Hi,

On 16 Mar 99 at 23:36, Dave VanHorn wrote:
<snip>
> First, is it a paralell resonant crystal?  It must be, or you will
> never get to sing the right tune, if it sings at all.

This isn't exactly correct.  If the crystal is specified for a serial
oscillator it should run fine in a parallel one but slightly off
frequency (typically about 0.02% above its specified frequency).

> Second, what loading capacitance value does the datasheet on the
> crystal say to use?  I don't know how uC could pull those c values out
> of the air like that. The loading cap values are determined by the
> CRYSTAL, not the chip..  In digiKey, a paralell rock is one that has a
> loading C specified, if there is no C speced, it's series mode.

Microchip gives recommended typical values.  (I'm guessing uC stands
for Microchip 'cause it could also be micro-computer,
micro-controller or something else...)

> Assuming you have a paralell rock:
> Take the load C that is speced, subtract about 5pF for parasitics and
> chip capacitance, and try that value.

1/CL = 1/C1 + 1/C2

CL = Load C, C1 and C2 loading caps on both sides of the crystal.


The 20Mhz crystal should have run nicely with *all* the cap values
you described.  I would look for the problem somewhere else:
1. Bad connections/solder joints.  Is this on some kind of a
breadboard?  Check all GND and VCC connections with an ohm-meter.
 2. "Dirty" power supply? Decouple with a high value
(10uF-100uF) and a low value (0.1-0.01uF) with the low value as close
to the IC gnd/vcc as possible.
3. Place crystal and caps as close as physically possible to the IC.
4. Make sure you satisfy the start-up requirements in the datasheet
(Vcc rise time, reset etc.)

I don't think you need one but a series resistor on the order of 100R
from the oscillator output pin would keep the drive current a little
bit lower.

Guy