NRF52832 SAADC result is not in the right order

[Yi-Wei]

Hi,

Here is my code.

I plan to use 3 ADC (2 single ended, 1 differential) on NRF52832.

#define SAMPLES_IN_BUFFER (3)

#define TEMP_pin     (NRF_SAADC_INPUT_AIN7)
#define BAT_ADC_pin  (NRF_SAADC_INPUT_AIN6)
#define DIFF_P_pin   (NRF_SAADC_INPUT_AIN5)
#define DIFF_N_pin   (NRF_SAADC_INPUT_AIN4)

const nrf_drv_timer_t MY_TIMER_0 = NRF_DRV_TIMER_INSTANCE(0);

static const nrf_drv_timer_t m_timer = NRF_DRV_TIMER_INSTANCE(0);
static nrf_saadc_value_t     m_buffer_pool[2][SAMPLES_IN_BUFFER];
static nrf_ppi_channel_t     m_ppi_channel;

#define NRF_DRV_SAADC_SINGLE_ENDED(PIN_P, GAIN)        \
    {                                                  \
        .resistor_p = NRF_SAADC_RESISTOR_DISABLED,     \
        .resistor_n = NRF_SAADC_RESISTOR_DISABLED,     \
        .gain       = GAIN,                            \
        .reference  = NRF_SAADC_REFERENCE_INTERNAL,    \
        .acq_time   = NRF_SAADC_ACQTIME_10US,          \
        .mode       = NRF_SAADC_MODE_SINGLE_ENDED,     \
        .burst      = NRF_SAADC_BURST_DISABLED,        \
        .pin_p      = (nrf_saadc_input_t)(PIN_P),      \
        .pin_n      = NRF_SAADC_INPUT_DISABLED         \
    }
    
#define NRF_DRV_SAADC_DIFFERENTIAL(PIN_P, PIN_N, GAIN) \
    {                                                  \
        .resistor_p = NRF_SAADC_RESISTOR_DISABLED,     \
        .resistor_n = NRF_SAADC_RESISTOR_DISABLED,     \
        .gain       = GAIN,                            \
        .reference  = NRF_SAADC_REFERENCE_INTERNAL,    \
        .acq_time   = NRF_SAADC_ACQTIME_10US,          \
        .mode       = NRF_SAADC_MODE_DIFFERENTIAL,     \
        .pin_p      = (nrf_saadc_input_t)(PIN_P),      \
        .pin_n      = (nrf_saadc_input_t)(PIN_N)       \
    }
    
    
//=========================================================================
//=========================================================================
void saadc_sampling_event_init(void) {
    ret_code_t err_code;

    err_code = nrf_drv_ppi_init();
    APP_ERROR_CHECK(err_code);

    uint32_t timer_compare_event_addr = nrf_drv_timer_compare_event_address_get(&m_timer,
                                                                                NRF_TIMER_CC_CHANNEL0);
    uint32_t saadc_sample_task_addr   = nrf_drv_saadc_sample_task_get();

    /* setup ppi channel so that timer compare event is triggering sample task in SAADC */
    err_code = nrf_drv_ppi_channel_alloc(&m_ppi_channel);
    APP_ERROR_CHECK(err_code);

    err_code = nrf_drv_ppi_channel_assign(m_ppi_channel,
                                          timer_compare_event_addr,
                                          saadc_sample_task_addr);
    APP_ERROR_CHECK(err_code);
}

//=========================================================================
//=========================================================================
void saadc_sampling_event_enable(void) {
    ret_code_t err_code = nrf_drv_ppi_channel_enable(m_ppi_channel);

    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);

        printf("%5d, %5d, %5d\n", p_event->data.done.p_buffer[0], p_event->data.done.p_buffer[1], p_event->data.done.p_buffer[2]);
        
    }
}


//=========================================================================
//=========================================================================
void saadc_init(void) {
    ret_code_t err_code;
    
    err_code = nrf_drv_saadc_init(NULL, saadc_callback);
    APP_ERROR_CHECK(err_code);
    
    nrf_saadc_resolution_set(NRF_SAADC_RESOLUTION_12BIT);
    
    // channel 0
    // ADC range = 0.6V / GAIN = 0.6 / (1/6) = 3.6V
    nrf_saadc_channel_config_t channel_0_config = NRF_DRV_SAADC_SINGLE_ENDED(BAT_ADC_pin, NRF_SAADC_GAIN1_6);
    err_code = nrf_drv_saadc_channel_init(0, &channel_0_config);
    APP_ERROR_CHECK(err_code);
    

    // channel 1
    // ADC range = 0.6V / GAIN = 0.6 / (1/2) = 1.2V
    nrf_saadc_channel_config_t channel_1_config = NRF_DRV_SAADC_SINGLE_ENDED(TEMP_pin, NRF_SAADC_GAIN1_6);
    err_code = nrf_drv_saadc_channel_init(1, &channel_1_config);
    APP_ERROR_CHECK(err_code);

    // channel 2
    // ADC range = 0.6V / GAIN = 0.6 / 1 = +-0.6 V
    // ADC range = 0.6V / GAIN = 0.6 / 2 = +-0.3 V
    // ADC range = 0.6V / GAIN = 0.6 / 4 = +-0.15V
    nrf_saadc_channel_config_t channel_2_config = NRF_DRV_SAADC_DIFFERENTIAL(DIFF_P_pin, DIFF_N_pin, NRF_SAADC_GAIN1);
    err_code = nrf_drv_saadc_channel_init(2, &channel_2_config);
    APP_ERROR_CHECK(err_code);
 
    err_code = nrf_drv_saadc_buffer_convert(m_buffer_pool[0], SAMPLES_IN_BUFFER);
    APP_ERROR_CHECK(err_code);

    err_code = nrf_drv_saadc_buffer_convert(m_buffer_pool[1], SAMPLES_IN_BUFFER);
    APP_ERROR_CHECK(err_code);

}

//=========================================================================
//=========================================================================
int main(void) {
    nrf_drv_timer_config_t timer_cfg = NRF_DRV_TIMER_DEFAULT_CONFIG;
    nrf_drv_timer_init(&MY_TIMER_0, &timer_cfg, &my_timer_handler);
    nrf_drv_timer_extended_compare(&MY_TIMER_0,
                                    NRF_TIMER_CC_CHANNEL0,
                                    nrf_drv_timer_ms_to_ticks(&MY_TIMER_0, 50),
                                    NRF_TIMER_SHORT_COMPARE0_CLEAR_MASK,
                                    true);
    nrf_drv_timer_enable(&MY_TIMER_0);
    
    //
    saadc_init();
    saadc_sampling_event_init();
    saadc_sampling_event_enable(); 
}

Only the first time convert, I can get correct order output  ADC_ch0, ADC_ch1 and ADC_ch2,

otherwise,  I discover the output order is ADC_ch2, ADC_ch0 and ADC_ch1.

How to solve this problem?

[Jørgen Holmefjord]

Hi,

Please have a look at this answer. The same issue will occur if you use breakpoints to halt the CPU, and it can happen if you run calibration, without applying the errata workaround.

Best regards,
Jørgen

[nrf_user]

For posterity, I had a similar issue to yours where SAADC initiated from PPI would sometimes lead to out of order buffers when BLE was on. This was because the PPI frequency was too high, going from 100us to 200us solved the problem