I have a device where the crystal and associated network is external to
the CPU. Which isn't working. Looking across the two connections with a
scope there's no approx 30 kHz as should be - but what looks like very
dirty mains hum. Any way of easily testing the crystal other than by
substitution?

--
*Why is the man who invests all your money called a broker? *

Dave Plowman London SW
To e-mail, change noise into sound.

"Dave Plowman (News)" wrote:

I have a device where the crystal and associated network is external to
the CPU. Which isn't working. Looking across the two connections with a
scope there's no approx 30 kHz as should be - but what looks like very
dirty mains hum. Any way of easily testing the crystal other than by
substitution?


You need a special probe to keep from loading down the circuit, and
if it is a typical 32.768 kHz watch crystal the waveform isn't clean
even when it is working properly. It is a tuning fork type of crystal,
not the typical cut used at higher frequencies. A high input impedance
FET amplifier with a fraction of a pF coupling capacitor is typically
used to prevent loading, but you can often see a waveform at the drive
side of the crystal. They use an unbuffered inverter and a resistor to
bias it into the linear range, but the AC voltage in the circuit pushes
it out of the linear range.


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"Dave Plowman (News)" wrote in message
...
I have a device where the crystal and associated network is external to
the CPU. Which isn't working. Looking across the two connections with a
scope there's no approx 30 kHz as should be - but what looks like very
dirty mains hum. Any way of easily testing the crystal other than by
substitution?

--
*Why is the man who invests all your money called a broker? *

Dave Plowman London SW
To e-mail, change noise into sound.


The easiest test that I can think of is to breadboard a CMOS oscillator circuit.
Use one inverter out of a hex inverter like this:

View in a fixed pitch font such as Courier

|\
+--| 0---+---- OUT
| |/ |
| |
+--\/\/\--+
| 1 Mohm |
| |
| \
| / 2.7 kohms
| \
| /
| |
| Xtal |
+---|[]|--+
_|_ _|_
55pf ___ ___ 60pf
_|_ _|_
\ / \ /

If the circuit oscillates at the crystal frequency, then the crystal is likely
good. If it doesn't oscillate at all, or oscillates at the wrong frequency,
then it's likely bad.

--
Dave M
MasonDG44 at comcast dot net (Just substitute the appropriate characters in the
address)

Life is like a roll of toilet paper; the closer it gets to the end, the faster
it goes.

Plowman (News) wrote in message
...
I have a device where the crystal and associated network is external to
the CPU. Which isn't working. Looking across the two connections with a
scope there's no approx 30 kHz as should be - but what looks like very
dirty mains hum. Any way of easily testing the crystal other than by
substitution?

--
*Why is the man who invests all your money called a broker? *

Dave Plowman London SW
To e-mail, change noise into sound.


Testing crystals,ceramic resonators and filters
Signal generator (RF) and scope. Connect the grounds together
and then connect the sig gen to the scope via the crystal.
Sweep the oscillator and resonant response at fundamental
and overtones will show on scope trace. Use a f counter
as well if f is required. If testing while
in circuit (unpowered) use less then 1V sig gen o/p .
The response may be muted comnpared to free component
especially for ceramic resonators.

from my tips files below

--
Diverse Devices, Southampton, England
electronic hints and repair briefs , schematics/manuals list on
http://home.graffiti.net/diverse:graffiti.net/

DaveM (MasonDG44 at comcast dot net) wrote:
The easiest test that I can think of is to breadboard a CMOS oscillator circuit.
Use one inverter out of a hex inverter like this:

View in a fixed pitch font such as Courier

|\
+--| 0---+---- OUT
| |/ |
| |
+--\/\/\--+
| 1 Mohm |
| |
| \
| / 2.7 kohms
| \
| /
| |
| Xtal |
+---|[]|--+
_|_ _|_
55pf ___ ___ 60pf
_|_ _|_
\ / \ /

If the circuit oscillates at the crystal frequency,
then the crystal is likely good.
If it doesn't oscillate at all, or oscillates at the wrong frequency,
then it's likely bad.

....and the device number for the gate
needs to have a "U" in it (for "unbuffered").