Steve at Big Mess o’ Wires posted an update on his Custom 4-Bit CPU:
Enough with the vague design talk – here’s the circuit schematic for the Nibbler 4-bit CPU! Click the image to zoom in to a full size view. For simplicity, this version omits some details like bypass capacitors, clock generation, reset circuitry, etc. The whole system fits on a single page, including the CPU itself and the I/O devices, so it’s easy to wrap your head around.
Except for RAM and ROM, all the chips shown here are common 7400 series parts. I haven’t selected a logic family yet, but most likely they’ll be 7400HC or 7400HCT, which require less power while offering similar speed to the more common 7400LS family.