Thursday, November 24, 2016

Building a 68000 Single Board Computer - High Level Plan

After spending some time studying the schematic and accompanying text in the book, I came up with this overall project plan:

Phase 1:

  1. Enter the schematic into a CAD program, make some changes to simplify it, and check it carefully for errors.
  2. From the schematic, generate a parts list or BOM (bill of materials).
  3. Do a rough layout of the ICs for a wirewrapped prototype to estimate the board size needed.
  4. Find sources for and obtain the majority of the parts needed.

Phase 2:

  1. Breadboard a minimal circuit to get the CPU to run in freerun mode.
  2. Test the power on reset circuit.
  3. Test the pushbutton reset circuit.

Phase 3:

  1. Start building the circuit on a protoboard with wirewrap sockets.
  2. Get the CPU running from ROM (with a test program like a simple loop that will run without any RAM) and minimal chips.
  3. Check ROM, address decoding, DTACK and BERR timing.
  4. Test the single step circuit.

Phase 4:

  1. Add RAM, verify RAM is working with some simple test code.

Phase 5:

  1. Add baud rate generator, serial UARTs, and RS-232 ports.
  2. Program ROMs and get the existing monitor program working.

Phase 6:

  1. Fully test board including all features of the monitor program.
  2. Possibly add more circuitry that was omitted, like interrupt encoding and acknowledge.
  3. Maybe try some other software like Motorola's TUTOR monitor from the ECB board or a BASIC interpreter.
  4. Install in case, maybe add power supply or a 5V regulator.

The thinking is to just make one wirewrapped prototype. I have no plans for a printed circuit board, although that might be a fun project to learn more about PCB layout and might make it easier for others to duplicate the board. The intention is to make all of the design information (both hardware and software) freely available.

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