At 11:45 PM 11/27/02 +0300, you wrote:
>Bob Ammerman wrote:
> > Note: Don't expect too much accuracy out of your
> > run-of-the-mill crystal based oscillator. You will get
> > significant drift over time, temperature, and I believe
> > supply voltage.
>
>"...and I believe supply voltage" - Bob, two weeks ago
>I've received few PIC16F871 from local Microchip folks
>(www.microchip.com.ua) . Also received crystals from
>them for these PICs. Crystals were 20 MHz without any
>additional information.
>    If I were Olin, I'd rather just stick them to these PICs. :-)
>Look at his posting:
>"Re: [PIC]: Serial resistor for the oscillator of a PIC16F876"
>on 2002-07-09 :
>    > > The point is that I need to know some suitable value
>    > > for the serial RESISTOR
>    >  I always use 0Ohms.  Where did you get the idea from
>    >  to put a resistor there anyway?  I don't recall ever seeing
>    >  a resistor in any Microchip documentation.
>
>    But I asked local Microchip folks about crystals'
>parameters. They sent me datasheet from witch I've
>found out that crystals were
>
>Jauch Quartz HC49/U4H SS4 0.1 mW max drive level!
>
> From Microchip AN826 Crystal Oscillator Basics:
>Power dissipation of the crystal can be computed by
>
>P = E^2 / R1 where E is rms voltage across the crystal.
>
>R1 is known to be about max 100 Ohm at 20 Mhz.

Use 75% of the maximum R value from the crystal datasheet.

>So aprox max P would be 2v^2 / 2 / 100 Ohm = 20 mW.
>
>Too much for 0.1 mW max drive level crystal.
>
>Of course, "You will get significant drift over time..." :-)
>
>...........................
>
>Mike.
>Correct me if I'm wrong, please.

Right as rain, Mike. That's an upper bound, but the point is that
it's probably WAY over 100 microwatts. When you're whacking a
Stradivarius with a sledge-hammer you can't expect it to stay in tune.

Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com

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