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HP-45 simulator running on the HP-20B


A flash resident program that will simulate an HP-45.


Use SAM-BA or any other comfortable flash programming tool to install the .bin file. It's less than 18kB. Of course, you will lose any existing firmware in your HP-20B, so be sure you backup the stock HP firmware before erasing it.

Expect RAM to be corrupted if you decide to reload the stock HP firmware.

Install this firmware at your own risk.


Simulating an HP-45 on the HP-20B.


 1/x  ln   e^x  fix  X^2  ->P
 sin  cos  tan   %   sto  rcl
 enter     rdn  x-y  chs  eex
 off   7    8    9    /
       4    5    6    *
 gld   1    2    3    -
 clx   0    .    sg+  +

The source is present. It builds for me using the CodeSourcery gnu tools under windows 2000. It's a thin wrapper around Eric Smith's casmsim. To reduce the ram footprint, casmsim was used to dump both the opcode map and the microcode file from the HP-45 as C arrays.

The code has not been optimized. The PIT has been configured to run at about 3.5kHz, so the microcode clock rate should be pretty close to real time. The display update stuff still isn't quite right. For some reason, the HP45 appears to be toggling the display on and off while at idle.

There is no power management. I measure about 2mA while running, and very little (100nA?) while off. The CPU runs at about 2MHz and does not sleep or idle once the program starts, so you can attach a JTAG debugger at any time. There is no auto off, so it will chew down your coin cells if you leave it on. (At constant 2mA drain, I'd guess it's good for about 240 hours.) Press the up-arrow key to power off the calculator.

I've noticed that the blinking error display doesn't work either. Not sure what I screwed up there.

There are probably still some bugs in the keyscan and display handling areas. The stopwatch doesn't seem to work. I've tweaked some of the values in the csim code to make the display solid and the keys register reasonably well.

I'd love to hear your comments on this.

Scott Newell

20b/software-demo_45.txt · Last modified: 2008/08/12 21:24 by newell