topic Pierce Oscillator - Cap values for XTAL in S12 / MagniV Microcontrollers

Pierce Oscillator - Cap values for XTAL

JohnnyP — Wed, 11 Oct 2006 22:04:04 GMT

I'm an HC11 refugee, and am getting ready to test my first 9S12 design.

I'm a little confused about the value of the crystal load caps.

The Abracon crystal spec says 18pF, so I ordered 11pF caps, to take into account the given 7 pF at the EXTAL and XTAL pins.

Is this correct?

Message Edited by JohnnyP on 2006-10-1110:05 AM

Message Edited by JohnnyP on 2006-10-1110:09 AM

(Alban split and retitled post)

Message Edited by Alban on 2006-10-11 04:29 PM

Re: Pierce Oscillator - Cap values for XTAL

bigmac — Wed, 11 Oct 2006 23:53:36 GMT

Hello,

I assume that the specification of 18pF is the shunt capacitance that should appear across the crystal in order to obtain the calibrated operating frequency. For the Pierce oscillator configuration, the shunt capacitance is the series combination of the capacitances to ground at each pin, including the input capacitance at the pins, and also any stray capacitance associated with the PCB traces. Assuming equal value capacitors, and allowing 10pF for input and stray capacitance at each pin, the value of each capacitor would be (18 * 2) - 10 = 26pF, say 27pF or thereabouts.

Any error will simply cause a frequency shift by a few kilohertz at most. So unless you are doing time keeping, this sort of error probably won't matter. In practice, capacitor values of 22pF or 33pF would also likely be satisfactory.

Regards,

Mac

Re: Pierce Oscillator - Cap values for XTAL

JohnnyP — Thu, 12 Oct 2006 01:04:04 GMT

Thanks for explaining that.

Re: Pierce Oscillator - Cap values for XTAL

DanielM — Fri, 13 Oct 2006 22:18:11 GMT

Hi,

adding the capacitances together does not seem to be the right way of calculating the combined capacitance for a series combination...

This PDF seems to give a bit of detail:

http://www.crystek.com/appnotes/PierceGateLoadCap.pdf

Daniel

Re: Pierce Oscillator - Cap values for XTAL

bigmac — Fri, 13 Oct 2006 22:41:28 GMT

Hello Daniel,

My "rough" calculation for the capacitor values required seems to be comparable with the calculations in the application note that you cited - I actually allowed for somewhat greater stray capacitance.

To clarify my calculation -

If C is the required shunt capacitance across the crystal, and equal value capacitors are used, the effective value of each capacitor in series will need to be 2*C. The stray capacitance at each pin is then subtracted to get the actual value of the capacitor.

Regards,

Mac

Re: Pierce Oscillator - Cap values for XTAL

DanielM — Sat, 14 Oct 2006 00:27:05 GMT

now I got it! I am kind of slow today...

Thanks.

Daniel