One Bit CPU Runs At A Blistering 60Hz

If you really think hard about it, a CPU is just a very general-purpose state machine. Well, most CPUs are, anyway. The MC14500 is a one-bit computer that has only 16 instructions and was meant to serve in simple tasks where a big CPU wouldn’t work for space, power, or budget reasons. However, [Laughton] took the idea one step further and created a single-bit computer with no real instructions to control a printing press. The finished machine uses a clever format in an EEPROM to drive an endless program.

Honestly, we’d say this is more of a state machine, but we like the idea of it being a minimal CPU which is also true. The design uses the EEPROM in an odd way. Each CPU address really addresses a block of four bytes. The byte that gets processed depends on the current phase and the status of the one-bit flag register.

Each four-byte block has two sections. There are two instructions that read inputs and sets outputs. The first instruction executes if the flag register is false and the second executes if the flag register is true. All of the inputs and outputs are, of course, single bits. The second pair of bytes are two addresses for the next instruction. Again, the first instruction is for a false flag register and the second is for a true flag register.

The I/O is a bit peculiar. Each phase 1 instruction selects an input to store in the flag register and can set or clear one of 7 output bits. Of course, you might not want to write an output on each instruction, so there is an eighth bit that isn’t connected. Writing to it has no effect.

There’s no actual RAM memory, but the design loops back two output bits to the input bits so that you can store two bits and read them back later. Really, sort of an EEPROM-based state machine but still a fun idea. We’ve seen a few MC14500 builds, too, if you want to find out more about what inspired this machine.

12 thoughts on “One Bit CPU Runs At A Blistering 60Hz

  1. Old 2716 trick used to be driving 7-segment displays by putting in the segment combinations at each address point and multiplexing the output. That of course was when they were plentiful. I still have about 25 of them in my chip tray – don’t know why. Seems a shame to get rid of a device that’s so easy to use. But you could program for things like hex output, so that was cool.

    1. Yup, using parallel I/O (E)(P)ROMs as a look-up table (LUT) is a time-honored way to implement any combinatorial logic you want as long the # of inputs is <= # of address bits and the # of outputs is <= # of data bits. Hold onto the 2716's, I say.

    1. a one bit cpu is about the simplest possible computational circuit, though it doesn’t look that way with all the parallel eeprom address lines. I guess if you wanted to simplify it farther and dispense with those you would need to use a mercury delay line.

  2. I once built a lift controller using this chip. The controller used contacts wired in parallel on each floor to indicate when a new floor was reached and two extra contacts, one at the bottom to calibrate the floor counter and one at the top to prevent the lift trying to break through the roof.

    When turned on, the controller would recalibrate by sending the lift downwards until the bottom contact was closed, which would also reset the floor counter. Going up, each closure of a floor contact would increase the counter and going down each floor contact would decrease it. There were also contacts to check that the doors were fully closed.

    The whole setup worked flawlessly, it seemed, until I realised it had a minor problem: When turned on it would recalibrate (yes, I know I said it before) but — and what a BUT it is — it would not check that the door were closed and not issue the command to close before it set off downwards.

    Oops!

    Luckily, I found this fault during testing and no elderly ladies were harmed in the process.

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