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How to build your own homebrew computer: getting started

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Hi! Thanks! I have never "blogged" before so I am not really sure how to proceed. I think what I am going to just discuss the latest topics in my own homebrew computing projects and if people as questions or are interested in something I'll elaborate on the topic. Please note, I will try to keep the topic to homebrew computing and not just a ongoing description of the N8VEM project. Obviously I am biased towards it so I offer no pretense of objectivity regarding the subject. If people are interested in it then they know to contact me separately and we can discuss further.

How do you get started building your own homebrew computer? Well, that is a very important topic and it really depends on you. What are you interested in? What are your objectives? What will your system do when it is "done"? Those are really quite personal decisions but there are some things that will be in common regardless of your choices:
1. You need a goal.
2. You need a plan.
3. You need a design.
4. You should make a schematic and keep it updated.
5. You should make a prototype to verify your design.

My personal goal is to make a computer that can run FLEX or CUBIX. That means a 6809 CPU. My plan is to review existing 68xx designs for ideas on how to make a system but also leverage the investment of my current system. There are many good 68xx designs available for study and here are a couple of interesting projects:

http://www.anynewgizmos.com/simon680...simon6809.html

http://www.sbprojects.com/projects/n...2/nano6802.htm

I needed a plan so what I did was build each of the above systems on a breadboard and studied them. Then I made my own design and wrote a schematic. It would plug into the ECB backplane and communicate with the SBC via a parallel to parallel "bridge". The plan is good because it was simple and easy to design, build, and verify. It worked great but after using it for a while it became clear that it lacked sufficient IO and autonomy that I really wanted.

I studied some more and read some datasheets. I discussed the topic on comp.sys.m6809 and considered several approaches. Finally I settled on a technique of using a mezzanine board to host IO chips and an expansion bus. The IO mezzanine contains enough simple IO for the 6809 to run independently of the ECB because it has a power interface, a serial port, programmable timers, and many parallel IO pins. I built a prototype using prototyping boards to verify the design and once satisfied the design was sound designed a PCB and had it manufactured. I am currently testing the 6809 computer and it seems to work although testing is not yet complete.

My 6809 computer approach is a specific example of the approach which can be applied to any homebrew project. The technology involved is really not relevant for the most part. The CPU or microcontroller (6809, Z80, 8051, PIC, AVR, Propeller, etc) all behave in a similar way and have much in common.

Thanks and have a nice day!

Andrew Lynch

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  1. andrew's Avatar
    Andrew:

    I must say you are a genius. I tried the Simon6809 at the site: http://www.anynewgizmos.com/simon680...simon6809.html but it didn't worked like it should. The (line) assembler always return an error saying invalid op code even if i typed in rts. Do you remember having any problem while using the assembler?

    Thanks
    andrew (in lower case )