frank@zen.UUCP (Frank Wales) (02/17/88)
[This item is posted on behalf of Wlodek, who can't access Usenet directly;
please direct any replies to him using the addresses at the end,
rather than just typing 'r'. Thanks. -- Frank]
Several comments on the HP-28C/HP-28S, on its clock, and
on the book "Customize Your HP-28" prompt me to send this.
1. As Brett Maraldo wrote, the HP-28 CPU uses a clock signal provided
by an LC oscillator. This runs at about 640kHz. Real-time operations,
such as printer control and the time-out after 10 minutes use a quartz
clock which provides a more accurate signal, nominally at 32.768kHz.
The CPU and internal data bus of the HP-28C should be able to run at
1MHz, but HP were careful and set the LC circuit to run at 640kHz. By
changing the two capacitors one can make an HP-28C run at the full 1MHz.
HP warn that "some" CPUs might not be able to run at this speed, so it
is worth checking that yours is OK at this speed before glueing it
together if you make such a modification. By the time the HP-28S was
designed, HP were happy to make its CPU run at 1MHz by default. One
reason why they might be happier about this is that the CPU, ROM and
display drivers are all integrated onto one chip in the HP-28S. The
printer is designed to receive signals controlled by the crystal clock
frequency, but the signals are timed by the CPU, whose speed can vary
from HP-28 to HP-28, and according to external conditions. There is
therefore some software which times the CPU against the crystal clock,
and uses the result to determine how many CPU clock cycles are used to
transmit each bit. This software can allow for CPU speeds in the range
640kHz to 1MHz, and a little to either side. Thus it can run printing
from an HP-28C, from an HP-28S, and from an HP-28C speeded up to 1MHz.
Many HP-28Cs can be speeded up to well above 1MHz, but the printer
control software will not be able to handle this.
2. The hardware clock signal produced by the crystal oscillator
provides a clock value which can be read, as on the HP-71B and the
HP-18C (and the newest calculators too). The HP-28C and 28S provide
only a very rudimentary piece of software to read the clock value - this
is used to initialize the random number generator. Fortunately HP have
told user clubs how this piece of software can be accessed with SYSEVAL.
On version 1BB (the original HP-28C), this is done with #123E SYSEVAL
(clearly the address as given here must be entered in hexadecimal mode).
On version 1CC (HP-28Cs sold since about October) use #1266, and on
version 2BB (current HP-28Ss) use #11CAh. In each case the result is a
48 bit hexadecimal number representing a number of clock counts, at the
rate of 8192 counts per second. Make sure you have the binary word size
set to at least 48 bits, otherwise you may be surprised to find the
clock cycling. To use this as a clock you can write programs to convert
the clock reading into seconds, to display it as a time, to correct it
so it displays the actual time, or even to act as a calendar (this would
be a liitle ambitious on an HP-28C but the HP-28S provides enough memory
to do it). HP have provided a solution sheet with programs to use this
software system clock, and lots of user clubs have published their own
programs, and also programs to run the HP-28 as a stopwatch. One
interesting use is to read the time before and after a calculation,
which lets you select the fastest way of doing a calculation if several
methods are possible. Rather than calculating a correction and using it
in your programs, you can POKE the correct time right into the address
where the clock value is kept. Once you have done this you should find
the clock is accurate to within a few seconds a month.
3. Mention of POKE brings me to my book "Customize Your HP-28". It
contains machine language PEEK and POKE programs, explanations of how to
speed up an HP-28C and how to add more RAM to it, and lots more. A
preliminary version describing the HP-28C has been on sale to friends
since December, and I am now updating it to include some HP-28S details.
The preliminary version is still on sale to friends, and I consider
anyone who is interested in the 28 to be a friend! Prices including
post and packing are 9 UK pounds in Europe and 21 US dollars outside
Europe. I apologise for using a bulletin board for shoddy commercial
purposes like this but it seems to be the only way to provide detailed
answers to questions which have been posted here, short of posting the
whole book! If you are interested, please contact me through this
medium, or at the addresses below.
4. HP have not published full details of the HP-28 hardware and
software, but they have been very kind in providing information to clubs
and through HP Journal. A lot more information can be gleaned from the
HP Journal article on the HP-71B, and from the HP-71B IDS (Internal
Design Specifications) which can be purchased from HP. User clubs are
adding to the pool of information and software through research and
articles by members. Watch this space! (Or better still, join a user
club - a list was posted about a month ago.)
W.Mier-Jedrzejowicz, Ph.D.,
EARN/BITNET: MIER@SPVA.PHYSICS.IMPERIAL.AC.UK
JANET: MIER@UK.AC.IMPERIAL.PHYSICS.SPVA
UUCP: uunet!mcvax!ukc!spva.physics.imperial.ac.uk!mier
Smail: Physics Dept, Imperial College, London SW7 2AZ, England
Disclaimer: Neither my employer nor HP are to blame. As for me, I accept
no responsibility for what use anyone might make of my ravings.
Have a nice day!