Overview

components_EPROM.jpg
The classic 2716 2Kx8 EPROM.

I recently designed and breadboarded a simple Z80 based computer. This was the sort of thing I did for fun and profit years ago. This time it was, well, for fun and profit. But it was a little different. Back in the day I had a drawer full of Erasable Programmable Read-Only memory chips (EPROMs), as well as an EPROM eraser and programmer*.   This time I had none of that. So I'd have to get inventive.

When I designed the system, I put in 2K of ROM/EPROM and 2K of static ram. Naturally when I bought parts, I got extras. So, that meant I had an extra 2K static ram chip sitting in a drawer. That's a powerful thing: having extra parts sitting in drawers. What I decided to do was build an EPROM emulator using that ram chip, some transistor/transistor (TTL) logic and a microcontroller board.

I was using a Z80 assembler (z80asm from z80pack) on Linux to write code for the Z80, so using a microSD card seemed a reasonable way to get code from Linux to the emulator. Once I had decided to do that, I needed a way to navigate the file system on the card, select a binary file, and load it onto the RAM in the emulator.  See how things snowball!

To make the project a bit simpler and more modular, I chose to divide it into two separate parts: the controller/user interface (UI), and the emulator. To maximize the flexibility of that separation, I decided to connect them using an I2C bus. That proved to be a very good choice as I went through several iterations using different microcontroller boards: from an Arduino Mega2620 to an ItsyBisty M0 Express and finally to an Metro M4 Express.

While I designed this project to support my work on a Z80 system, it can be used to replace a 2716 EPROM in any system. In this guide, I'll call that system the host.

* I'm planning to build a new take on an EPROM programmer sometime in the near future which will share some features of this project, and in some ways could be considered as Part 2. It will most likely use the same controller/ui design.

Parts Used

1 x MicroSD card breakout board+
MicroSD card breakout board+
1 x Monochrome 128x32 I2C OLED graphic display
Monochrome 128x32 I2C OLED graphic display
1 x Rotary Encoder + Extras
Rotary Encoder + Extras
1 x Adafruit Metro M4
Microchip ATSAMD51
1 x 4-channel I2C-safe Bi-directional Logic Level Converter
4-channel I2C-safe Bi-directional Logic Level Converter - BSS138
1 x MCP23017 - i2c 16 input/output port expander chip
MCP23017 - i2c 16 input/output port expander
1 x Silicone Cover Stranded-Core Wire 30AWG
Silicone Cover Stranded-Core Wire - 50ft 30AWG Black
This guide was first published on May 14, 2018. It was last updated on Nov 14, 2018. This page (Overview) was last updated on May 13, 2018.