A couple of years ago, I encountered the breadboard computer project by Ben Eater over at eater.net. I found the idea interesting so I purchased his kits for the 8 bit breadboard computer project. I then embarked on the project and put together all the modules. It “worked” for some value of “work” but there were clearly some issues with it. Given the complexity of it and the number of bits where it violates good design practices (it’s a hobby learning project), I was unable to definitively trace the problems. I have some theories, however.
I have now decided to embark on a new project. I’m still going to build a breadboard computer with an 8 bit processor. However, I’m going to build my own design rather than Ben’s. His was fun, but it’s my hobby so why not?
So, I have a couple of rules for this project:
- All ICs get decoupling capacitors. In the worst case, that does nothing but it should help a lot with instability.
- All unused inputs on logic gates get tied high or low. A similar situation for other types of chips where it makes sense. Basically, avoid floating inputs.
- Lines should be buffered between a module and a bus to avoid feedback and other complications. Where possible and practicable, high impedence (tri-state) should be used. This is to minimize the load for anything driving the bus.
- Components are mostly limited to basic parts (resistors, capacitors, diodes), 74-series logic (and other) chips, and items where appropriate (RAM for instance). Components used should not be listed as “obsolete”.
The overall goal is to create a computer that has roughly the capabilities of a MC6809 CPU. That’s a long way down the road, however. What that means, however, is that there will be an 8 bit data bus and a 16 bit address bus between the processor and the memory and any memory mapped I/O devices. What the internal connections within the processor look like is undetermined at this time.
My general plan is to add entries here as I work through the project. As we all know, however, I’m likely to forget or get bored with entries so if they stop, well, maybe poke me and see what’s going on.
The next post will be about the actual build.