SAADC - Real sample rate is a bit higher than the configured sample rate

RE: SAADC - Real sample rate is a bit higher than the configured sample rate

Michael01101 — Fri, 09 Oct 2020 06:31:37 GMT

Thank you very much for your reply.

I will keep that in mind.

RE: SAADC - Real sample rate is a bit higher than the configured sample rate

Kenneth — Thu, 08 Oct 2020 15:16:18 GMT

You can call hfclk request from anywhere, before starting timer is preferred though. If you want low power you should use rtc (running from lfclk) instead of timer (running from hfclk), but then that will affect the accuracy.

RE: SAADC - Real sample rate is a bit higher than the configured sample rate

Michael01101 — Thu, 08 Oct 2020 06:41:09 GMT

Hello Kenneth,

thank you very much for your response.

I added this request into my saadc_sampling_event_init(void) function:

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_config = NRF_DRV_TIMER_DEFAULT_CONFIG;
    timer_config.frequency = NRF_TIMER_FREQ_125kHz;                                                             
    err_code = nrf_drv_timer_init(&m_timer, &timer_config, timer_handler);
    APP_ERROR_CHECK(err_code);

    nrf_drv_clock_hfclk_request(NULL);

    /* setup m_timer for compare event */
    uint32_t ticks = nrf_drv_timer_ms_to_ticks(&m_timer,SAADC_SAMPLE_RATE);
    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);
    
    ...
    
}

It looks like, this fixed my problem. Thank you for that. It seems that the HFCLK consumes about 150µA.

Do you recommend to call nrf_drv_clock_hfclk_request(NULL) in the saadc_sampling_event_init(void) function or somewhere else?

Thank you in advance.

Michael

RE: SAADC - Real sample rate is a bit higher than the configured sample rate

Kenneth — Thu, 08 Oct 2020 06:07:05 GMT

Can you try to start the HFCLK here by nrf_drv_clock_hfclk_request(NULL). The internal HFCLK is not accurate (typical 1-2% tolerance).

Kenneth

RE: SAADC - Real sample rate is a bit higher than the configured sample rate

Michael01101 — Wed, 07 Oct 2020 13:23:52 GMT

Hello,

I was able to fix the problem for my nrf52832 on my development board. It works just fine there.

But on my custom board with the nrf52832 chip it does not work. Maybe it has something to do with the timer source. On the custom board, the SoftDevice uses NRF_CLOCK_LF_SRC_RC instead of NRF_CLOCK_LF_SRC_XTAL.

Here are my questions:

1. Is this a problem of the timer source?

2. Which timer does the SAADC use by default?

Thank you so much in advance.

RE: SAADC - Real sample rate is a bit higher than the configured sample rate

Michael01101 — Wed, 07 Oct 2020 11:27:23 GMT

Hello Turbo J,

thank you very much for your fast reply.

It looks like you fixed my problem.

I wish you a great day. :)

RE: SAADC - Real sample rate is a bit higher than the configured sample rate

Turbo J — Wed, 07 Oct 2020 11:15:14 GMT

Relevant code:

timer_config.frequency = NRF_TIMER_FREQ_31250Hz;

That frequency has no available integer divider in order to achieve exactly 1 kHz.

Just use NRF_TIMER_FREQ_125kHz or higher.