MicroMicro

The MicroMicro (working name) is a project designed for a series I inteded(intend) to write, about how microcomputers work and how to go about developing one. It was designed to use a relatively small amount of integrated circuits (13), not use any programmable chips, microcontrollers or obselete parts, and to be a self-contained architecture that handles its own input and output (ie. not serial based).

With these requirements in mind, the overall specs are humble but capable:

• an 65c02 8bit cpu, running at 2Mhz
• 32KiB of ram, 8K of which are video ram
• 256x240 black and white display, via vga
• 1bit "beeper" audio
• Ultra-compact 30 Key keyboard using the "gherkin" layout
• Tape Interface for loading and storing data
• Cartridge port for rom programs and peripheral expansion
• 2 atari-style joystick ports

Potential Features

• custom case made from pcbs

The software side is still up in the air, but I'm aiming for the following features:

• A text editor
• An high-level programming language (undecided on which)
• A built-in assembler
• Saving and loading via the tape interface

This code would likely be custom and documented to follow the article structure of the hardware side of things.

Memory Map

+---------------------+-------+-----------+
| Name                | Size  | Location  |
+---------------------+-------+-----------+
| Zero Page & Stack   | 512   | 0000-01FF |
| Video Data          | 7680  | 0200-1FFF |
| General Purpose RAM | 24576 | 2000-7FFF |
| IO Registers        | 8192  | 8000-9FFF |
| Expansion ROM       | 8192  | C000-DFFF |
| Bios ROM            | 8192  | E000-FFFF |
+---------------------+-------+-----------+ 
* the area from A000-BFFF is an invalid bus conflict region

Video Data Format

Each byte corresonds to a line of 8 horizontal pixels on the screen. The screen addresses are organized as 32x30 character cells of 8x8 pixels each. 8 Consecutive bytes build one character.

This non-linear format was chosen to speed up the rendering of characters to the screen.

IO Register

The state of the keyboard and joysticks can be read at the following addresses:

+---------------------+-------+-----------------------+
| Name                | Location  | Keys              |
+---------------------+-----------+-------------------+
| Keyboard Col 1      |    9FFE   | Q  A  Z  X  S  W  |
| Keyboard Col 2      |    9FFD   | E  D  C  V  F  R  | 
| Keyboard Col 3      |    9FFB   | T  G  sh al H  Y  |
| Keyboard Col 4      |    9FF7   | U  J  B  N  K  I  |
| Keyboard Col 5      |    9FEF   | O  L  M  sp en P  |
| Joystick 1          |    8FFF   | up dw lf ri tl tr |
| Joystick 2          |    97FF   | up dw lf ri tl tr |
+---------------------+-----------+-------------------+ 

Beeper

The Beeper is a 1bit register that is used for both audio generation and to save programs to tape. Its state is toggled whenever a write to the upper half of memory occurs $8000-$FFFF- Its current state can be read in bit 6 of the IO Register.

IC List

The design uses only 12 integrated circuits:

• 65c02 - 8bit CPU
• 62256 - 32KiB SRAM
• 39SF010 or 28C64 or 27C256 - 8KiB Program ROM
• 74hc10 - Tripple 3inNAND
• 74hc14 - Hex inverter
• 74hc32 - Quad 2inOR
• 74hc166 - 8bit parallel-to-serial shift register
• 74hc174 - 6bit d-type latch
• 74hc541 - 8bit buffer
• 74hc590*2 - 8bit Counters
• 74hc4520- Dual 4bit Counters

Operating System

The 8k bios provides a simple command line operating system with the following commands:

Command	Description
help	Lists all available commands
clear	Clears the display
file	The Size and Name of the currently loaded file
new	Create a new file
name <name>	Rename the current file
edit	Open the file editor
save	Save file to tape
load	Load file from tape
run	Run Loaded File
peek <addr> <bytes>	Read contents of Memory
poke <addr> <bytes>	Write to Memory
prun <addr>	Run a Machine Code Subroutine

Changelog

A2

Interactive BOM

• Reset is now performed using a MCP-100 voltage supervisor, this primarly serves to replace the 74HC14 Schmitt Trigger Hex NOT with a 74HCT04, reducing delays, improving reliabiliy of the clock oscillator circuit, and freeing 1 NOT gate for the video circuit
• Video glitches (leftmost pixel column doubled on right, junk at the bottom of the screen) are now gone
• VGA video output is now better centered on the screen
• The small 6mm buzzer has been replaced with an header for a small speaker, improving volume and sound clarity
• Filtering capacitor added to data output
• Needless diodes removed
• Fixed footprint for headphone jack
• 2 additional M2 screw posts added to the center region of the board
• Schematic rearranged