Hello

I have been working on this computer project for quite a while.  I had not originally intended it as an entry to the Hackaday prize.  My original intent was to finish the project, and document the schematics and code online.  I saw another couple computer entries entered into the contest, so I figured, if I was going to give away the design anyway, I might as well post it all here, and get a ~ 1/300 chance of blasting off into space.

I'm am going to make a series of posts in the build log, showing from the beginning of the project, what has happened.  I've collected notes and pictures along the way, and I will be presenting these in a time-compressed manner.

Why another computer?

I have been playing around with computers since a young kid.  One of the first computers I played with was the Commodore Vic-20.  It was very limited in its abilities, but it was a start.  I eventually got into PCs, and now I'm a software enginner.  I've also been very interested in electronics.  For a long time, I've been wanting to build my own computer.  In college we simulated computers in FPGAs; that's probably the closest to realizing my own computer design in hardware.

Why 8-bit, why AVR?

I wanted to build it myself.  32-bit and 64-bit devices tend to be surface mount with many pins.  I also wanted a device that has a lot of support, was reasonably performant, and with fairly modern components.  I wanted to use all through-hole parts, making it more likely that others can learn from, copy and modify the design.  Large parts of this project can be built, and run, on a breadboard without even soldering anything.  Also, I am considering that someday this could be made as a kit, similar to the Uzebox of Maximite kits available online.

When it comes to microcontrollers, there's 2 big camps: the AVR and the PIC.  Both are awesome, and come with flash and itty bitty ram, letting you start off right away.  Why did I choose AVR?  For two reasons.  1) an AVR can execute most instructions single cycle; PIC cannot, and  2)Arduino is AVR-based, and there is a wealth of information about Arduino.  If you can connect an Arduino to a whatevery-do, you can connect an AVR to it as well.

Also, before someone mentions it, I do know about the Propeller.  It is a 32-bit 8-core DIP chip, has comfortable 32k of RAM, and can generate VGA signals.  I did not want to use this, because simply too much of my project is already done for me. 

Why VGA and PS/2?

Why not VGA?  Most homebrew computers offer either composite video output, OR a simple terminal.  I don't want a TV on my desk too!  With VGA output, it can use the same monitor my desktop computer uses.  With the right KVM, your keyboard too!

Connectivity

Well, this is a connected device competition!  There is a lot of inter-device communication going on in this project:

internal:

• bit-bang SRAM interface: don't take RAM for granted.  address, page latch, read and write enable, are all controlled by software

external:

• RS-232: The classic interface.
• VGA:  mixed analog/digital output with extremely precise timing required
• PS/2 keyboard protocol:  Unusual in both software and hardware.  A PS/2 keyboard communicates over a clocked, open collector bus, and the clock is generated by the keyboard.
• SD Card interface: SPI bus with level conversion required
• Planned: Ethernet.  This is a stretch goal for the project.

Anyone who builds and programs a Cat-644 is going to learn a lot!  

Why SD cards?

SD Cards can be put in an SPI mode.  Conveniently, AVR microcontrollers have hardware SPI.  In SPI mode, SD cards have no minimum data transfer speed, and a maximum speed of 20 or 25 megabits.  That is perfect for a 20 MHz system, which may have a very frazzled CPU that needs to slow down.  Data is also read/written in blocks of 512 bytes, which very easily fits in the 4k internal RAM of the atmega.   I considered compact flash/IDE, but that takes too many I/O lines.  USB mass-storage and SATA are too complex and fast for the poor little AVR.  There's also SPI flash chips, with which I could build cartridges, but that's pretty much what the SD card already is!