Resonators

Mike Huselton

I have many 3-legged resonators in my parts bin. I know crystals are superior, but what is the schematic for using a resonator - 4mhz, 5mhz, 6mhz, 8mhz & 20mhz values? Crystals are good - resonators bad?

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JMH

Phil Pilgrim (PhiPi)

You can use a resonator. You shouldn't expect to produce any color composite video output if you do, though. The frequency just won't be accurate enough.

-Phil

Mike Huselton

I know about ceramic resonator accuracy. Can you tell me what to do with the center pin of the resonator? I think that it represents a T-bridge capacitor network - center tap in the middle, ~47pf on each outside leg. Can I just ignore the center tap and use the Resonator as a crystal?
I would prefer someone with actual experience, not guesswork. Phil, have you physically used a resonator?

I am grateful for all of the support in this forum - really, just excellent group maturity and professionalism

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Post Edited (James Michael Huselton) : 2/24/2009 4:21:38 AM GMT

mctrivia

it has been a while since I used them. Crystals are not expensive and much more acurate. i am pretty sure you ground the center pin and apply the other 2 to xo and xi

Mike Huselton

This is not a matter of cost. I just want to know if anyone has actually used a ceramic resonator. I have 5.0 mhz crystals on hand - I just want someone with actual experience to reply. My guesswork is as good or better than yours

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JMH

Post Edited (James Michael Huselton) : 2/24/2009 4:18:15 AM GMT

mctrivia

what i do know for sure is it will not damage the prop to wire it up as i said. try it.

Mike Huselton

Yes - I'll try it.

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Phil Pilgrim (PhiPi)

JMH,

I've used many a resonator with PICs and SXes but, until a few minutes ago, not with the Propeller. Just to double-check that I wasn't feeding you a bunch of blather, I plugged a resonator into my Prop Demo Board and grounded the center pin. I then loaded a color video demo. The program ran fine, but the video was not in color. This happens when the frequency is not accurate enough to provide a correct chroma clock and color burst.

I also tried it without the center pin grounded, and the Prop did not run at all.

Now are you satisfied?

-Phil

Mike Huselton

Phil,

Thanks for graciously testing this out. I just now duplicated your results. This issue can be put to bed now .

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Beau Schwabe

James Michael Huselton,

Phils' resonator connection is correct, the center terminal goes to GND. You might also want to pay attention to the "XTAL" settings in the Propeller software depending on the frequency and load capacitance value of the resonator that you are using as these could make a difference with some resonators.

From page 29 in the Propeller Manual:

XTAL1 - 36pF load capacitance - 4 to 16 MHz Resonator
XTAL2 - 26pF load capacitance - 8 to 32 MHz Resonator
XTAL3 - 16pF load capacitance - 20 to 60 MHz Resonator

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Beau Schwabe

IC Layout Engineer
Parallax, Inc.

Phil Pilgrim (PhiPi)

One additional note: "GND" in this case is "AC ground", which can be any constant, low-impedance voltage point between, and including, Vss and Vdd. In some PIC and SX chips, for example, the crystal/resonator pins are on the same side of the chip as Vdd, which is opposite Vss. Depending on the PCB layout, making a direct connection from a resonator's center pin to Vdd is often easier than finding an equally direct path to Vss. But it works just as well. This is because, compared to the resonator's 20pF or so internal caps, a local 0.1uF bypass cap from Vdd to Vss looks like a dead short at oscillator frequencies.

Bottom line: Use Vss if you can make a short, direct connection; otherwise, Vdd should work just as well.

-Phil

Post Edited (Phil Pilgrim (PhiPi)) : 2/25/2009 6:32:39 AM GMT

Mike Huselton

I stand chagrined and corrected.

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JMH