microcontroller - how to select capacitor for a crystal oscillator

Thursday, 27 December 2018

Here is a 32 MHz crystal from one of TI's reference design

enter image description here

The BOM list defines the crystal as

" CRYSTAL, OSCILATOR, 32MHz, 10pF, 10PPM/+10PPM, ‐40DEGC/+85DEGC, SMD"

It is for a MCU (ARM M3) and the datasheet has this sub-chapter:

enter image description here

Here is the link to 32 MHz Crystal Oscillator chapter:

enter image description here

When I use the above CL formula:

CL = 1 / (1/12pF + 1/12pF) + Cp = 6 pF + Cp

Another reference design of TI that uses a chip from the same family has the below crystal enter image description here

all the other definations are the same as above formulas and notes. The BOM defines the crystal as:

Crystal, 32.000MHz, NX3225DA, 10/15ppm, 16 pF ! Temp range -30 - +85 deg C !

CL = 1 / (1/27pF + 1/27pF ) + Cp = 13.5pF + Cp

I could not really understand how C341 and C351 values are chosen, could anyone please explain it?

And How do you assume Cp's value?

Answer

Cp is the input capacitance plus stray capacitance. You can use a few pF (3-5pF) for the value unless something is really strange.

So, for a crystal rated with a 10pF load, Cl = (10pF-Cp)\$\cdot\$2, so if we use 4pF for Cp, we get 12pF for the load capacitors.

For the 16pF crystal, Cl = (16-Cp)\$\cdot\$2, so using 4pF for Cp we get 24pF, using 3pF we get 26pF.

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