Daniel -

Short of fully characterizing a crystal that works against a
crystal that doesn't work in a crystal text fixture using
appropriate test equipment -

Does a change to the parallel cap values have any effect
in getting a bad crystal to work?

Can you change just one of the parallel caps and get
the crystal to work?

By now you should have a pile of 'bad' parts - when a
difference that 'cures' the problem is found - try several
different bad xtals. I have used small machine pin sockets
with xtals successfully in the past to 'socket' xtals.

These are the kind of 'general' things I would try in order
to 'see' what works then go from there ...

Jim P


----- Original Message -----
From: D. Schouten <daniels@XS4ALL.NL>
To: <PICLIST@MITVMA.MIT.EDU>
Sent: Friday, June 09, 2000 7:29 AM
Subject: [PIC]: XTAL startup issues on PIC 73B


Hi All,

In one of my applications I'm using a PIC16C73B 20MHz part
running at 18,432MHz. The xtal is connected the standard way
i.e. with two 33pF caps (loadcapacitance spec of xtal is 32pF)
to ground and layout wise everything as close as possible to
the PIC chip.

The problem is that from the 1000 boards I test, about 25 aren't
running at all. Replacing the xtal usually solves this problem.
Sometimes the board works well in the pretesting phase, but fails
at the final test. Current xtal brand is TXC, but we've had similar
problems with other brands too.

My question is, is this 2.5% failure rate normal, or is something
wrong with my application?

Is the PIC16C73B sensitive in HS mode, or do I have to add small
series resistance with the xtal, or large parallel resistance?

The application which has his problem is quite price sensitive, so
there isn't any room for varicaps or other tweaking stuff.

Thanks for any help.

Daniel...