topic Re: Help Understanding Clock Rate of K66 in Kinetis Microcontrollers

Help Understanding Clock Rate of K66

C0490FDB — Sun, 23 Jan 2022 17:28:48 GMT

Trying to understand how the microcontroller is being clocked, sorry, the datasheet isn't clear to me. Right now I'm setting the LPUART baud rate by measuring bit duration on my scope but would like to document the precise calculations to derive the register settings.

I am working with a MK66FN2M0VLQ18 on a RDDRONE-FMUK66 running PX4. It appears that during initialization the PLL is selected and I estimate the CPU is clocking at about 100 MHz, but again this is measured on a scope.

The schematic shows a 32KHz crystal on board but perhaps it's using the internal IRC48M 48 MHz source?

Thanks!

Re: Help Understanding Clock Rate of K66

C0490FDB — Sun, 23 Jan 2022 18:43:28 GMT

I set the peripheral clock source to the internal 48 MHz source and the baud rate settings make sense now. I'm still unclear how to derive the processor speed using the PLL or not.

Re: Help Understanding Clock Rate of K66

jingpan — Mon, 24 Jan 2022 07:41:26 GMT

Hi @C0490FDB ,

Please see this picture. If input internal or external 32k osc clock, MCG can use FLL to boost up to 96M clock.

Regards,

Jing

Re: Help Understanding Clock Rate of K66

C0490FDB — Mon, 24 Jan 2022 17:44:11 GMT

Thank you, this diagram is very helpful.

Re: Help Understanding Clock Rate of K66

mjbcswitzerland — Mon, 24 Jan 2022 18:17:21 GMT

See this practical guide too:
https://www.utasker.com/kinetis/MCG.html

Regards

Mark

Re: Help Understanding Clock Rate of K66

C0490FDB — Tue, 25 Jan 2022 01:54:46 GMT

That looks helpful as well, thanks!