Power usage with SAADC

RE: Power usage with SAADC

Jason — Tue, 17 Sep 2019 13:17:45 GMT

Perfect this is exactly what I want.

Thank you Thank you Thank you

RE: Power usage with SAADC

Jørgen Holmefjord — Tue, 17 Sep 2019 11:38:55 GMT

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.

RE: Power usage with SAADC

Jason — Tue, 17 Sep 2019 09:45:52 GMT

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?

RE: Power usage with SAADC

Jørgen Holmefjord — Mon, 16 Sep 2019 14:40:13 GMT

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