Donovan Magney

Overview

The 8-bit breadboard computer is a Ben Eater inspired project to study basic functions of a CPU. This project will also serve to bridge the gap between using ICs (integrated circuits) on their own performing their own functions, to combining them together to accomplish a much bigger task. There are only 2 major constraints to the project. Constructing it on a breadboard, which is essential to allow easy modifications for fixing mistakes or hardware bugs. And slow clock, about 1-3hz to allow easier debugging of code.

Final Product

The computer took approximately a week to construct. The final product includes:

• Variable astable clock (1 - 300 hz) with the option for a button triggered monostable clock.
• 16 bytes of random access memory (RAM)
• 3kBs of read only memory (ROM)
• 4-bit program counter
• An arithmetic logic unit (ALU) that is capable of 8-bit addition and subtraction
• A 3 digit 7 segment output display with the ability to display in either signed or unsigned digits
• 11 CPU Instructions
• 8-Bit main bus

Also added (not shown) is an Arduino Nano connected to the RAM module which programmed the RAM on power up while the computer is in programming mode. It does this by first by writing to every memory address, then verifying by reading each address. After programming, the Arduino would flash an LED to indicate completion and slowly cycle through each address to allow for human verification until the computer was switched to run mode. The code for this can be found here. The clock can then be started to run the program and a halt is triggered at the end of the program.

As stated earlier, the computer has a total of 11 possible assembly instructions. These are:

• Binary - Assembly - English
• 0000 - NOP - No Operation
• 0001 - LDA - Load A register
• 0010 - ADD - Add
• 0011 - SUB - Subtract
• 0100 - STA - Store A register value in memory)
• 0101 - LDI - Load Immediate (value) to A register
• 0110 - JMP - Jump
• 0111 - JC - Carry Jump
• 1000 - JZ - Zero Jump
• 1110 - OUT - Display Output
• 1111 - HLT - Halt Operation

It is possible to add up to 5 more instructions, due to the 4-bit instructions 16 total are possible. Through these instructions 3 test programs were created before retiring the project. A "Test Sequence", which is just doing basic arithmetic, cycling through the Fibinachi numbers up to 255 (maximum possible with 8 bits), and a multiplication program.

Conclusion

This project was very successful in completing the objective. I learned how to construct a basic CPU using bus architecture and all the require modules needed to accomplish it. This project also gave lots of experience in being able to select integrated circuits (IC), and combining them to collectively perform any task, then being able to write custom assembly code to give it a basic instruction set. However, the breadboards caused power issue at this scale due to their high resistance, which caused a few bugs along the way with ICs not being powered properly. Future large scale breadboard projects would involve soldering each module to at least a perfboard as they are completed.