On Sat, 19 Oct 2002, Robert Rolf wrote:

*>Been there, done that, and found that a RTC chip like the DS1302
*>just makes a LOT more sense, if you can pursue that option. 3 pins (2 of
*>which you can share with other functions if you're clever) and a
*>energy source (MnO, Li or supercap) gets you a hassle free solution.

Me too except I prefer to use a 12C508A as timekeeper (with 32kHz xtal).
It sends 600Baud TTL level RS232 data on a pin (using open drain drive to
avoid loading the supply). The setting is done using two buttons (time
honored shift & set) which can be wired to another cpu is necessary. The
timer runs in packed BCD. The 300Baud tx does most of the constant run
time timing ;-) ;-) Getting the years to work right took some work, but I
do not need years usually.

The tx format is HHMMSSDDMMYYYY in packed BCD (7 characters per second).

Some RTC chips (from Seiko etc) operate like this but use 3 wire format.
They also use 2 set buttons as above.

Peter

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