Kinetis Microcontrollers中的主题 Re: K22 crystal tolerance calculation

K22 crystal tolerance calculation

huijiejiang — Tue, 28 Nov 2017 01:15:55 GMT

We are using the MK22DX256VMC5, with external crystal + internal PLL as shown below. Our question is how to select the tolerance of the crystal?

for example, we select the 8MHz external crystal, whose freq tolerance is 100ppm, and the main clock is 48MHz using PLL. When we use UART(115200Hz, 1 start, 1 stop, 8 data, no parity, totally 10 bits), how previous mentioned 8MHz crystal 100ppm tolerance will affect the UART communication?

Is the crystal tolerance OK?

How to calculate the tolerance at the UART viewpoint? ( tolerance of crystal -> OSCSEL -> PLL -> PLLS? -> MCGOUTCLK? -> OUTDIV? -> System clock? -> UART error)

Re: K22 crystal tolerance calculation

Robin_Shen — Wed, 29 Nov 2017 08:27:11 GMT

Hi Huijie,

When you use 115200Hz buadrate error is only 0.006%.

Within the 1.5% maximum error UART operation should be OK.

8MHz crystal 100ppm tolerance will not affect the UART communication.

Best Regards,

Robin

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Re: K22 crystal tolerance calculation

huijiejiang — Mon, 04 Dec 2017 00:19:28 GMT

Thank you, Robin!