Re: Crystal circuits

Just to follow up, I did a search for a document that can explain the detail
better than me, here it is:
http://www.saronix.com/Document_Files/AppNote-BasicOscillatorDesign-0305.pdf

The Pierce type is more common (at least until many recent extra low power MCU's)
and is more immune to noise, drives a higher frequency and does consume
a bit more power than Colpitts.

The Colpitts type is usually used where very low oscillator power consumption
is needed (standby small battery powered devices) and is less noise
immune and more sensitive to oscillator component values, and can't
drive as high a frequency as Pierce.

Your MCU data book should have some details on example crystal hookup and
parts selection.
Some MCU's actually have software selection between Pierce and Colpitts modes,
but I don't think yours does (what MCU is is again?)

Cheers,

Marc

----- Original Message -----
From: David Shoemaker
To: vectorlist@vectorlist.org
Sent: Tuesday, October 21, 2003 3:43 PM
Subject: VECTOR: Crystal circuits

While building up the two beta 9100 keyboard wedge boards I realized I didn't have any 22pf caps. So I tossed a pair of 100pf caps
onto my breadboard of the circuit and everything worked great.

Then I went to build the perfboard version and realized I only had two of the 100pf caps and as I wanted to leave the breadboard
version intact for now so I grabbed a pair of .1uf caps.

Hooked everything up and it didn't work.

So I checked everything twice more and still no go. Got to wondering if maybe the value of those caps is really important and
realized I have NO idea how to spec them nor even what they do.

So can anyone enlighten me?

I have seen circuits use 18pf, 22pf and 27pf for the crystal caps. The 100pfs do work, but I wonder now if I just got lucky.

David

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Received on Tue Oct 21 02:31:18 2003