I was just having crystal problems myself and did a mini-research on
the subject...

On 16 Feb 99 at 11:28, Barry King wrote:
<snip>
> Also, AN588 has a (very technical) discussion of the electronics
> behind crystal and oscillator specs.

Personally I didn't like this app note.  It seemed to dodge around
the questions I needed answered (start-up times and drive levels).

A very good reference on microprocessor oscillators is Intel
AP-155 (you can get it on their web site).
Fox electronics (www.foxonline.com) also have some good app notes on
their site.
Also various data sheets and app notes for National Semiconductor RTC
chips contain some information on component selection for real time
clock oscillators.

> The rule-of-thumb value for 8 MHz is 15 to 30 pF.  This is a
> starting point that works fine for the majority of situations.

As someone else has said, the load capacitance has to match the one
specified by the crystal manufacturer and is the series combination
of the capacitance on both sides of the crystal (and in parallel with
stray capacitance).  At 8Mhz the exact values are not critical.

<snip>
> Vdd.  And like drive level to the crystal, especially if the drive
> level spec of your crystal is low, like a strip-cut crystal for 8
> MHz.  Overdriving a crystal can cause some intermittant problems and
> or rapid aging and destruction of the crystal.

Usually crystals specified as "Microprocessor Crystals" will have
a drive level spec that can be used without a series resistor but I
think emissions and stability would benefit from a series resistor -
possibly around 400 ohm for 8Mhz  (never used one myself).

Just fried some 32.768Khz crystals myself. (EPSON ones, recommended
drive level 0.1uW. yes microwatt).  I have a feeling the method
suggested by Microchip (testing signal level at CLKOUT) does not do
the job properly.  I did find a real time clock chip with a similar
oscillator that has a built in series resistor of 200K for 32.768Khz
crystals. (NSC DP8570)

<snip>

Another practice recommended by various sources is guarding the
oscillator circuit (surrounding the oscillator, caps and pins with a
ground conductor).

Hope this helps,
Guy - mlsirton@inter.net.il