myelin-acorn-electron-hardware

The Acorn Electron is a British computer that was very popular in the UK and New Zealand in the 80s, and was the first computer I ever used, in 1985. I acquired one on eBay in 2016, and started hacking on hardware expansions for it a little later. In the meantime I’ve also acquired a BBC Model B, and some BBC Master 128 motherboards. This repository contains various hardware expansions I’ve designed for the three machines.

Background: https://www.theregister.co.uk/2013/08/23/acorn_electron_history_at_30/

Designed by Phillip Pearson (philpearson@google.com)

This is not an official Google product.

Acorn Electron projects

• standalone_cartridge_programmer: USB Acorn Electron cartridge interface, allowing read/write access to cartridges without an actual Electron / Plus 1. (Built, verified.)

• elk_pi_tube_direct: Acorn Electron cartridge using a CPLD to provide address decoding and level shifting for a Raspberry Pi running PiTubeDirect. (Built, verified.)

• expansion_minispartan_breakout: Breakout board for the Electron’s rear expansion connector, with buffers and footprint to attach a Scarab miniSpartan6+ LX25 board. (Built, verified.)

• minus_one: Acorn Electron expansion that provides three Plus 1 workalike cartridge slots. (Built, verified.)

• 32kb_flash_cartridge: Simple Acorn Electron cartridge with two 16kB flash banks. (Built, verified standard PCB. Mini PCB built but had a PCB error.)

Cross-platform projects

• cpu_socket_expansion: Mezzanine board that attaches to the CPU socket of a BBC or Electron and translates between 5V TTL and 3.3V CMOS levels. (Built, tested.)

• cpu_socket_minispartan_daughterboard: Attaches to a cpu_socket_expansion board and connects a miniSpartan6+ FPGA board and a Raspberry Pi Zero running PiTubeDirect. (Built, tested.)

• spi_sd_card: Experimental implementation of the interface used by MMFS to access SD cards, for the CPLD on an elk_pi_tube_direct board. (Code only.)

• fx2_tube_cartridge_adapter: Adapter to allow connecting an FX2-based logic analyzer board to the BBC Tube interface or as an Electron cartridge. (Partly tested.)

• bbc_power_distribution: Small board to make it easy to power a Model B or Master motherboard from a 5V power supply with a barrel jack or Micro USB output. Uses an LTC1983 to generate -5V, so you don’t need to provide it externally.

BBC Model B projects

• new_bbc_bringup: Notes on modernizing a BBC Model B, with flash and sideways RAM, and an emulated disk interface.

• upurs_usb_port: Code to run on an ATMEGA32U4 to use it as a USB to serial adapter for UPURS. (Flaky.)

• serial_sd_adapter: VHDL that runs on an elk_pi_tube_direct board and provides a fast serial port and SD card interface. Also, a board that attaches to a BBC using the 1MHz Bus and provides the same interfaces. (Built, tested serial port. SD card interface untested.)

• econet_from_scratch: USB Econet interface. (Untested.)

• econet_hub: Clock and bias circuit, with five Econet sockets, to connect a simple network together. (Untested.)

BBC Master 128 projects

• master_updateable_megarom: BBC Master 128 MOS ROM replacement that can be reprogrammed while in the machine. (Untested.)

• emulated_keyboard: Code and VHDL to allow an attached computer to act as a keyboard for a BBC Master 128. (Code only, verified.)

• new_master_bringup: Notes on getting a BBC Master 128 motherboard to run without a keyboard or standard power supply.

Miscellaneous things that happened along the way

• atsamd11_pro_micro: Tester board to experiment with USB on the Atmel ATSAMD11C14 chip. (Untested.)

• atsamd21_usb_host: Tester board to experiment with USB hosting on the Atmel ATSAMD21E18A chip. (Untested.)

• cherry_mx_keyswitch_tester: Tester board to verify my Cherry MX keyswitch footprint before I order anything expensive. (Built, verified.)

Future project ideas

• dual_ported_ram: Acorn Electron cartridge using a CPLD and a 128kB SRAM chip to implement dual ported RAM that can be read or written by both the Electron and a USB-attached computer. (Working on the PCB layout.)

• expansion_debugger: Pro Micro, CPLD, and edge connector that can be connected to a Plus 1 or other Electron expansion for testing. (Just an idea so far.)

Installation notes

I use my own Python scripts for generating netlists, rather than entering schematics through Kicad’s eeschema package. If you change the .py file in any of the pcb folders, running ‘make’ (or ‘make net’) should rebuild the netlist.

The Makefile will also plot gerbers once a .kicad_pcb file has been created. This requires some messing around on macOS – you have to copy the system Python executable into the pcbnew folder.

Generating the previews linked from the README.md files requires pcb-tools, which you can install using python setup.py install in the third_party/pcb-tools folder (after running git submodule update --init). On macOS, first install its dependencies with brew install cairo pango.