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K22FN128 FLL reference clock questions

Question asked by Isao Takashima on Dec 21, 2015
Latest reply on Jan 3, 2016 by Isao Takashima

We will use the MK22FN128 MCU with 20 MHz external crystal, and set core clock to 100 MHz in FEE mode.

I believe that we can set core clock to 100 MHz as below.

- FLL reference clock = 39.0625KHz (= 20 MHz / 512)

- FLL DCO output clock = 39.0625KHz * 2560 = 100MHz.


As you know, the maximum FLL reference frequency range is 39.0625 KHz.

Can I use crystal which 50 ppm frequency tolerance?

Because frequency of crystal is 20 MHz +/- 1 KHz and FLL reference clock is 39.0625 KHz +/- 1.95 Hz, some case exceed 39.0625 KHz.

Then I want to make sure that I can use 50 ppm crystal or not.


Please reply as soon as possible.


Best regards,