Subject: CP290 Clock Mod
Newsgroups: comp.home.automation
Distribution: world

[last modified: 15-May-95]

The latest version of this file can be accessed my web page at:
http://www.rdrop.com/users/billmc

CP290 Clock Mod
Bill McFadden
billmc@rdrop.com

Here is a way to increase the the accuracy of the CP290's clock at least
tenfold.  Instead of drifting 30 seconds per week, my CP-290 now drifts
less than half a second per month!

The CP-290 is clocked by a two-transistor crystal oscillator running at
32.768 kHz.  The power consumption of this oscillator is very low, which
allows the CP-290 to run on battery backup for long periods.  The
oscillator is used to run the clock whether or not it is connected to AC
power, i.e., the power line frequency is not used.

To open the CP-290, unplug it and remove the battery.  Next, remove the
four screws holding the case together, one in the battery compartment and
three under the rubber feet in the remaining corners.  Remove the battery
compartment and the screw near the power transformer that holds the main
circuit board in place.  When removing the main circuit board, watch out
for the ribbon cable.  It's pretty stiff, and the wires might break if you
flex it too much.

To improve the accuracy of the clock, I removed the original 100 ppm watch
crystal and replaced it with a 20 ppm part (Digi-Key sells several).  Next,
I removed 33 pF capacitor C17 and replaced it with a 15 pF fixed capacitor
in parallel with a 5-15 pF variable capacitor.  I used NP0 ceramic
capacitors for maximum temperature stability.

The oscillator circuit is located along the right side of the circuit
board, about halfway between the ribbon cable and the battery compartment.
The crystal is a small cylinder about 1 cm long and a few mm in diameter,
and is covered by a blob of silicone to keep it in place.  After installing
the new parts, I removed the solder flux with alcohol so the stray
capacitance of the flux wouldn't affect operation.

To trim the oscillator, I used a frequency counter with 0.1 ppm accuracy.
The clock signal can be probed at the end of R35 nearest to the battery
compartment, or at wire jumper L21 near tuned transformer TC3, or on pin 39
of the 80C48 microcontroller IC1.

People who don't have access to a good frequency counter/standard will have
to trim the oscillator the old fashioned way: by trial and error.  If this
is the case, you might want to program the CP-290 to turn off a
non-existent module once an hour.  The LED on the front panel will blink
each time the CP-290 transmits, which will allow you to determine how
quickly the clock is drifting.

When adjusting the frequency, keep the CP-290 at the same temperature at
which it will normally be operated, and allow the circuitry to warm up for
at least 10 minutes first.  It doesn't matter whether you run it on battery
power or AC because the oscillator's supply voltage is regulated.  I
recommend using battery power for safety.

You may have to use different capacitor values depending on how far and in
what direction the frequency is off.  Lower values of C17 make the
oscillator run faster, and higher values make it run slower.  If you make
C17 too small, the oscillator won't run at all.  You can tell whether the
oscillator is running because the front panel LED will blink every few
seconds when the CP-290 is on battery power.  If it doesn't blink, the
oscillator isn't running.  Also, if you try to adjust the frequency too
far, the oscillator will become unstable and drift with temperature.  This
happened to me on the original crystal, which is why I replaced it with a
better one.

Disclaimer: If you break your CP-290 while working on it, don't come crying
to me!

Good luck.