Re: How to use a crystal


"Peter S. May" <psmay@xxxxxxxxxxxx> wrote in message
news:bKKdnaWMJPj97QLbnZ2dnUVZ_sGdnZ2d@xxxxxxxxxxxxxx
I've never used a timing crystal before--awesome, right?--so I'm trying
to decode some information from the data *** of my micro (PIC 16F88)
and the catalog page for the crystal (11.0592MHz, 32pF impedance) and
find that I don't know enough about the analog circuit to reconcile them.

The PIC *** says that this is how to hook a crystal up to the PIC:

C1, ca. 15pF in . (inside PIC)
+----||----------+---------> >----+------+
| | . \ |
GND |--+ [=] XTAL . / Rf V inverter
| | out . \ o
+----||----------+--/\/\---< <----+------+
C2, ca. 15pF Rs .

The same ***, however, is very careful not to actually commit to
anything, saying the the values for C1 and C2 may not be optimal, and
that Rs may or may not be necessary, but with no real guideline to
decide its value if it is. It defers to the specifications of the
crystal itself. The crystal's *** contains no sample schematics, but
the catalog page does...but that looks like this:

XTAL C3, 32pF (match impedance of crystal)
+----[=]------||------+
| |
+--/\/\---------------+
| 2.2K |
| |
+----------+----->o---+---->o----> output
| | inv. | inv.
= C1, \ = C2,
| 150- / 4.7K | 220-820pF
| 470pF \ |
| | |
+----------+----------+---| GND

That's easy enough for me to understand if I'm trying to clock some
device with just a plain CLK pin, but I'm not sure how it translates to
the PIC's two oscillator pins, which seem to be intended to factor out a
few parts. (And even this schematic gives no guidance how to choose C1
or C2, but then they probably assume that buyers of crystals already
know what to do with them.)

Anyway, I could use some guidance: I have (or will be buying) a crystal
with 32pF impedance, and I want to hook it up to a PIC. How do I do it?

Thanks
PSM


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