# Crystal Capacitor Formula (Parallel Mode)

Discussion in 'Electronic Basics' started by Costas Vlachos, Jul 3, 2003.

1. ### Costas VlachosGuest

Hi all,

I'm looking for the formula to calculate the value of the two capacitors C1
and C2 used in crystal oscillators for microcontrollers (you know, the ones
you put between the Xtal terminals and GND). I've found this on the net:

Crystal Capacitor Formula (Parallel Mode)
-----------------------------------------
Let: CL = Crystal's load capacitance
CP = Circuit's parasitic capacitance
CI = Chip's input capacitance
C1 = Crystal oscillator capacitor 1
C2 = Crystal oscillator capacitor 2

Then: C1 = C2 = 2 * CL - (CP + CI)

Example: For a crystal with 12pF load capacitance (CL)
and assuming CP + CI = 5pF (typical), we have
C1 = C2 = 2 * 12pF - 5pF = 19pF. We can then
use two 18pF capacitors (closest standard
value).

Is this correct? Can someone please confirm?
Many thanks.

Costas

2. ### Spehro PefhanyGuest

Yes.

Best regards,
Spehro Pefhany

3. ### Larry HatchGuest

On a side note. There is a fellow on the internet (Ricardo Richi, or
something like that). He shows how to make pf caps with wirewrap. I
have done about 10 PIC and AVR projects, with a xtal and two little
twisted wires to run it. And no errors in the uP running ever. One is
on a motorcycle that does 150MPH wheelies and viberates a lot. Only
a cell phone too close have cause a problem, and I do not believe it is
in the wire caps, but the lack of caps and the long wire from the back
on the bike to the front.

Anyway, I though the wire twisting was interesting, now to have a good
meter to tune them. Any ideas on that? Good way to measure 33pf, etc caps.

4. ### Michael A. TerrellGuest

The trick to measuring low capacitance is to build a small insulated
test jig that plugs directly into your meter to minimize stray
capacitance, and to keep the value stable. Pomona, Keystone, and other
companies make banana plugs with either a threaded stud, or threaded
hole for a 6-32 screw. A small piece of PC board with a v shaped
section of the copper clad scored and peeled off gives you a good
surface to check surface mount capacitors, or leaded parts. You can
tack solder gimmicks or leaded caps to the board for critical
measurements, or use a cheap, self wiping IC socket cut in half to let
you plug leaded parts in for test. Just remember that the sockets wear
quickly, and you need to make sure you can replace them easily.

Yes, but that is a pretty cheap method for test equipment.