68000 Tablet - First test

Written by Stephen April 21, 2020

I have finally received enough parts to run a basic test of the 68000 tablet computer I designed. The main thing that I was waiting on was female headers, so that I could install the 68000 into the board. Here it is running:

I don't have enough parts for the address decoding circuit, so I'm unable to use ROM or RAM (nor has the ROM arrived). Instead, I used 16 220 ohm resistors to pull the data bus to 0x00. This corresponds to the instruction OR.b #0, d0 , which does nothing. The result of this is that the CPU races up the memory address space, executing the same instruction as it does so. The green LEDs are connected to the address bus, and we can see it counting up in binary, looping back to 0 when the bus overflows.

A few other jumpers can be seen around the board, pulling CPU pins high and low to compensate for chips that aren't installed.

Overall, this was a success! I'm looking forward to the arrival of the rest of the parts so I can get this board finished.

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