The Design

Paul's Digital Computer

A few weeks ago, I got this crazy idea to build a CPU from scratch without using a micro processor, but construct it entirely from discrete logic circuits. I have chosen to use TTL ICs to implement the logic and CMOS technology for memory. With Google as my best friend to start off with, I came across many similar DIY projects. The Nibbler project got my attention and confirmed that my initial thought on finishing such a build would be feasible.

My goal was not to come up with an entirely new design, but create it from scratch and understand every single bit of the design. I will call it the PDC-1 which stands for Paul's first Digital Computer and are also my initials :-)

I created the entire circuit in Proteus 8 that can also simulate the entire circuit.

The high level requirements

• real 4-bit processing: the ALU and databus are 4-bits
• harvard architecture: program and data in separate memory
• 12-bit memory addressing (32kbit RAM; 4096 x 8-bit)
• instruction set contains 16 instructions
• microcode for instructions stored in 2 EEPROMS
• 2x 4-bit input and 2x 4-bit output
• only 1 register, the Accumulator