How to predict how much CPU time the SAADC needs on the nRF52832?

RE: How to predict how much CPU time the SAADC needs on the nRF52832?

Susheel Nuguru — Thu, 07 Feb 2019 13:40:45 GMT

most probably you are using the burst mode then. I am not sure if the oversampling rate is linearly proportional to the time it takes to finish the full samples. But it definitely seems to take more. Here is the text from the PS

The accumulator is controlled in the OVERSAMPLE register. The SAMPLE task must be set 2OVERSAMPLE number of times before the result is written to RAM. This can be achieved by:

Configuring a fixed sampling rate using the local timer or a general purpose timer and PPI to trigger a SAMPLE task
Triggering SAMPLE 2OVERSAMPLE times from software
Enabling BURST mode
CH[n].CONFIG.BURST can be enabled to avoid setting SAMPLE task 2OVERSAMPLE times. With BURST = 1 the ADC will sample the input 2OVERSAMPLE times as fast as it can (actual timing: <(tACQ+tCONV)×2OVERSAMPLE). Thus, for the user it will just appear like the conversion took a bit longer time, but other than that, it is similar to one-shot mode.

If you have enabled Burst mode, then it seems that taking more time for the whole sample to finish seems reasonable.

RE: How to predict how much CPU time the SAADC needs on the nRF52832?

heehaw — Wed, 06 Feb 2019 04:11:17 GMT

In the main loop of the SAADC sample, I've added a toggle of GPIO 24:

while (1)
{
nrf_pwr_mgmt_run();
NRF_LOG_FLUSH();

nrf_gpio_pin_toggle(24);
}

In saadc_sampling_event_init() of the SAADC sample, I've set the PPI timer to 1ms:

/* setup m_timer for compare event every 1ms */
uint32_t ticks = nrf_drv_timer_ms_to_ticks(&m_timer, 1);

With this setup, GPIO 24 in the main loop toggles every 1 ms. If I set oversampling from OFF to 2x, GPIO 24 toggles every 2ms. If I set oversampling to 4x, GPIO 24 toggles every 4 ms. As I understand it, the SAADC sample is non-blocking, so I would expect GPIO 24 to toggle as fast as possible, and not be directly related to the SAADC configuration.

RE: How to predict how much CPU time the SAADC needs on the nRF52832?

Susheel Nuguru — Mon, 04 Feb 2019 10:49:59 GMT

I am not sure how you are toggling the GPIOs but SAADC is a peripheral with its own EasyDMA, the CPU usage should be only to configure the peripheral and process any callbacks. It would give us a better view if we see exactly where you are toggling the GPIOs