Crystal problems/questions

From: Brett (bsa22_at_cornell.edu)
Date: 08/07/04 

Date: 7 Aug 2004 06:21:42 GMT

Hi All,

I'm working on building a small prototype for a PCB we would like to
have built. It is based on an ST7262 uP, and my first step has been to
put this processor on a breadboard, hook up some of the basics like Vdd,
reset, etc, and then attach the crystal. As far as I can tell on my
first attempt, the crystal is not resonating, at least when I use an
oscilloscope to probe it. I have a few ideas about what could be wrong,
but nothing I'm really sure about. I would appreciate it if anyone
could check over what I'm doing and let me know if it makes sense. My
circuit looks like this:

     ------------OSCIN
     | |
    --- |
33pf--- ----
     | xtal
    GND ----
     | |
    --- |
33pf--- |
     | |
     ------------OSCOUT

I'm using a 12Mhz Citizen can type crystal with an 18pF load capacitance
and an 80 Ohm ESR. First off, since I have seen no indication anywhere
that the pins on the crystal mean anything I'm assuming it is
symmetrical.

The ST7262 data*** has a little table listing capacitor values based
on varying ESR values of potential crystals. The closest entry is for
70 Ohms ESR, and it recommends 22pF capacitors.

So, does the crystal's load capacitance of 18pF mean I should use those
for the capacitors, or the 22pF recommended in the uP data***?

You'll probably notice I'm using 33pF in my schematic above...this is
the closest we had in the lab. Will a value this far off prevent the
crystal from resonating at all? I'm thinking about going to Radio Shack
and getting a capacitor kit that includes the 18pF capacitors I think I
should be using, but I'm not sure this is the problem yet. I could also
try combining 33pF's to get 16.5pF which is pretty close to 18...

Does a mistake in this kind of circuit just cause the crystal to not
work, or will it ruin it permanently?

Finally, I did read while googling that the extra capacitance introduced
by an o-scope probe could cause the crystal to stop...I've probed the
crystal on an evaluation board sucessfully, but it uses a different
crystal so I'm not sure this is the problem. I just found that I can
enable a clock output on one of the uP pins so I may update the firmware
and try probing that pin instead.

To sum up...
1) It doesn't matter which crystal pin is attached to which uP input,
correct?
2) is the load capacitance value of the crystal referring to the same
thing as the recommended capacitor values in my uP data ***?
3) is a 33pF capacitor a problem when an 18 or 22 is specified, or is
this "close enough"?
4) If it is a problem, will I need a new crystal or should it be ok?
5) How much of a problem will putting an o-scope on one of the crystal
pins be?

Thanks for any help you can provide!

Brett

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