Path: utzoo!utgpu!news-server.csri.toronto.edu!rpi!zaphod.mps.ohio-state.edu!swrinde!cs.utexas.edu!execu!sequoia!uudell!bigtex!texsun!geraldo.Central.Sun.COM!cronkite!exodus!oobleck.Eng.Sun.COM!bender
From: bender@oobleck.Eng.Sun.COM (memory fault - core dumped)
Newsgroups: sci.electronics
Subject: XTAL not oscillating at marked frequency
Keywords: wierd crystal stuff
Message-ID: <15604@exodus.Eng.Sun.COM>
Date: 21 Jun 91 08:00:17 GMT
Sender: news@exodus.Eng.Sun.COM
Organization: Sun Microsystems, Mt. View, Ca.
Lines: 55

I had an interesting experience these past two days.  Last night I was
writing some 8051 code for a project that I'm working on and loading the
code into my 8051 emulator; the code was nothing complex, just a timer setup
to interrupt at what I thought was 2mS intervals, but instead of getting
interrupts every 2mS, I seemed to be getting them around every 6mS or so.
So, I checked my code, made sure that I understood the correct value to
stuff into the timer registers, made sure that I wasn't doing something
silly like flipping a port pin every OTHER interrupt, and all of the other
usual software stuff, but still, I wasn't getting interrupts at the
frequency that I thought I should be getting them.  I then decided to get
down to basics, and just drive a pin high, execute 4 1-cycle instructions,
then drive it low, and jump to the top of the loop.  I reasoned that at my
processor's clock speed of 12 MhZ, the port pin should be high for around
5uS, and low for around 3uS or so, but I was still getting much longer high
and low times.  Back out with the 8051 data book, check the instruction
cycle times, even check my scope (!), but I could find nothing wrong, it
just seemed that the processor was executing much more slowly than it should
have at a 12 MhZ clock!

So, I called the emulator company (American Automation) and talked to a
designer (Joe), and he ran the same code loop on his system and his times
were what I thought mine should have been.  He then suggested checking the
emulator's clock, since I was using the emulator's internal clock rather
than providing my own XTAL/cap combination, and sure enough, the emulator's
clock was running slow; as a matter of fact, the 24 MhZ XTAL that was in the
emulator was oscillating at about 8 MhZ!  I replaced the 24 MhZ XTAL with a
12 MhZ XTAL, and the oscillator fired right up at 12 MhZ, so I'm pretty
confident that the oscillator itself is OK.  Joe said that sometimes crystals
go bad and won't oscillate at their marked harmonic frequency, but rather
oscillate at their fundemental frequency, and this sure seemed to be what
was happening.  I haven't replaced the 24 MhZ XTAL yet, but I did provide my
own 12 MhZ XTAL/caps and got the processor working at the correct speed.

What exactly is the failure mode here?  Do crystals go "bad"?  Here's what
the oscillator circuit looks like:

	+-------------------||-------------------+
	|                 0.1uF                  |
	+--/\/\/\--+                  +--/\/\/\--+
	|    330   |                  |    330   |
	|   |\     |                  |    |\    |
	+---| o----+---+---|[]|-------+----| o---+
	    |/         |  24 MhZ           |/
           LS04        |                  LS04
                       |    |\
                       +----| o--------> 24 MhZ out to divider
                            |/
                           LS04
any ideas?

mike
--
------------------------------------------------------------------------
1985 Honda Shadow VT1100                                   | DOD #000007
1989 Honda NX-650                                          | AMA #511250