SAADC acquisition time measurement

[hesquibet]

OS : Ubuntu

SDK: 15.3.0_59ac345

Base example : peripheral/saadc

HW : nRF52832 DK PCA10040 1.2.1 2017.49 682663245

Hello, i would like to make sure the ADC acquisition speed.

As seen in this post, it was recommended to use GPIOTE to toggle a pin on start and end of an sampling to measure precisely the acquisition frequency with a logic analyser.

• I have updated the peripheral/saadc base example to attache two channels on EVENTS_DONE  and TASKS_SAMPLE
   to clear and set the GPIO 20.
• I have changed the acquisition time to  NRF_SAADC_ACQTIME_3US
• I have changed the timer to 100µS.

 

void saadc_sampling_event_init(void)
{
    ret_code_t err_code;

    err_code = nrf_drv_ppi_init();
    APP_ERROR_CHECK(err_code);

    nrf_drv_timer_config_t timer_cfg = NRF_DRV_TIMER_DEFAULT_CONFIG;
    timer_cfg.bit_width = NRF_TIMER_BIT_WIDTH_32;
    err_code = nrf_drv_timer_init(&m_timer, &timer_cfg, timer_handler);
    APP_ERROR_CHECK(err_code);

    /* setup m_timer for compare event every 5µs */
    uint32_t ticks = nrf_drv_timer_us_to_ticks(&m_timer, 100);
    nrf_drv_timer_extended_compare(&m_timer,
                                   NRF_TIMER_CC_CHANNEL0,
                                   ticks,
                                   NRF_TIMER_SHORT_COMPARE0_CLEAR_MASK,
                                   false);
    nrf_drv_timer_enable(&m_timer);

    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_alloc(&m_ppi_channel_saadc);
    APP_ERROR_CHECK(err_code);

    err_code = nrf_drv_ppi_channel_alloc(&m_ppi_channel_saadc_start);
    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);

    // err_code = nrf_drv_ppi_channel_fork_assign(m_ppi_channel, nrf_drv_gpiote_out_task_addr_get(LED2));
    err_code = nrf_drv_ppi_channel_assign(m_ppi_channel_saadc,
                                          (uint32_t)&NRF_SAADC->EVENTS_DONE,
                                          nrf_drv_gpiote_clr_task_addr_get(LED2));

    APP_ERROR_CHECK(err_code);

    err_code = nrf_drv_ppi_channel_assign(m_ppi_channel_saadc_start,
                                          (uint32_t)&NRF_SAADC->TASKS_SAMPLE,
                                          nrf_drv_gpiote_set_task_addr_get(LED2));

    APP_ERROR_CHECK(err_code);
}

From my understanding, i expect a 3µs toggle every 100µs, but i have a nothing toggleing on the pin.

If now i change the events from :  EVENTS_DONE  and TASKS_SAMPLE to  : EVENTS_DONE  and EVENTS_STARTED i do have a toggeling ( so my PPI channeling is working ) but i still don't have the expected result ( i have 87.5µs every 0.4ms instead of the expected 3µs toggle every 100µs)

Am i understanding something wrong ?

[haakonsh]

The started event is triggered after the saadc driver has prepared the next buffer. 

You should use the sample task and the done event. 

Alternatively, you can connect the done event to the sample task, as well as toggling a gpiote pin with it. This will sample the SAADC as fast as it can.
Remember to set a large buffer.