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Address-Datapath PCB Design

A project log for LCPU - A CPU in LED-Transistor-Logic (LTL)

This projects tracks my efforts to develop discrete LED-Transistor logic building blocks and designing a CPU from them.

TimTim 02/22/2020 at 11:250 Comments

Designing PCBs with regular structures using free tools turned out to be surprisingly tedious. In the end I used the free version of Eagle due to it's block feature which allowed replicating of circuit and layout units. The other options I looked at were EasyEDA and KiCad.

Unfortunately, the free version of Eagle is limited to a single sheet, so I ended up transferring my nice hierachical LTspice implementation into a single paged mess of a schematic. The image above shows a one bit slice.

Curiously enough, none of the many LEDs in the LTL gates represented the actual state of the address output pins. Therefore I introduced two additional indicator LEDs.

The final PCB layout is shown above. I managed to fit a four bit wide slice of the adress path into one 8,5x7cm² PCB.

A populated PCB is shown above. Since I am not too fond of placing all those parts by hand, I used an SMD assembly service. The finals stats are: 52 transistors, 48 diodes, 24 capacitors, 56 LEDs and  88 resistors.

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