NRF52 real sampling rate different

[Jong yoon lee]

Hello, I was wondering why I am not getting a theoretical sampling rate as I set them to with a timer.

I am using PPI to trigger a SAADC sampling with the timer to set to 8ms. However, I am not getting a sampling rate with 125Hz. My Connection intervals for BT are Min = 20 and Max=30. Also, I am using 2 buffers to ensure that sampling happens even if other higher priority events than saadc_callback

   uint32_t ticks = nrf_drv_timer_ms_to_ticks(&m_timer, 8);
    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_event_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_event_addr);
    APP_ERROR_CHECK(err_code);

I am wondering if there are any solutions for the inconsistency of the sampling rate or a way I can calculate the inconsistency and optimize for it. My project highly depends on constant sampling rate and having no blind spots for sampling.

Thank you, John

[Øyvind Karlsen]

Hi,

Since you are using 8ms sample intervals the HFCLK based TIMER is not recommended as it adds a lot of current consumption. Instead you can use the LFCLK.

The sample rate is relatively slow, so you should not need to be worried about being interrupted by SoftDevice activity. To ensure that you do not collide with the SoftDevice you can read the buffers from a lower priority interrupt when the application has time.

Instead of using the SAADC directly have you tried to set a GPIO using the task and measuring the frequency, if so, what did you get?

Best regards,

Øyvind

[Øyvind Karlsen]

Well, running a lot of things at the same time means there are more things that can potentially go wrong. However I think this should be fairly simple to get running. By the way, it may be worth looking into the Application timer tutorial if you will use the LFCLK.

The SoftDevice uses the HFCLK at two times, when the radio is running and when it has to calibrate the LFRC (Low frequency RC clock source, if you do not have a separate LF crystal). The HFCLK adds a lot of current consumption when running, and should therefore be left off when not in use.