Meanwhile, while I'm waiting for my 65C02 to arrive, I've been trying to implement a hexadecimal 7-segment decoder to visualize what's going on at the data bus. Initially I wanted to use GAL20V8. Here's what I came up with:
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GAL20V8
Hex-7Seg
CLK A B C D A1 B1 C1 D1 NC NC GND
/OE NC Q a b c d e f g NC VCC
a = A*/B*/C + /A*B*D + A*/D + /A*C + B*C + /B*/D
b = /A*/C*/D + /A*C*D + A*/C*D + /B*/C + /B*/D
c = /A*/C + /A*D + /C*D + /A*B + A*/B
d = /A*/B*/D + /B*C*D + B*/C*D + B*C*/D + A*/C
e = /B*/D + C*/D + A*C + A*B
f = /A*B*/C + /C*/D + B*/D + A*/B + A*C
g = /A*B*/C + /B*C + C*/D + A*/B + A*D
DESCRIPTION
Hexadecimal decoder for 7-segment displays
So I decided to try and 1) implement driver with good old 7400 ICs in emulator (because why not?) and 2) try to multiplex lower and higher nibble of 8-bit input to each of the two displays.
I ended up with a huge matrix of AND & OR gates (I heard you guys like *big* schematics!) and simulated it in Digital (https://github.com/hneemann/Digital).
Initially I had 35 terms total, but there were some duplicates, so the final count of AND gates went down to 28:
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>>> terms = re.findall('([A-D*/]+)', s)
['A*/B*/C', '/A*B*D', 'A*/D', '/A*C', 'B*C', '/B*/D', '/A*/C*/D', '/A*C*D', 'A*/C*D', '/B*/C', '/B*/D', '/A*/C', '/A*D', '/C*D', '/A*B', 'A*/B', '/A*/B*/D', '/B*C*D', 'B*/C*D', 'B*C*/D', 'A*/C', '/B*/D', 'C*/D', 'A*C', 'A*B', '/A*B*/C', '/C*/D', 'B*/D', 'A*/B', 'A*C', '/A*B*/C', '/B*C', 'C*/D', 'A*/B', 'A*D']
>>> len(terms)
35
>>> len(set(terms))
28
>>> set(terms)
{'/A*B*D', '/C*D', 'A*/B', '/B*C', '/A*C', 'B*C*/D', 'A*/D', 'A*B', 'A*/C', 'A*D', 'A*/B*/C', '/A*/C', '/B*/D', 'A*C', '/B*/C', 'A*/C*D', '/A*B*/C', '/A*C*D', 'B*C', '/B*C*D', '/A*/C*/D', '/A*D', '/C*/D', 'B*/D', '/A*B', 'C*/D', '/A*/B*/D', 'B*/C*D'}
Digital circuit: Animated simulation (GIF, click to play):
Still, I think that NOR can be made with 2 diodes, a NAND, and a pull-up: LCDEN = NOT(Ф1 DIODE_OR PULLUP(LCD/EN)). Will this work?
