Many years ago I completed the challenge to fit a CPU into the smallest available CPLD on the market - the MCPU. Ever since then I have been pondering about a new challenge in minimalism in CPU-Design. I had completed a TTL-based CPU even before the MCPU. Clearly, the only direction left is to go fully discrete and build a minimal CPU out of discrete transistors.
To make things interesting and add a modern twist, I decided to investigate a logic family that uses light emitting diodes (LEDs) as an active element. LTL is a logic family from a past that never happened. It combines 1950s transistor logic with low current green InGaN LEDs that were invented in the 1990s.
I already completed the design up to a prototype of a sub-system (see header image) and will use this project to document the steps I have taken to get there.
Start with the first log by using this link or use this link to view all project logs on one page.
4way LUT XOR'd against the prior slice's carry looks good. Not so thrilled by ripple carry. Combinatorial works fine for exceptions that replace carry, but search you some "Muting Transistors" and propagate Manchester pass-though style.