# Calculated crystal capacitance is out of range

I know, another question about crystal capacitor values.

I would like to use this crystal in the 20 MHz version to clock an ATmega1284.

According to the crystal's datasheet, it has a load capacitance of 20 pF.

Using the formula $C = 2(C_L - C_S)$ and assuming a stray capacitance of 2 pF or 5 pF, I get a value for each capacitor of 36 pF or 30 pF, respectively.

However, this is outside the recommended range for the capacitors of 12-22 pF according to page 46 of the ATmega1284 datasheet.

So, can I even use this crystal, and if so, with what capacitors?

## 2 Answers

It will probably work with 22pF capacitors, however the frequency will be a bit off (high). Maybe 50ppm or 75ppm, not enough to worry about the maximum frequency of the MCU.

Better to buy a part that is calibrated with a lower load capacitance.

• What would happen if I used 36 pF? Jan 11, 2018 at 2:06
• Might have trouble starting when cold or hot, things like that. Jan 11, 2018 at 2:07
• If I were to use either 22 or 36 pF, which would you recommend? (This is for a hobbyist project. Not frying my chips is my main concern.) Jan 11, 2018 at 2:10
• Suggest 22pF... Jan 11, 2018 at 2:11
• As Microchip says: Care should be used in selecting values of (load capacitors). Large values increase frequency stability but decrease the loop gain and may cause oscillator start-up problems. Jan 11, 2018 at 3:20

They are implying you should choose a part with low load capacitance and not 20pF. This also helps reduce power consumption.

A=Series
E=10pF
G=12pF
U=13pF
L=18pF
M=20pF
N=22pF
Q=30pF
R=32pF
S=33pF