2 layers122 × 81 mm 8-bit cpu, the printed circuit board in 3D

8-bit cpu

An 8-bit CPU running on a TangNano 9k FPGA. Written in SystemVerilog and tested with PYUVM.

Size: 122 × 81 mm
Copper layers: 2
Thickness: 1.6 mm
Parts: 20
Nets: 52
Pads: 185
Tracks, vias: 165, 0
Pours: 2
Routed: Yes
KiCad: KiCad 8.0 (file 20240108)
Created: 2 Jul 2025
Updated: 9 Dec 2025

Overview

8-bit cpu is an open-source printed circuit board by Robert-Riordan-UCD, from the Robert-Riordan-UCD/8_Bit_CPU_FPGA repository on GitHub. It is a 2-layer board of 122 × 81 mm with 20 parts on 52 nets, routed with 165 tracks and 0 vias.

Its parts include 8 switches, 4 resistors, 2 ICs and 2 connectors. It belongs with the fPGA and programmable logic and retro computing and games boards and the FPGA and CPLD boards of this library.

README

README.md

8 Bit FPGA CPU

This is an 8-bit CPU design running on a Tang Nano 9k FPGA. The CPU design is based on Ben Eaters Breadboard CPU, which is based on the Simple As Possible Computer.

The CPU is made of 11 modules, which are designed in SystemVerilog and verified using pyuvm.

This is a gif of the CPU calculating the fibonacci sequence and displaying it (in hexadecimal). Video of the CPU calculating the fibonacci sequence

Getting Started

This project is not fully complete yet and is currently only tested on my personal laptop and raspberry pi. However, this is the current process. (I am developing on Ubuntu 24 and used this guide to get up and running using an open source toolchain)

Installation and Setup

The following programs are required:

yosys for synthisis.
nextpnr for place and route, and bitstream generation.
openFPGALoader for loading the bitstream.

sudo apt insatll yosys nextpnr-gowin openfpgaloader

Clone and navigate to the repository

text

git clone https://github.com/Robert-Riordan-UCD/8_Bit_CPU_FPGA
cd ./8_Bit_CPU_FPGA/

Running

make all upload

Testing

pyuvm is required for testing.

sudo apt install pyuvm

To test each module navigate to the verif directory for that module and run make.

text

cd ./verif/<module>/
make

Current Design Status

Module	RTL	Verified	Runs on FPGA
Clock	✅	✅	✅
Program Counter	✅	✅	✅
A register	✅	✅	✅
B Register	✅	✅	✅
Memory Address Register	✅	✅	✅
Instruction Register	✅	✅	✅
Random Access Memory	✅	✅	✅
Arithmetic Logic Unit	✅	✅	✅
Control	✅	✅	✅
Output	✅	✅	✅
Bus	✅	✅	✅
Bootloader	✅	✅	✅

Top	✅	❌	✅

The output is currently in hexadecimal, because there was not enough LUTs on the Tang Nano 9k to implement a naive approach to decimal conversion using division. I will hopefully update this to a better approach soon.

Category and platform

FPGA and programmable logic Retro computing and games FPGA and CPLD

Topics

fpga pyuvm systemverilog tangnano9k

Project files

Show this board on your page

https://pcbkit.io/boards/Robert-Riordan-UCD/8_Bit_CPU_FPGA

The board's own page, for a message or a post.

Source

Repository: Robert-Riordan-UCD/8_Bit_CPU_FPGA
Board file: pcb/8-bit-cpu.kicad_pcb
Commit: 3d0091c
Homepage: robert-riordan-ucd.github.io/this-is-rob/posts/fpga-8-bit-cpu
Licence: Not stated

Parts

Qty	Value	Footprint	References
4	1k	Resistor_THT:R_Axial_DIN0309_L9.0mm_D3.2mm_P12.70mm_Horizontal	R1, R2, R3, R4
4	MountingHole	MountingHole:MountingHole_3.2mm_M3	H1, H2, H3, H4
2	Conn_01x24_Socket	Connector_PinHeader_2.54mm:PinHeader_1x24_P2.54mm_Vertical	J1, J2
2	~	footprints:4xDoubleThrowSwitch	SW7, SW8
1	BOOTLOAD EN	footprints:DoubleThrowSwitch	SW3
1	CA56-12EWA	Display_7Segment:CA56-12EWA	U2
1	CLK MODE	footprints:DoubleThrowSwitch	SW1
1	CLK PULSE	footprints:tactile_switch	SW2
1	RAM MODE	footprints:DoubleThrowSwitch	SW4
1	RAM PULSE	footprints:tactile_switch	SW5
1	Tang Nano	footprints:TangNano	U1
1	~	footprints:8xDoubleThrowSwitch	SW6

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Questions

How big is the 8-bit cpu PCB?

It measures 122 × 81 mm and has 2 copper layers, 1.6 mm thick.

What parts does the 8-bit cpu use?

20 parts. The full list of its parts is on this page, with links to each part's symbol and footprint where PCBKit's library has them.

Where can I download the 8-bit cpu KiCad files?

From the Robert-Riordan-UCD/8_Bit_CPU_FPGA repository on GitHub, at the commit this page was made from: the board is pcb/8-bit-cpu.kicad_pcb and its schematic pcb/8-bit-cpu.kicad_sch. PCBKit opens the board in the browser, with nothing to install.

Can I use the 8-bit cpu design in my own project?

Its repository states no licence, so its author keeps all rights: ask Robert-Riordan-UCD before reusing the design.