A working 6809 Computer on a 5x5 cm PCB that could fit on your keychain ( and even has a hole for that :) )

This is version 2 that fixes some issues of the first version.

A tiny and minimalistic 6809 board that is mostly based on designs and ideas from Jeff Tranter, Dave Collins and Grant Searle.

To fit this into 5x5 cm i used a 6 layer PCB as JLCPCB offers them in a promo for no additional cost compared to 4 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 5 is a solid 5v plane leaving 4 planes for routing.

Parts were mostly selected based on what JLCPCB had in stock and could populate, as hand soldering the tiny smd parts would be a hard job.

Routing was only done by hand for more critical tracks and to guide the autorouter, 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.

Features :

• 6809 CPU @ 1MHz or  6309 @ 5MHz 
• 32K RAM (SMD)
• 32K 27C256 EPROM ( 24K usable, 16K + one 8K block addressable )
• Assist09 ROM monitor and Microsoft extended Basic
• Independent Baudrate generator
• 6850 ACIA @ 230400 Baud + simplified hardware handshaking
• USB C Connector for power and data ( protected with 500mA resettable fuse and TVS diode )
• USB to Serial converter onboard
• 30 PIN 2.54mm expansion header
• Reset button and separate reset connector for use with EPROM emulator
• Tiny 50x50mm 6 layer PCB utilizing two solid planes for power distribution

I used the "combined" ROM from Jeff Tranter.

The 16K ROM code is placed in the upper half of the EPROM (0x4000 offset). The first 8K of the lower half are mapped to 0x8000 - 0x9FFF.

The expansion port has one chip select and enougth address lines for another 4K block to use.

I also created an 6821 expansion board fitting this one:

https://oshwlab.com/andreasjakob/6809-cube-cpu_copy_copy_copy

Attached is a short documentation (German only in the moment) and the ROM binary modified for hardware flow control.