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Details

The Hitachi 6309 is a really interesting processor. It was originally sold as a direct drop-in replacement for the Motorola 6809 but fabricated in CMOS, allowing much lower power consumption and higher clock rates. Unbeknownst to the public for quite a while, the Hitachi engineers did a lot more than just re-implement the processor in CMOS. They added a whole host of undocumented features, including more registers, many additional opcodes, faster processing of many instructions, and much more -- but these features had to be enabled by setting a hidden bit in a particular register. Some Japanese hobbyists discovered this, but it would be a few years before one of them posted a document on Usenet that revealed all the secret features to the wider world.

Motivation

There are a lot of 6809-based single board computers, including Matt Sarnoff's Ultim809 which is quite possibly one of the most amazing single board (solo!) projects I've ever seen. The Tandy Colour Computer used the 6809 and can take a 6309 as a drop-in replacement, allowing use of the NitrOS-9 system. But there really aren't that many dedicated 6309 single board computers, at least not that I have come across. So in the middle of the night a little while ago I spontaneously started creating a schematic for one, starting with the creation of a 6309 symbol in KiCAD.

Design Goals

• Easy to use and program
• Possible expansion interface
• Inexpensive
• A decent ROM with callable routines
• Open hardware and software