[time-nuts] Timebase Replacement - Seiko Epson RTC-72421

[Chris Caudle]

On Thu, October 3, 2019 12:00 am, Corey Sukalich wrote:
> My question pertains to a timebase used in a Schulmerich carillon bell
> tower system from the 1990’s.
>
> The Seiko Epson RTC-72421 Real Time Clock Module (4-bit) is used, but the
> clock ends up walking to a noticeable degree (minutes) over a period of
> months.  Is this expected for the device in question, or could it possibly
> have a defect?

The datasheet you linked shows 5ppm max aging in the first year, but no
aging curves after the first year.
Using the worst case value just for a rough estimate, mid-90's would mean
some devices could be 25 years old:
5ppm * 25 yrs -> 125ppm off nominal
Note that  nominal starts at anywhere from +/-10 to +/- 50 ppm depending
on what model  you have, and aging could be in the same direction as
initial offset, opposite direction as  initial offset, or could change
direction at some point during the device lifetime.

Given all those caveats, 125/1,000,000 * 30 days/month * 24hrs/day *
60min/hour -> 5.4 min/month

So if the clocks are losing or gaining anything less than 5 minutes per
month, then I would say they are probably beating the worst case value
guaranteed in the datasheet.

> From what I can tell it uses an internal oscillator, so it sounds like I
> would need to find a drop-in replacement or build a module that will
> replace it with a significantly better time standard.

Or build an interface which will push in the correct time at some time
that is unlikely to be noticed (e.g. 2:42AM,  some time not right on an
hour or  half hour).

> Also, on the off chance someone is versed in EPROM data recovery or knows
> of a resource for it, I would appreciate that information as well.

There are a lot of devices which can read data from an  EEPROM, that
aspect will be nearly trivial, the trouble is usually interpreting the raw
data, or filling in any data which has degraded over time and can no
longer be read.

-- 
Chris Caudle