First computer with real-time clock? - test-drb@ccmp.vtda.org

29 Jul 2004

...
   This is
basically a "digital clock" chip with BCD outputs (intended to
 drive BCD to 7-segment decoders) on an S-100 board. The fact that it
 is a chip intended for a clock display gives rise to certain odd
 characteristics: 
The chip used in the HP 98035 (RTC module for the 9825) is even worse. 
It's got 7 segment outputs... 
Egad! - that is definately "worse".

...
   - To set the
time, the software has to "hold down" Fast and Slow time
   set buttons, and watch the time value scroll by until the desired
   setting is reached - just the way a human would set a digital clock
   from that era. 
YEs, that's done in the 98035 too. In fact it's just like a digital watch 
with one button to select hours/minutes, and a second press for secondes, 
and one button for setting (or something like that). Quite fun to watch 
the signals when it's 'writing' the time to the battery-backed clock chip.

This was the older fast/slow set style - the kind that used to drive me
craze in clocks - you hold down "fast" and the digits count like crazy
(minites and hours), you switch to "slow" when you get close. It was really
annoying if you overshot, because you had to "go round" again.

When I first started writing the software to set the clock, I assumed that
the computer would be plenty fast to simply watch the time in "fast" mode
and stop "right on the button" - I quickly found out that the display
refresh in the clock chip didn't keep up with the "fast" counter, and
sometimes (rather often) it would "miss" a digit, going for example
directly from '1' to '3' --- So I had to implement detection when it
got "close" and switching to "slow".

Regards,
-- 
dave04a (at)    Dave Dunfield
dunfield (dot)  Firmware development services & tools: www.dunfield.com
com             Vintage computing equipment collector.
                http://www.parse.com/~ddunfield/museum/index.html