Recreating a HP-25

[Alejandro Paz(Germany)]

I've been working lately on the code quite a bit and on a prototype board with keys and 7-segment displays. The code runs till the entry loop at 0742..0745. I have to implement keyboard scanning now. Debouncing seems to happen entirely on hw. If this where some microprocessor, I'd just use some software counters to scan the matrix and store key changes. I'd like to avoid using one counter per key, because I don't think that it was done like that. There is enough logic in this chip, I just think that is a waste of logic to do it like that....
I had some problems with the carry. It gets set and cleared in the same instruction, not a behaviour I have to reproduce,... on the 1LF2 I used an extra register for the comparison result. Here I used the carry, but it resulted in awkward code. There is a bit to improve. I follow the principle: make it work, make it fast, make it good . The 1LF2 has hw registers and the ws has memory-mapped registers, there is not enough logic for all the muxes one needs in a 8x64 bits register file with two access ports. Two R/W ports are not really needed, they are just convenient. It is a behavioural model after all, not a gate-level model (one could also do this but... why ?).
The code posted to github is up-to-date. There is a stream of serial data, pc, opcode and P register coming out of the FPGA (numbers in octal except P). I'm using a MachXO2-1200ZE breakout board, it has a TTL33 to USB adapter built in. It is clocked from the internal 2.08 MHz clock, serial data is output at 500 kbps, no problems for the FT2232, they can also have 2 Mbps .
I'll post a couple of pictures of the prototype once I've finished soldering the keys to the connectors, and the displays... quite a bit of wiring going on.

If you study a bit the code, there is another surprise there , it only needs another row of keys .

Code:


0000 1173 0
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1362 0656 C
1363 1673 C
1356 1116 C
1357 1657 C
1353 0344 C
1355 0620 C
1356 1116 B
1357 1657 B
1353 0344 B
1355 0620 B
1356 1116 A
1357 1657 A
1353 0344 A
1355 0620 A
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