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8-bit computer – control logic

Over the last few weeks, I’ve continued work on the 8-bit breadboard computer. I first added the control signals.

The bundle of yellow wires in the bottom right are the control lines that run to each module. The blue LEDs underneath the bundle will show the active signals.

The next step was to define the microcode and add the control logic using two eeproms. The code snippet below shows the instructions as a binary representation of the control signals above. The data array below that defines the opcodes.

In the image above, the bottom left breadboard contains the two eeproms that hold the control logic. Above that is a counter that counts the cycles in each opcode. The additional circuitry added to the left of the instruction register is the reset circuit. This is tied to the reset line of each module and is triggered by the push button.

I’m having issues now as the breadboard is expanding. I’m getting unreliable connectivity and garbage data at times. In the image above, I’ve added additional power lines and alligator clip leads to try and distribute the power more evenly, but this computer is still unreliable at best. I’m thinking of rebuilding it on perfboard and soldering it in place and I’ll probably add some bypass caps to the chips. That will hopefully fix some of the issues, but I may work on that later on down the road.

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