topic Re: LPTMR keep time and generate hardware trigger in VLPS in Kinetis Microcontrollers

LPTMR keep time and generate hardware trigger in VLPS

ppd — Wed, 30 Aug 2017 14:47:02 GMT

Hi,

I want to generate a hardware trigger every 10ms or so from LPTMR to TSI while the MCU is in VLPS. But I also want to know for how long (measured in microseconds) I've been in VLPS once I get out of it. Can I do both of these things at the same time with the LPTMR? If not, is there any other way of achieving this? I'm using a KL05.

Thanks,

Paul.

Re: LPTMR keep time and generate hardware trigger in VLPS

mjbcswitzerland — Wed, 30 Aug 2017 22:09:41 GMT

Paul

I would use the RTC (you can also run it from the same clock as the LPTMR) but then has no restrictions, such as the amount of time that it can measure.

http://www.utasker.com/kinetis/KL_RTC.html

Regards

Mark

Re: LPTMR keep time and generate hardware trigger in VLPS

ppd — Thu, 31 Aug 2017 04:54:48 GMT

Hi Mark,

Unfortunately the RTC increments every second (if external an external 32.768kHz clock is used) or every 32 seconds (if the internal LPO clock is used). Not good enough. I need the time in at least multiple of tens of milliseconds.

I'm afraid I'm going to have to use TPM with external clock source (for lowest power). Unfortunately this means adding an external crystal (which I don't have right now and the PCB is already very crowded) and accepting an additional ~30uA in current consumption to keep the TPM running in VLPS.

Anyone having a better idea?

Thanks!

Re: LPTMR keep time and generate hardware trigger in VLPS

mjbcswitzerland — Thu, 31 Aug 2017 11:13:12 GMT

Paul

The RTC has a pre-scaler register called RTC_TPR which counts at the clock rate. This allows 1ms resolution from LPO or 30us resolution with external 32kHz.

I have crossed out the 32kHz clock since it actually increments at the "selected" clock rate.

Beware that the LPO is not very accurate so there may be several % of drift.

Regards

Mark

Re: LPTMR keep time and generate hardware trigger in VLPS

ppd — Thu, 31 Aug 2017 14:57:57 GMT

Ah, I overlooked the prescaler register.... Well then this is just what I want. I'm not looking for accuracy so this is just perfect. Thank very much for your help!