2024-09-01: This retroshield will work w/ Arduino Mega 2560 or Teensy 3.5 only. Unfortunately Teensy 4.1 support is too much work due to weak processor output buffers and/or incompatible port assignments on teensy 4.1 (vs 3.5). Sorry for the inconvenience.

My Brain-In-A-Jar Project

• A daughtercard with a real 8bit microprocessor, plugged into Arduino Mega.
• Arduino Mega emulates system hardware such as ROM, RAM, I/O devices.
• Real microprocessor executes code.
• Clock speed is about 200kHz.
• 200KB ROM space available in Arduino.
• 6K RAM available in Arduino (can be expanded to 128KB - see my blog).
• Any existing Arduino shields (LCD, SDCard, etc.) can be used to add new features.
• Compatible w/ my Teensy Adapter for RetroShield for faster clock (>1 Mhz) and more memory (256 KB RAM).

NOTE About UARTs: UART on this RetroShield is slower compared to other processors because most monitors use 8031's internal UART hardware, which means Arduino has to implement a processor cycle-based soft-uart on 8031's TxD/RxD pins. Current monitor UART speed is 300 cpu cycles/baud, meaning each character will take 2400 clock cycles to be transferred. As a comparison Z80+ emulatd 8251 UART takes a few clock cycles to send/receive. If you are writing your own monitor program, I suggest using an emulated 8251.

What can you do with it?

You can run the following:

• PAULMON2 v2.1 by Paul Stoffregen

Or design your own hw platform in Arduino.

What do you get?

Three options:

• Assembled and tested board w/ microprocessor*
• Kit for you to build: PCB w/ surface mount parts soldered + microprocessor + socket/header/LED

Sorry no PCB's. 8031 is running below min clock freq specified in datasheet and I need to send you screened microprocessors.

NOTE: ARDUINO PICTURE SHOWS 6502, BUT YOU WILL GET RETROSHIELD 8031.

Why did you make it?

First, I have been building SBC's with old microprocessors since high school. For every SBC, I had to spend time wire-wrapping the ROM, RAM, peripheral chips over and over again. Then it was more hassle to expand the SBC. I am now lazy; Arduino Mega is capable enough so I now do my wire-wrapping in software :)

Second, I wanted to give back to the retro community. So all hardware and software design files are open source.

What makes it special?

• My goal is to make a daughtercard for every 8-bit microprocessor out there. so far I built six (6502, 6809, z80, 8031, 8085, 1802).
• I got chips for 4004, 4040, 8008, 8080, 8088, 2650, 8X300, 6800, 6803, 68008, F8, SC/MP, 6100, etc. So these are in the pipeline.