In The Words of Tom Petty, I'm Free Runnin

A project log for Lacey Mk I

A Homebrew 68k Computer for Shits & Giggles

matt-laceyMatt Lacey 04/16/2017 at 13:120 Comments

Well it was close enough, and I don't like that song, but I do like the fact that this old chip is free running on the breadboard now.

What's free running? Well it's basically hooking up the bare minimum to make it work. Power lines, inputs all zero'd out (high in some cases), a basic reset circuit and a clock. The key is that the datalines are all grounded, resulting in the processor executing the instruction "ORI.B #0,D0", in other words OR the immediate value zero with register D0. The program counter runs this then reads the next instruction which is still the same and so on, cycling through the address space forever, meaning if you hook an LED to some of the higher address lines you'll see it flash at slower and slower intervals as it's being used to represent more significant bits in the PC address.

I'm going to be using an Altera MaxII CPLD to implement some of the glue logic (though this may change due to it using 3.3V logic), and right now it's the oscillator in there that's driving the clock line of the CPU. I'm not sure how long I'll be able to use this CPLD for, it's not rated to drive 5V logic, but given this CPU is isn't CMOS it sounds like it might be doable.

The electrical specs for the MC68000P12 CPU say the minimum high level for input is 2V, and it's high for output is 2.5V, so the ranges do overlap with the MaxII which has a range of 1.7-4V for the high input voltage and the output high has a minimum of the IO supply volatage (which should be 3.3V on this dev board). I'm not an expert, and people may read this and pull their hair out, but as long as things aren't on fire and function correctly then I'll keep on going.