Wednesday, December 28, 2016

Building a 68000 Single Board Computer - Freerunning on Protoboard

I'm now at the stage with the wirewrap version where I was on the earlier breadboard -- I have the clock, power on and pushbutton reset circuits, and CPU wired up so that I can free run the CPU. The power switch and power LED work, as well as the reset pushbutton and reset/halt LED.

By tieing AS* to DTACK* and pulling the 16 data bus lines low, it will continuously execute the instruction $0000 (which corresponds to ORI.B #0,D0) and cycle through all of memory. If I pull some data lines high, it gets an odd address exception and halts with the RESET/HALT LED lit, as expected.

Only five chips are wired up, but the rest have power and ground and bypass caps.

The next step is to wire up most of the remaining circuitry and attempt to get it running out of ROM and RAM. This will take some time as it involves almost everything in the circuit except the two UARTs. I will also only need half of the RAM and ROM chips to run the Teesside monitor program.

1 comment:

Doug's Theology BLOG said...

Hi Jeff,

Fantastic retro project. I made an FPGA build of the card. It runs at 25 Mhz (FPGA CPU clock speed). The 68000 Core is very fast - I think it uses less clocks per instruction, too. It's up in GitHub. Everything fits in a single EP4CE15 FPGA.