HP98035 Real Time clock and AC5954N clock chip

On Friday (06/07/2019 at 11:01AM -0700), CuriousMarc via cctalk wrote: FWIW, there was an article in Kilobaud magazine, perhaps 1977 or '78 that described connecting a TI LED digital watch to the SS-30 bus in the SWTPC 6800. I built this then, wrecking the TI watch as a watch but it made an excellent RTC for this machine. Two AA size NiCADs were used to power the watch and charged through a simple trickle charge when the machine was powered up. The interface to the host was through a 6820 PIA, with just the segment lines and a couple of output bits. The output bits indeed "pressed" the buttons on the watch and the code would set it just like a human would by stepping through each digit in sequence and "pressing" the other button to increment that digit. When reading the clock, the code would pretend to set the watch but never increment any digit. It would just press the button to advance to the next digit, stepping through all of the digit positions until it had read them all (both time and date) and then return this buffered result. By pretending to set the clock but not actually changing it, it allowed the code to know which digit was being displayed and therefore it did not need to have the digit select lines brought into the host. Chris -- Chris Elmquist