View Full Version : Cheap Propeller Crystal ?

11-20-2010, 02:43 PM
Can we use this kind of crystal with Propeller?


They are series resonant, have 18pf load capacitance, and 30ppm nominal.
About the same size as the HC49-USX, but only $.15 each.

Mike Green
11-20-2010, 02:58 PM
They should work, but you'll have accuracy problems. The crystals you've shown are calibrated for an 18pF load capacitance while the Prop's oscillator is designed to provide a higher load capacitance (about 30pF). You could try the XTAL3 setting rather than the XTAL1 setting, but I think that also shifts the drive level, not just the load capacitance. Stick with the recommendations in the Propeller Manual's discussion on the crystal oscillator and _CLKMODE.

11-20-2010, 03:19 PM
Series-resonant crystals don't have load capacitance.

11-20-2010, 10:48 PM
Steve, basically no. IIRC you require a parallel with 18pF. The spec sheet has full requirements.

william chan
11-20-2010, 11:03 PM
Why don't you buy some and try since they are cheap.

I always use 18pf and 20pf crystals on the Prop and they always work for me, even on xtal1 setting.

Sometimes I think 18pf equals to 36pf because some manufacturers on specify one capacitor but Parallax specifies both capacitors.

11-20-2010, 11:24 PM
Thanks guys. What's the difference between that and the attached crystal spec besides the footprint and case?

11-20-2010, 11:48 PM
Hi jazzed.

Them not have BIG differences.
1-one have 18 pF Total and R1=150R-Max
2-one have 20 pF Total and R1=120R-Max

In my opinion I have be tested 1-one to be sure Propeller support it --> But I think it will RUN - maybe as anyone said little of its supposed Frequency BUT not nessesary

Thanks guys. What's the difference between that and the attached crystal spec besides the footprint and case?

11-21-2010, 12:01 AM
Thank you Sapieha.

11-21-2010, 09:59 AM

As Leon mentioned , series resonant crystals don't have load capacitance specifications (CL), so these crystals are suspect to be parallel resonant.

For 18pf CL, using a 36pf capacitor located on either side of the crystal results in an effective capacitance of 18 pf. Bear in mind that the actual circuit being resonated by the oscillator drive circuit is one crystal in series with two 36pf caps (ground---36pf cap---xtal---36pf cap---ground).The XTAL1 setting looks perfect for the crystals you found as it has two 36pf caps internal to the Propeller.