topic Re: Clocking MCU in Kinetis Microcontrollers

Clocking MCU

martindusek — Wed, 25 Sep 2013 10:32:53 GMT

Hello,

can I use RTC 32 khz oscillator as an input for FLL and in the same time main oscillator for PLL (MK20DN512VLL10)?

Re: Clocking MCU

hectorsanchez — Wed, 25 Sep 2013 15:25:53 GMT

Not exactly , but you can use the Multipurpose Clock Generator (MCG) as an external reference clock for the PLL and the RTC

You can see this in the chapter 25 of the attached document.

In the same chapter you can find this:

External clock from the Real Time Counter (RTC):

• Can only be used as a source for the FLL.

If you need the features of the RTC Oscillator you can find in the chapter 27 of the attached document.

You can find this document in the Freescale Official Page here:

http://cache.freescale.com/files/32bit/doc/ref_manual/K20P100M100SF2V2RM.pdf?fpsp=1

Re: Clocking MCU

martindusek — Wed, 25 Sep 2013 16:51:12 GMT

So, the answer to my question is NO, am I right?

Re: Clocking MCU

hectorsanchez — Thu, 26 Sep 2013 13:42:56 GMT

No,

You can use the MCG to have the same clock speed on them instead.

Re: Clocking MCU

john_suchyta — Fri, 13 Dec 2013 21:56:16 GMT

No. The PLL requires a higher frequency reference than the 32.768kHz. The datasheet shows the PLL reference frequency range to be 2MHz to 4MHz. This is selected by using the high range setting of the main system oscillator (OSC0) and dividing that by the PRDIV0 value in the MCG_C5 register.

Re: Clocking MCU

dave408 — Sun, 21 Jun 2015 17:19:53 GMT

hectorsanchez I am trying to understand the different clock configuration, i.e. what is possible with each type of clock source. I'm looking at the datasheet that you have posted to understand what info there supports your statement that the 32.768 RTC clock cannot be used as the clock source for the PLL. I don't see anything in Chapter 27 that says this... furthermore, when I look at the clock diagram, it looks like it can feed into the PLL. Can you please explain what I am missing?

Thanks!

Re: Clocking MCU

Jorge_Gonzalez — Sun, 21 Jun 2015 20:17:09 GMT

Hi Dave:

The info is in Chapter 25 as stated by Hector:

Colleague John Suchyta gives the explanation of this, please refer to his response below about PLL reference range.

It is true that the block diagram may imply it is possible, but that's just the design, so the System Oscillator can reach both, PLL and FLL, also to select between System Oscillator or RTC for FLL.

Regards!

Jorge Gonzalez

Re: Clocking MCU

dave408 — Mon, 22 Jun 2015 05:17:42 GMT

Thanks, sorry I saw Chapter 27 and completely missed Chapter 25. It's very clear there. Ok, I'll keep that in mind and keep plugging away at this.