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'crystal drive level question'
1995\11\13@200647 by David G. Schmidt

How do I know if I am going to overdrive a crystal?  I am planning on
using a 16c54xt with a AT cut 4mhz crystal.  The ones I am getting list a
drive level of 10uW (fundamental) and 50uW(3rd).  Looking at the data
sheet on the PIC16C54, I can't find the section that lists how hard the
'54 drives crystals or what size series resistor to use in series to
limit power to a desired level.

Thanks in advance!

( Dave Schmidt       DSchmidt Technologies )
(     Ventura, CA        )

1995\11\23@023007 by Dan Matthews

picon face
Hi Dave,

The best way to check your oscillator for over/under drive is to look at the
osc out pin (OSC2) with a scope. This is the "driven" pin, as opposed to osc
in (OSC1) which is a high impedance input. Connecting to the input with the
load of the scope probe can dampen or kill the oscillator.

Anyway, the signal should be a nice clean sine wave that goes below 0.3 X
Vdd (1.5Vdc @ 5V Vdd) and above 0.7 X Vdd (3.5Vdc @ 5V Vdd) for the
PIC16C5X. If the clock is hitting the rails and squashing at the top or
bottom, you are overdriving the crystal.  Remember to take the capacitance
of your scope probe into account when deciding your final capacitor values.

One last note: Look at the oscillator output at the lowest temperature
combined with the highest Vdd your circuit will see. These two test
parameters assure the highest gain for the oscillator circuit. Conversley,
check the output while at the lowest Vdd and highest temp to make sure your
circuit will meet the minimum input requirements in the lowest gain

See ya,

       Dan Matthews

At 05:05 PM 11/13/95 -0800, you wrote:
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