FreeRTOS Tickless Idle vs Tick Current Consumption

[WestCoastDaz]

We have been comparing the current consumption of the nRF52 running FreeRTOS Tickless Idle versus a constant tick and using the idle task to sleep and the results are interesting, see below.

June 7th Edit: SDK 11, SoftDevice S132 2.0.0.0, Silicon is QFAABB (Rev C)

Tickless Idle:

• Average current is 390uA

Tick with Idle Task:

• Average current is 303uA

Both runs were identical with the floating point interrupt handler implemented as in this post devzone.nordicsemi.com/.../, same number tasks/execution profile, same tick rate, advertising off. The only difference is tick mode uses the idle task hook to put the processor to sleep.

Note the follow images were captured using our own Labview tool sampling at 8kHz. Ignore the bottom trace

Tickless Idle

Tick with Idle Task

[RK]

I also can't figure out what on earth would produce those results.

What's running on the chip, do you have the softdevice loaded and active, is it doing anything at all? The tickless mode should be showing less than the other one, if it has nothing scheduled then it should put itself to sleep for seconds or minutes or basically forever.

Are you sure you've got this test right? Because it really makes no sense at all.

[RK]

I can't think up a scenario which would give those results. OK so the SD is on but not even advertising, which means it generates no interrupts, it's really doing nothing at all. What code do you have running, just a few tasks which run on a timed basis or are there async bits to it, like external interrupts, SPI/TWI/UART data transfers etc? If it's just tasks running on timers, there's nothing to cause the chip to wake from sleep apart from the RTC tick, which runs (in the nordic port) at the RTOS tick rate.

I wondered if the tick idle was being entered with interrupts turned off (BASEPRI set) but if that were the case, the RTC tick wouldn't wake you either, so that can't be it.

I assume in the idle handler you're just calling sd_app_event_wait(). The reason it makes no sense is because the Nordic RTOS port also just calls sd_app_event_wait() in the tickless function ...