October 28, 2014

Designing a 68K Single Board Computer

Hackers of a certain age love the 68000 family of CPUs, and I’ve begun sketching out designs for a 68K-based single board computer. Take a vintage off-the-shelf CPU, add ROM and RAM and an SD card and some type of I/O, and Bob’s your uncle! I plan to start with bare metal programming to test out the hardware and peripherals, then try to get text mode Linux running. I/O will probably be exclusively through a serial port, but I hope to also include a USB controller to see what I can make happen through Linux. For inspiration and direction, I’m drawing heavily from Simon Ferber’s (Kiwi) and Luis Alves’ 68K-based homebrew designs. Here we go!


I have an old MC68008P8 CPU gathering dust in a drawer that would be perfect. The P8 version is rated for 8 MHz, and the 68008 is essentially a 68000 with an 8-bit data bus instead of the standard 16-bit bus. That means its memory throughput is only half the 68000′s, and since it has no cache and most instructions require a memory access, its performance is only slightly better than half that of a 68000 at the same clock speed. But what it lacks in speed, it makes up in simplicity. The 68000 requires twice as many data lines to be routed, and either twice as many RAM and ROM chips, or physically larger chips with 16-bit interfaces.

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