HP98035 Real Time clock and AC5954N clock chip

[Paul Berger]

On 2019-06-08 5:40 p.m., Chris Elmquist via cctalk wrote:
> On Friday (06/07/2019 at 11:01AM -0700), CuriousMarc via cctalk wrote:
>>> I wondered if it's actually a digtal watch chip (2.5V could have been a couple
>>> of mercury cells in series, LED watches were not uncommon back then).
>>>
>>> In which case it would not normally have come in a 0.6" wide DIP. Perhaps
>>> normally it was a bare chip directly mounted on the watch circuit board or
>>> something.
>>>
>>> The DIP version would be unusual, which is perhaps why we can't find data
>>> on it.
>>>
>>> -tony
>> I think you are on to something. That would make perfect sense.
> 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

That article "Let Your Computer Wear a Watch" was in the October 1978 
Kilobaud available on archive.org

Paul.