Re: x10 - CM12U (CM11) Clock Accuracy

[Charles Sullivan <cwsulliv@xxxxxxxxxxxx>]

On Tue, 25 Sep 2007 15:09:08 +0100, Martin Davies wrote:

> Hi - I'm looking for some advice about CM12U clock accuracy.
>
> Here's the situation. I've downloaded a simple sequence of timed events to
> the CM12U using Activehome. The computer is then disconnected from the
> interface. To begin with the events work ok but very quickly (a couple of
> days) the timing of events drifts significantly (it appears the internal
> clock in the interface is not very accurate?) When you reconnect the
> interface to Activehome and check the interface time it appears correct -
> maybe a second or twos difference only - this seems strange since the timing
> of events may be out by upto an hour or two.
>
> I've seen elsewhere information about a mod to the clock in a CM11a but this
> doesn't appear to be relevant for the CM12U? There appears to be no 80C48
> chip in the CM12U.
>
> Any help or guidance much appreciated. Thanks...

The CM11/CM12 clock is usually kept roughly in sync with the AC
line frequency, which can fluctuate. In the USA the power companies
tweak the frequency up or down so that AC clocks will be accurate
on the average over the long term.

When disconnected from AC and running under battery power, the
CM11A clocks I've tested tend to lose about 6-8 minutes per day.

When you disconnect the CM12U from the PC, you _must_ disconnect
the cable at the CM12U end.  The circuitry is very susceptible to
stray pickup from a dangling cable which can screw up the works.

Note: If AC power to the CM12U is interrupted even momentarily,
then once power is resumed it sends a request for a clock update
to the PC.  ActiveHome silently complies with this request.

Regards,
Charles Sullivan