_event->data.done.p_buffer[1];
        output[2] = p_event->data.done.p_buffer[2];

        if(output[0] > 50000) output[0] = 0; //We don't want 65535 etc negative about 0 to get into average
        if(output[1] > 50000) output[1] = 0;
        if(output[2] > 50000) output[2] = 0;

        voltage_reg_ave = ((output[0] - voltage_reg_ave) / mov_ave_divisor) + voltage_reg_ave;
        voltage_in_ave = ((output[2] - voltage_in_ave) / mov_ave_divisor) + voltage_in_ave;
        current_sense_ave = ((output[1] - current_sense_ave) / mov_ave_divisor) + current_sense_ave;
    }
}

[haakonsh]

See https://devzone.nordicsemi.com/support-private/support/120290#permalink=79052 

1. What is your sample rate? 
2. Do you experience the issue if no other CPU activity but the SAADC driver is running?

[Philip]

Hi Haakonsh.

Sample rate is about 50Hz.

Only have the issue when the CPU is under heavy load, i.e. lots of BLE comms and UART serial comms. It can be running for days with no issue, but as soon as lots going on the values swap.

I tried the following so sample can only happen when callback finished, but didn't help;

        if(saadc_sample_count < SAMPLES_IN_BUFFER)
        {
            nrf_drv_saadc_sample(); //Tigger saadc samples
            saadc_sample_count++;
        }     

I am going to try this next;

        nrf_drv_saadc_sample_convert(0, &output[0]);
        nrf_drv_saadc_sample_convert(1, &output[1]);
        nrf_drv_saadc_sample_convert(2, &output[2]);

Phil

[haakonsh]

If you're only going to sample at 50Hz using nrf_drv_saadc_sample_convert can be an acceptable workaround. 

Otherwise I recommend writing a new SAADC driver, it's actually not as hard as it seems since it will only need to support a subset of the SAADC features.

[Philip]

nrf_drv_saadc_sample_convert not working. Think because I put NULL below;

err_code = nrf_drv_saadc_init(NULL, NULL);
APP_ERROR_CHECK(err_code);

Why is the saadc_callback needed for blocking sampling? All I want to do is take a reading from the ADC using nrf_drv_saadc_sample_convert.

Thanks

Phil

[Philip]

Get NRF_ERROR_BUSY error on nrf_drv_saadc_sample_convert(0,&output[0])

[Philip]

Working now! Had to comment out the below from the saadc_init function

//err_code = nrf_drv_saadc_buffer_convert(m_buffer, SAMPLES_IN_BUFFER);
//APP_ERROR_CHECK(err_code);

[haakonsh]

Glad to hear, let me know if you need any more help.