HP48G Clock Variability

[Giuseppe Donnini]

(02-10-2021 10:56 PM)Jonathan Busby Wrote:  NOTE #1 : The above reads like one only has to change the connections or other electrical properties of special pads on the Yorke SoC to "make" an S/SX or a G/GX, but that is not what I meant -- Instead, one only has to change the electrical properties ( eg. connections ) of the Yorke SoC's pads in order to transform the Yorke SoC into, what was for all intents and purposes, a Clarke-compatible SoC for an S/SX, *but* that only works for the SoC and not the rest of the calculator. The main PCB would still be completely different than the corresponding G/GX model, but, one would not need a Clarke SoC as the Yorke can be switched, by means already discussed, to a fully Clarke compatible mode that only runs at around 2MHz.

Thanks for pointing that out, Jonathan.

In view of the initial yield problems, it is immediately apparent why HP had to provide the Yorke chip with a dual speed option for the projected enhancement of the S series, but it may be less obvious why they continued to do so for the G series.

There are actually three reasons, as explained by Dave Arnett in another message sent to comp.sys.hp48 on October 15th, 1996 (Google Groups Link):

1. We were not certain whether there would be a demand for the 48SX after
the G and GX were introduced. I have jumpers on the board which would
allow you to build an HP48SX on this circuit board. That task requires
rerouting the card detect and select lines and (optionally) running the
CPU at 1X speed rather than 2X speed.

2. There were problems with the yield of Yorke ICs from NEC which would
run at 2X speed at all temperatures. We had a contingency plan that would
have made all HP48G units run at 1X speed until the problem was resolved.
The jumpers allowed us this option, had it been necessary. Thankfully,
the yield improved and we didn't have to exercice the option.

3. The old RAMBOX development systems used by the software team would
only support 1X operation. The ability to select 1X speed when runnig
code from the RAMBOXes, and then switch to 2X speed when running from the
EPROM was a design requirement. I saw no reason to delete this feature on
the production board. My EPROM unit and Charlie Patton's unit had a
gearshift for switching speed on the fly without a soldering iron.