toymcu

This repository contains the Verilog implementation of a microcontroller, based around a CPU core which uses an extension of the Princeton TOY instruction set architecture, as well as an assembler and example program.

How it came to be

During my 2nd year of university, I studied the basics of digital hardware design, using VHDL. Out of personal curiosity and passion for the subject, I had the idea of implementing a small CPU design. While looking for possible instruction set architectures to use, I came across the TOY ISA, developed by Princeton University, to use in their Computer Science courses. I settled on it, considering its simplicity and the fact that nobody had (publically, at least) implemented it in hardware previously.

During the 3rd year, I decided I wanted to improve my digital hardware design skills and learn Verilog. The CPU core was my first Verilog design, having translated the older project from VHDL, while also making some significant and useful modifications.

After I was pleased with the state of the CPU core, I started to work on the assembler, in order to be able to write programs without having to type machine code by hand directly into memory. I also implemented the transmit part of a UART, at this point being able to output characters to a serial console.

With the CPU core and assembler mostly functional, I decided to implement more peripherals. I ended up implementing a full bidirectional UART, a timer, GPIO port and interrupt controller, basically turning the project into what can universally be considered a microcontroller.

Repository contents

Build system

This project uses makefiles and various open-source tools for its build system. The usage of this build system is described in this file.

Hardware design

The hw folder contains the Verilog implementation of the toymcu, simulation testbench and a top-level design meant for the Colorlight 5A-75B V8.2 board.

Assembler

The assembler folder contains the assembler for the toymcu. This file explains the features and usage of the assembler.

Boot ROM and loader tool

The toymcu starts executing code from a ROM memory. This memory contains a bootloader which loads code into RAM, over the UART. Code is sent using the loader tool.

Example program

The program/hello folder contains an example program for the toymcu, which showcases UART and GPIO output, as well as UART and timer interrupts. The program source is split across multiple assembly files

Documentation

• The build system
• Improvements made upon the original Princeton TOY ISA
• The TOY ISA
• The toyasm assembler
• Memory map
• Peripherals
  • Interrupt controller
  • UART
  • Timer

To do

• Documentation
  • document GPIO peripheral
• Implement GPIO interrupts