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'Crystal Tutorial; RC+crystal combination idea'
1998\10\22@123308 by Morgan Olsson

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At 10:28 1998-10-21 -0500, you wrote:

>Generally, the only RC oscillator setups I've seen with micros
>have had a very loose (+/- 20% or so) frequency accuracy.  I
>would think, though, that RC oscillators should be able to get
>within the 2% or so needed for serial communications

Precise caps are more expensive than 3-pin cer resonators, and for the
latter you can skip the Resistor.
And still you have problem vith the temperature drift of the cap.
Also the trig level of the oscillator in the chip is not very exact.

>...while offering instant wake-from-sleep behavior, etc.

If we have an app that need to start immediately, and then have precise
(Example: wake on change and then serial transmit or whatever)

I feel it would be possible to somehow add a crystal or resonator to a
RC-cirquit, in a way that on wake-up it immediately starts up as a RC
oscillator, then the crystal takes over.

The RC is to oscillate in a freq close to crystal resonance, so the crystal
quickly starts to resonate, lowering its impednce, and thus connecting
itself into the cirquit.

Maybe just use a parallel resonant crystal parallel to the RC cap?

If the xtal is designed for two 30pF caps (one each side to GND), then the
parallel cap should be 15pF.

Measure the capacitance of xtal+cap, and calculate R for RC oscillator
to hit crystal frequency with that capacitance.

What do you think?

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