Major battery life improvement

[rbaron]

It turns out the SoftDevice lets us put our application in deep sleep while it keeps advertising.

In the current firmware, we have this piece of code:

  prst_adv_start();
  nrf_delay_ms(PRST_BLE_ADV_TIME_IN_MS);
  prst_adv_stop();
  symbolic_go_to_deep_sleep_for(PRST_DEEP_SLEEP_IN_SECONDS);

We can change it into something like this:

prst_adv_start();
symbolic_go_to_deep_sleep_for(PRST_BLE_ADV_TIME_IN_MS);
prst_adv_stop();
symbolic_go_to_deep_sleep_for(PRST_DEEP_SLEEP_IN_SECONDS);

Impact in power consumption

To see the difference, I set:

#define PRST_DEEP_SLEEP_IN_SECONDS 3
#define PRST_BLE_ADV_TIME_IN_MS 1000

Before:

After:

This trick brought the average current while awake from 7.48 mA to 1.63 mA. This drop has a meaningful impact on the oveal battery life.

In both cases the full deep sleep (no app + no advertising), hovers around 3.24 uA:

If we plug these new values in the battery estimation spreadsheet, for a cycle of 10 minutes:

Before: After:

These numbers are optimistic, but the relative increase is very real. I am looking forward to putting this to test in the real world.

[JoelWise]

Very cool! Has this been completely implemented in sleep-adv branch yet? The code still seems to just delay during advertise rather than deep sleep.

prst_adv_start(); nrf_delay_ms(PRST_BLE_ADV_TIME_IN_MS); prst_adv_stop();

[rbaron]

@JoelWise yes, it's implemented in full there. The old delay has been removed - you can check the main.c in that branch.

It's a little different from what I illustrated in the issue here. The firmware wakes up from deep sleep after a timer. So when it wakes up we just need to check whether it's waking up from full deep sleep (SLEEPING state) or just from the "advertising sleep" (ADVERTISING state):

• If it wakes from the SLEEPING state, it runs the application, starts advertising and go into the ADVERTISING sleep state for PRST_BLE_ADV_TIME_IN_S (in other words: configures the timer to wake the system up after PRST_BLE_ADV_TIME_IN_s seconds)
• If waking up from ADVERTISING, it means we finished advertising, so we just stop advertising and go into the SLEEPING full deep sleep for PRST_DEEP_SLEEP_IN_SECONDS (in other words: configures the timer to wake the system up after PRST_DEEP_SLEEP_IN_SECONDS seconds)

[JoelWise]

Got it. I flashed one of my probes and found similar battery life results. My Power Profiler suggests at a default 5 minute update frequency the change improves battery life by 3x!

[rbaron]

I've reflashed all my b-parasites with this code and I'm happy to say they've been working well. I merged #49, but I'll leave this PR open for a while for visibility.