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 hate that comments are limited) ... and the tickless function is called from the tick idle, just with the tick turned off and a compare interrupt turned on for a future time. So what you're doing sleeping in the idle handler is a superset of what tickless idle is doing, assuming you're even getting into tickless idle mode.

The bottom trace, are those spikes at your RTC tick frequency, it's tempting to assume they are, but perhaps they are only every 10 or 20 ticks. How often are you doing work, every tick? Less than 2-3 ticks of idle, you're not actually getting into sleep mode.

I've just spent bits of this week instrumenting things with SystemViewer, not only Nordic's FreeRTOS port but my own and also the softdevice itself. Would be interesting to apply that analysis, although having the debugger attached may mess up the results.