Power usage with SAADC

[Jason]

Testing with the NRF52840 dongle and the Power Profiler Kit. I see a reading of ~5uA when sleeping without SAADC enabled. 

I use this code to enable SAADC 

void saadc_init(void)
{
    ret_code_t err_code;
    nrf_saadc_channel_config_t saadc_channel_config = NRF_DRV_SAADC_DEFAULT_CHANNEL_CONFIG_SE(NRF_SAADC_INPUT_AIN0);

    nrf_drv_saadc_config_t nrf_drv_saadc_config = NRF_DRV_SAADC_DEFAULT_CONFIG;
     
    err_code = nrf_drv_saadc_init(&nrf_drv_saadc_config, saadc_callback);
    APP_ERROR_CHECK(err_code);

    err_code = nrf_drv_saadc_channel_init(0, &saadc_channel_config);
    APP_ERROR_CHECK(err_code);

    err_code = nrf_drv_saadc_buffer_convert(m_saadc_samples, SAADC_SAMPLES_IN_BUFFER);
    APP_ERROR_CHECK(err_code);
}

My power usage when sleeping is now 913uA

My final product will sleep for 2 minutes, wake by timer, read the SAADC value, advertising the value, sleeping for another 2 minutes. repeat.

My expectation is that I should uninit the SAADC before I call nrf_pwr_mgmt_run(). And then call saadc_init() when my product wakes. I'm not sure how to uninit the SAADC. 

[Jørgen Holmefjord]

Hi,

Please have a look at this GitHub example, which does exactly this. There is also an untested branch with SDK 15.2.0 support.

Best regards,
Jørgen

[Jason]

Fantastic, thank you.

It turns out that all I needed to do was set SAADC_CONFIG_LP_MODE to true in my sdk_config.h.

I also didn't realise I ought to periodically re-calibrate the SAADC

I'm a little confused about what SAADC_CONFIG_LP_MODE does though. I'm only doing single samples without oversampling, burst mode etc. I just call nrf_drv_saadc_sample() from a timer created with app_timer_create(). and yet I see the power drop from 900uA to just over 4uA just from setting SAADC_CONFIG_LP_MODE. I'm not sure why it's not on by default.

This discussion seems to suggest that SAADC_CONFIG_LP_MODE ought not to do anything in this situation.

https://devzone.nordicsemi.com/f/nordic-q-a/32885/meaning-of-saadc_config_lp_mode-misleading-comment-in-nrf5_sdk_14-2-0_17b948a-config-sdk_config-h/127454#127454

Is there any downside to using SAADC_CONFIG_LP_MODE?

[Jørgen Holmefjord]

You are correct, the only change required is to set SAADC_CONFIG_LP_MODE. The difference between the low-power mode and normal mode is that in low-power mode the START task is not triggered until the sample function is called, while in normal mode, START task is triggered when the buffer_convert function is called to setup the buffers. The SAADC peripheral support EasyDMA, allowing sampling of channels directly to RAM using PPI, without CPU intervention. When low-power mode is enabled, the CPU is required to trigger sampling, and it will only work with a buffer size of 1 sample. In case of high sample rate, there is little or no benefit from the low-power mode.

The low-power mode is implemented exactly for the scenario that you describe.

[Jason]

Perfect this is exactly what I want.

Thank you Thank you Thank you