# how to select capacitor for a crystal oscillator

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

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:

Here is the link to 32 MHz Crystal Oscillator chapter:

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

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?

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.