Clocking MCU

[martindusek]

Hello,

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

[john_suchyta]

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.

[hectorsanchez · Accepted solution]

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

[dave408]

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!

[Jorge_Gonzalez]

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

[dave408]

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.

[martindusek]

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

[hectorsanchez]

No,

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