Andreas Jakob's Tiny 6809 Board Puts a Working Microcomputer on Your Keychain
Built on a six-layer board to minimize footprint, this tiny little eight-bit includes Microsoft Extended BASIC and a monitor in ROM.
Classic computing enthusiast Andreas Jakob has built what is likely to be one of the absolute smallest Motorola 6809-based microcomputers around — so small, in fact, it includes a mounting hole to fit it on your keychain and carry some computing history in your pocket.
"[This is a] tiny and minimalistic [Motorola] 6809 board that is mostly based on designs and ideas from Jeff Tranter, Dave Collins and Grant Searle," Jakob explains of the project. "To fit this into 5×5cm (around 1.97×1.97") I used a six-layer PCB as JLCPCB offers them in a promo for no additional cost compared to four layers at the moment of writing this, including free filled and plated vias and gold plating. This allows via in pad placement to save space. The top layer is a solid ground plane and layer five is a solid 5v plane leaving 4 planes for routing."
The resulting tiny board has a footprint barely larger than the two DIP-packaged chips which dominate its surface: a 32kB 27C256 EPROM holding a copy of Microsoft Extended BASIC and the Assist09 ROM monitor plus a compatible clone of the Motorola 6809 — an eight-bit chip with a few 16-bit features first released back in 1978 as a high-priced but high-performance alternative to the Zilog Z80.
"Parts were mostly selected based on what JLCPCB had in stock and could populate, as hand soldering the tiny SMD [Surface Mount Device] parts would be a hard job," Jakob notes. "Routing was only done by hand for more critical tracks and to guide the auto-router, which did most of the work. The bottom side ICs are placed in precision female headers so the pads for the CPU socket can still be reached for soldering. Normal sockets would block access."
The completed tiny machine features the 32kB of RAM and 32kB of ROM, a 6809-compatible CPU running at 1MHz or a 6309 running at 5MHz, the monitor and BASIC in ROM, and a serial port capable of running at 230,400 baud with a USB Type-C connector for data and power via an STMicro interface chip. A 30 pin header provides room for expansion, too, and can play host to a second and equally-compact board Jakob has designed: a Motorola 6821 peripheral interface adapter (PIA).
Jakob's tiny computer is available on the Open Source Hardware Lab (OSHW Lab) under the CERN Open Hardware License (CERN-OHL), as is the PIA expansion board.