MicroMicro

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.

Keyboard and Joystick

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

+---------------------+----------+-------------------------+
| Name                | Address  | Value (bits 0-5)        |
+---------------------+----------+-------------------------+
| Keyboard Col 1      |   9FFE   |  Q   A  Shf  z   S   W  |
| Keyboard Col 2      |   9FFD   |  E   D   X   C   F   R  |
| Keyboard Col 3      |   9FFB   |  T   G   V   B   H   Y  |
| Keyboard Col 4      |   9FF7   |  U   J   N   M   K   I  |
| Keyboard Col 5      |   9FEF   |  O   L  SPC ALT ENT  P  |
| Joystick 1          |   8FFF   |  ↑   ↓   ←   →   L   R  |
| Joystick 2          |   97FF   |  ↑   ↓   ←   →   L   R  |
+---------------------+----------+-------------------------+

Beeper and Tape Interface

The Beeper is a 1bit register that is used for both audio generation and to load and save programs to tape. It can be accessed through the following registers:

+---------------------+-------------+-----------+
| Name                | Address     | Value     |
+---------------------+-------------+-----------+
| State               | 9FFF (read) | TB.. .... |
| Toggle Beeper       | FFFF (write)| 0000 0000 |
+---------------------+-------------+-----------+
T = Tape Input
B = Beeper State (top 2 bits of input register)

The tape interface is a 4-pin trs connector that be connected directly to a computer using an appropriate aux cable.

Note: While these registers can be accessed from alternative locations, the addresses and values on this table ensure no bus conflicts occur.

Expansion Interface

The MicroMicro has an expansion interface that allows extending the base capabilities of the system in various ways:

Generic Banked Expansion Rom

An 8KiB region is set aside in the memory map for expansion roms ($C000-$DFFF), the emulator implements a simple banked rom cartridge where the current bank can be switched by reading from $A0xx, where xx is the bank number, the selected bank is also automatically set to zero on reset.

The bios will automatically jump to address $C008 on boot if it detects an expansion rom that begins with the sring CART-RUN.

Currently expansion roms can be loaded by dragging and dropping a file into the emulator window.

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:

More info on the bios can be found here

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