A hobbyist documents the design of a 6809-based single board computer (SBC) intended to run the OS-9 operating system. The post covers the journey from a minimal Grant Searle reference design to a custom PCB, including lessons learned about the 6809E external clock requirement, generating a quadrature clock with D flip-flops, selecting the MC68681 DUART for dual serial ports and GPIO, memory mapping decisions (64K bankswitched RAM, 8K EPROM, I/O page at $FF00), and address decoding logic using 74LS-series gates. SD card storage is also integrated via the DUART's GPIO pins. Design files are available on GitHub.
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