HP48G Clock Variability

[Giuseppe Donnini]

Hello Jonathan,

The message you are looking for was posted on September 22nd, 1994. Somebody asked whether it is possible to "change the speed of the HP-48SX." Here is Dave Arnett's response:

Faster SX? Don't quote me on this, and don't ask for further
information. All the 48s operate from a 32768 Hz crystal. It is
possible, though unlikely, that damage to that crystal could change its
tuning by small amounts and remain within the functional limit of the
CPU. But we wouldn't expect big changes.

The 2x speed YORKE chip was originally intended for an enhancement
of the S series, but there were some issues with the foundry of controlling
process parameters to achieve good yields on 2x-capable chips. The original
Yorkes were generally, though not exclusively or completely, capable of
running 2x speed at room temperatures. To meet project and contract goals,
some of the very late S series machines were built using Yorkes running at
1X. We originaly put together the Rev H ROM for a 2x S-series machine.
When the 2x yield was in question, we put out Rev J, which is a Yorke chip
running 1x, the only speed at which that batch of Yorkes is fully qualified
to run. So if you have a Rev J machine, you also have Yorke.

Some of the Rev J machines would actually run at 2x speed, if pin
37 were tied to Vdd rather than Gnd. By experience, I have found that
making pin 37 float will also cause 2x operation. However, the Rev J code
will not expect that operation speed, and some operations will be
affected. Also, processor operation on a Rev J unit may be flaky if it
is run at 2x speed, since these chips were not fully qualified for 2x
operation.

We actually considered making the G run at 1X, with the GX at 2X,
until the 2x yield issues were resolved. In fact, the G-series circuit
board is equipped to build either an S-series OR a G-series on the same
PCB. At several locations on the board (for those who are daring enough
to have voided warranties and opened their units), you will see some
solderable jumper pads. A solderable pad is soldered automatically by a
process I won't bother to describe, and it simply consists of two capital
Us interlocked. The two of interest are X1, located between the Yorke
and the serial connector, and X2, located roughly in the center of the board
near the lower middle LCD twist tab. Soldering both of these would
create a short circuit. Soldering X1 only runs York at 1X, and soldering
X2 only runs Yorke at 2X. There are other pads elsewhere on the PCB
which reorganize the internal port controllers and bypass some of the
bank select circuitry to achieve an equivalent SX configuration. Note
that if you have a Rev J S series, you have a completely different PCB,
so this discussion of jumper pads is of no help.

So that's the general rundown of clock speed switching. Note that
I am not speaking in any official context, and certainly not as an HP
representative. HP does not recommend that you modify any of these
parameters or connections, and neither do I. This posting is simply a
personal attempt to provide insight into how the 48 operates, and to provide
some historical and trivial background for entertainment and amusement.

I do not feel obliged to provide further information on this topic,
and I am known to ingnore e-mail from the outside world whenever I feel so
inclined. If you think I've ingnored you, please do not feel slighted.
You are in VERY good company!

Good Day!
Dave Arnett