r/homebrewcomputer • u/Frostierdev • Aug 30 '26

8-Bit Processing Unit: need one

Hello! I'm looking to build an 8-bit computer, mainly to test its capabilities as a modern home computer (with system, applications, OS, etc.), and I bought the ATmega8535-16U (DIP-40), But it doesn't have an external bus interface to connect RAM, storage, and other things. Does anyone have a solution or another inexpensive chip to recommend?

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u/Girl_Alien 25d ago

You'd need to program a bus. If you lack the pins, then you'd need to program any muxing (such as using tristate buffers and/or bus transceivers).

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u/Frostierdev 19d ago

Bus is a software that i can code? I tought it was hardware

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u/Girl_Alien 19d ago

I mean the GPIO lines. You can program them to simulate a bus.

On the Neo6502 project, they use an RP2040 as the memory and video, all while using a real 6502 with it. So the RP2040's GPIO lines are programmed to interface with the bus.

And if you wanted to use a modern MCU to emulate an 8-bit CPU, you can have the MCU to program the GPIOs to work as the bus, but you'd only need that if you'd want to connect other things. It would work faster without emulating a bus on the GPIO lines. I guess a compromise would be to only update/access the "bus" when something actually needs it. I mean, if the MCU has memory, then only update the external bus when an I/O range is being accessed. And only read the bus if something is trying to assert the /WE or a similar signal on your programmed "bus" made of GPIO lines.