xtal capacitance question

Don M

I've used a 10Mhz crystal like this on a project- http://www.mouser.com/ProductDetail/TXC-Corporation/7A-10000MAAJ-T/?qs=sGAEpiMZZMsBj6bBr9Q9aWUu9Vmt5DeoQ8wKllyl4wA%3d

It's rated at 18pf. They are out of stock and I need some this week. Can I use one that is 12pf?

Mike Green

It depends. The frequency that a crystal oscillates at is partially dependent on the capacitance in parallel with it. The different crystal settings of the Propeller are intended to provide specific parallel capacitance values for typical crystals. XTAL1 provides 36pF. XTAL2 provides 26pF. XTAL3 provides 16pF. I think these are from XIN and XOUT to ground, so the capacitance in parallel with the crystal is half that. These don't include wiring capacitance which is pretty small if the crystal is close to the Propeller. If the crystal is designed for a different capacitance, the frequency will be off. Use the XTAL2 clock setting in your project which will provide a pretty close capacitance to what the crystal is made for. If an exact frequency isn't that important, you could use XTAL1 as well.

Don M

Thanks Mike