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LLS mode on K20 processor

Question asked by Chris Paynter on Mar 5, 2013
Latest reply on Mar 6, 2013 by Martin Kojtal

I am using a MK20DX256VLL7 on a board of our own design, and trying to operate it in LLS mode with MQX.  I have created a custom BSP based on the TWRK20DN72M board support package, and I see that it has LLS mode mapped to LPM_OPERATION_MODE_STOP.   I have copied the relevant code from the low power demo for that tower board, and if functions fine in LLS mode, and I have managed to modify the init_cm.c file to allow wakeup from either the LPO or an external pin transition. 

The problem I have is that the processor still draws about 1.5 mA when in stop mode.  The external oscillator is still running off a 10 MHz crystal, and I do see a small reduction in current if I clear the EREFSTEN bit just before the processor goes to sleep, but then it doesn't wake up again.  I think in a general sense I must have to make some OS calls to set up a different clock setup before going to sleep, but am a bit vague on the mechanics of how that works. I also think that there must be some other sources of power consumption that I need to address, which I'm continuing to experiment with.

Are there any general tips for optimizing low power operation on the Kinetis parts (apart from AN4503 and AN4447), and tracking down where the consumption is coming from?  From reading the specs, I would expect that I should be able to get down in the tens of microamps in LLS mode, and still be able to wake on the LPO or an external input transition.