Crystal capacitors

Vinnie

Hi folks,

last night·I tried using a 4mhz crystal on my sx-tech board.. to my surprise the test-setup·appears·to function fine in stand-alone mode when using the OSCXT1 setting and only a crystal·in the resonator socket (no capacitors and no sx-key!).. I just tried it because the capacitors hadn't arrived yet -·not expecting it to work.

is this coïncidence or·normal·behaviour?

if it works, is there a·specific·reason to still add the suggested 15 and 22 pF·capacitors?

greetings,
Vinnie

ps AFAIK the crystal doesn't contain any capacitors.. but no way 2B sure

Beau Schwabe

This is normal behavior, however the main reason this is done is to help guarantee startup and frequency stability of the specified crystal
from one board design to the next. Sometimes there is enough parasitic capacitance within the circuit configuration itself that makes using
additional capacitors negligible. By changing the external load capacitance on the crystal, you can persuade the crystal to oscillate slightly
above or below it's operational frequency. Typically this is why crystals are designed to operate with a specific range of load capacitance.

Suppose your board has a parasitic capacitance of 5pF, and then on a different board design perhaps it's only 2pF. The difference here is 43%
between the two different boards. Now with the same boards place a 18pF on the crystal pins. Effectively these would be placed in parallel
with the existing parasitic capacitance of the board, so with the first board instead of 5pf, now you have 23pF. Likewise with the second board
design, you would have 20pF. Now the difference is only 7% between board designs.

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Beau Schwabe

IC Layout Engineer
Parallax, Inc.

Vinnie

Beau,

Thanks for the explanation.. It makes total sense now..

When the capacitors arrive, I'll incorporate them in the design.

I thought making a micro-PCB with the crystal + capacitors - pincompatible with the resonator - would be neat... I'll probably try that.

greetings,
Vinnie