SoftDevice preventing low-power state

RE: SoftDevice preventing low-power state

rnelson — Tue, 26 Jun 2018 15:54:09 GMT

It turns out our main issue was Anomaly 87 (FPU exception preventing low power), but I also implemented Austin's suggestion since I was sometimes seeing the SoftDevice bounce out of sd_app_evt_wait() immediately.

RE: SoftDevice preventing low-power state

Bjørn Kvaale — Tue, 22 May 2018 12:23:11 GMT

You are of course correct. I read through the code a bit too fast & did not notice the else block.

RE: SoftDevice preventing low-power state

rnelson — Fri, 18 May 2018 13:46:40 GMT

Thanks Austin! That sounds very likely to be what I'm seeing as well. I'll give your code a try as soon as I'm able to.

RE: SoftDevice preventing low-power state

Austin — Thu, 17 May 2018 23:19:17 GMT

I also saw this behaviour with nRF52840 and S140 where sd_app_evt_wait() would return immediately on the first call when entering tickless idle. I proposed a different method for entry/exit to idle in this post. The code attached to that post will stay in an inner loop around sd_app_evt_wait() until there is a runnable task or the idle time has expired. From memory, the profiling counters in the inner loop show that sd_app_evt_wait() is almost always executed at least twice when entering idle, showing that the first sd_app_evt_wait() drops through. There are some current consumption numbers listed which show the difference between normal tickless and my proposed changes.

RE: SoftDevice preventing low-power state

rnelson — Wed, 16 May 2018 20:28:57 GMT

Bjorn - thanks for the quick response. I'm looking at nrf_pwr_mgmt.c in SDK 14.2.0, and it does NOT appear that _WFE() is called after sd_app_evt_wait(). Relevant code snippet:

// Wait for an event.
#ifdef SOFTDEVICE_PRESENT
if (nrf_sdh_is_enabled())
{
ret_code_t ret_code = sd_app_evt_wait();
ASSERT((ret_code == NRF_SUCCESS) || (ret_code == NRF_ERROR_SOFTDEVICE_NOT_ENABLED));
UNUSED_VARIABLE(ret_code);
}
else
#endif // SOFTDEVICE_PRESENT
{
// Wait for an event.
__WFE();
// Clear the internal event register.
__SEV();
__WFE();
}

Note the "else" before the block containing _WFE().

Any other suggestions?

Thanks,
--Russ

RE: SoftDevice preventing low-power state

Bjørn Kvaale — Wed, 16 May 2018 11:54:14 GMT

In most SDK examples, the call to nrf_pwr_mgmt_run() inside the main for or while loop calls the sd_app_evt_wait() function & then afterwards calls the _WFE() function, which puts the CPU into a low power state. Take a look at the blinky central example inside the sdk/ble_central folder for example. You should not have to disable the softdevice in order to get power savings.