HP calcs with clock

[Christoph Giesselink]

Quote:What about 28C/S and 42S? Do they keep counting in the OFF mode?

In addition to Katie's reply some more information.

Yes, all three models (HP-28C, HP-28S and HP-42S) have a timer running with 8192Hz, even if the calculator is switched off. Because there's no possibilty to access this timer value on the HP42S only clock programs for the HP-28 are available.


Years ago I wrote two articles in the HP Articles Forum

275: HP28S: Hardware I/O basics
313: HP28C: Hardware I/O basics

and under section "Special considerations to master/slave operation" you can find some information about these timers.

The hardware timer is 32 bit wide and running @8192Hz, so the timer could hold values for (2^32)/8192Hz = 524288s = ~6d. That's of course too short for a clock with absolute time value. So the hardware timer is expanded by a 12 nibble (12*4 = 48bit) RAM area. To update the RAM content the interrupt capabilities of the timers are used. So the maximum timer time without an interrupt is reduced from 524288s to 524288s / 2 = 262144s = ~3d. So after maximum 3 days the CPU is awaked from sleep state (without turning display on) udpate the clock RAM, preset the timer for new interrupt and shutdown CPU again.

So only when you remove the batteries or put the calculator into "deep sleep" (aka. coma / storage) mode, the clock timer will stop. It's not recommended to put a HP-28 calculator into deep sleep mode, because of a hardware design bug the current drain is higher than in "off" mode.

At http://users.skynet.be/luc.pauwels/luc/H...clock.html is a clock program for the HP-28 series.