At 12:08 PM 7/14/2005 -0400, you wrote:
>Spehro Pefhany wrote:
>>The capacitors
>>are effectively in series
>
>No, this is a common Crystal Myth.

Not a myth, but so common that it's in all the technical literature.

>If the crystal is driven with a 0 impedence sine wave on one side, the
>specified load capacitance is that value to ground on the other side that
>will cause the desired phase shift at the rated frequency.

Sure, but that's nothing that remotely resembles the real situation.

>Therefore, if
>the PIC oscillator output pin has a small impedence compared to the load
>capacitance, then the capacitor on the oscillator output pin doesn't enter
>into consideration.  In that case, the best oscillator input pin capacitor
>value would be the rated load capacitance minus the inherent existing
>capacitance due to the leads, circuit board, etc.
>
>If the PIC oscillator output had a very high impedence, then both capacitors
>need to be considered in series as you said.

The output acts like a *current* source. This is where you go astray. The
crystal is acting like an inductor in a Pierce oscillator, and resonating
with CL (the series of the capacitors, plus in/out/stray capacitance), which
is why the frequency is always a bit higher than the series resonant
frequency of the crystal.

The only hard thing is that the output capacitance of the current source
and the input capacitance need to be accounted for, so really
a few pF (5 maybe) should be added to each capacitor before the
usual CL = Cin*Cout/(Cin+Cout) + Cstray calculation.

More to the point for Mark, the drive power of the crystal can be approximated
by this forumula:

Pdrive ~= Rs * 0.5 * (2*pi*f * CL * Vdd)^2  (CL is the actual load
capacitance as calculated above using 5pF for input and output
capacitance and 3pF for stray)

So, reducing the supply voltage, using lower value capacitors
(resulting in a small shift in frequency), or using a lower Rs crystal
will all reduce the power dissipation of a given frequency of crystal.

BTW, running those numbers with the *maximum* Rs of Mark's crystal (taken
from the datasheet) and a calculated load capacitance with 33pF caps
(calculated CL = 22pF) yields an estimated maximum drive power of 3.8mW, well
above the datasheet 1mW maximum rating of the crystal. That's the problem,
Euros to donuts.

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
->> Inexpensive test equipment & parts http://search.ebay.com/_W0QQsassZspeff


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