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A project log for Cat-644

An 8-bit computer based around an AVR ATMega-644 and junk-box SRAM.

Mark Sherman 09/26/2016 at 01:560 Comments

I had gotten super-busy with other things, and this project got shelved... I have a renewed interest in it since visiting the Vintage Computer Festival. I have create a to-do list to once the for all finish the project:

  1. Remove ethernet board. It was exciting to get ethernet added, but was premature and I don't like the way it is interfaced. The extra wiring also makes for a lot of noice on the bus.
  2. Finalize a base software image. The code I have on github is old, basically just tests the hardware. I want to at least have a base image (.hex) file that brings up all the hardware, and dumps the user in an interpreter environment.
  3. Filesystem: I have a driver that allows reading and writing of blocks on an SD card, and I have a simple block allocation/delteion/freelist scheme. A file is a linked list of disk blocks. If you allocate a block, you can chain many blocks off of it, and if you know the starting block number of a file, you can read the whole file. I considered FAT, but FAT has 4k clusters, and I'd rather work with 1 disk block at a time (512 bytes). Also, the fun of the project is building it yourself. A directory of files will likely be a regular file that just happens to have a list of files in it and their starting block numbers.
  4. Hardware rebuild: The physical hardware is a mess. It works. I will probably uncover problems as the software develops. I would like to buffer-isolate the external ram address bus from the spi bus as best as possible. Right now just too much happening directly off of PORTB. MISO definately needs really special handing, as it needs to accept: input logic from 3.3 devices, needs to be able to output 5v logic to ram addresses.
  5. re-add ethernet

I have also considered adding a attiny84 or atmega328 to the project as a coprocessor to handle the sound, keyboard, disk, ethernet (its a "southbridge"), but someday there might be a CAT-644 R2.0.

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