[riot-notifications] [RIOT-OS/RIOT] kinetis: Implement low power modes (#7897)

Thomas Stilwell notifications at github.com
Fri Jun 21 09:36:58 CEST 2019

> I need to actually know the power consumption on the LED to calculate better the expected values.

I attempted to calculate these current consumption values from the frdm-kw41z schematic, however, as is common for low-power boards, the LEDs are being run at much lower current than the LED datasheets show on the I/V graph. Therefore the practical way to obtain the values is empirically. The calculations below are based on extrapolation of the curve, at exactly the point in the curve where it's known to turn nonlinear, which is a really bad idea.

The schematic gives part number CLV1A-FKB-CJ1M1F1BB7R4S3 for LED 1-3 and `generic red LED` for LED 0, with R=220 ohm for all 4 LEDs. I assumed VDD is 3.30 V.

LED0 = (3.30 V - 1.8 V) / 220 ohm = 6.8 mA
LED1 = (3.30 V - 1.8 V) / 220 ohm = 6.8 mA
LED2 = (3.30 V - 2.8 V) / 220 ohm = 2.3 mA
LED3 = (3.30 V - 2.8 V) / 220 ohm = 2.3 mA

But these values would be best obtained by measuring the voltage across each 220 ohm resistor. Then you can get a nice precise value of `v / 220 ohm = x.xx mA`. However it is still the case that the LEDs consume much more current than a sleeping microcontroller, so it isn't really practical to measure sleep currents while any LEDs are on.

> However, I have the impression the current value is not really affected in modes 1 to 3.

That is probably explained by a combination of LED current as well as possibly having the debug circuitry active as I mentioned in the previous post. Kinetis (in hardware) will translate a PM_VLPS request to a PM_STOP request whenever the debug circuitry is active, however it does not interfere with a PM_LLS request (it just powers off the debug hardware until `mode > LLS`). Additionally, the debug circuitry adds about 4 mA any time it's powered.

> kernel panic

That's just what happens when you unblock a mode that wasn't blocked. There's an assertion to check for that: https://github.com/RIOT-OS/RIOT/blob/968e53b74f51779530f331c41e46d81d0f09c59e/sys/pm_layered/pm.c#L83

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