Acorn Atom

Acorn Atom
	Acorn Atom
Specifications
Manufacturer	Acorn Computers Ltd
Type	Home Computer
Released	March 1980
Discontinued	1982
Intro price	Kit: £120, Assembled: £170 (1980)
CPU	MOS Technology 6502 @ 1 MHz
Memory	2 KB RAM (expandable to 12 KB), 8 KB ROM
Storage	Cassette tape interface, optional floppy disk
Display	256×192 monochrome graphics, 32×16 text mode
Sound	Internal speaker (software-driven)
Dimensions	381 mm × 241 mm × 64 mm
Weight	1 kg
OS / Firmware	Acorn MOS with Atom BASIC
Predecessor	Acorn System 3
Successor	BBC Micro Model A/B

The Acorn Atom was a home computer manufactured by Acorn Computers Ltd from 1980 to 1982. Designed around the MOS Technology 6502 processor, the Atom served as Acorn's entry into the home computer market and established the technical foundation for the subsequent BBC Micro series.

Architecture

The Atom employed a straightforward single-board design with the 6502 processor running at 1 MHz. The system architecture provided direct CPU access to all peripherals through memory-mapped I/O, eliminating the need for complex decoding logic.

Memory Organization

**Acorn Atom Memory Map**
Address Range	Size	Function
$0000 – $09FF	2.5 KB	Block Zero RAM (lower text space)
$0A00 – $0AFF	256 bytes	Assembler workspace
$0B00 – $0BFF	256 bytes	Input buffer/Floating point workspace
$0C00 – $0FFF	1 KB	System variables and stack
$1000 – $1FFF	4 KB	Extension ROM socket A0
$2000 – $21FF	512 bytes	Catalog buffer
$2200 – $27FF	1.5 KB	Floating point variables
$2800 – $28FF	256 bytes	Graphics workspace
$2900 – $2FFF	1.75 KB	User BASIC programs (lower)
$3000 – $3BFF	3 KB	Extension RAM (optional)
$3C00 – $3FFF	1 KB	User BASIC programs (upper)
$8000 – $81FF	512 bytes	Video RAM (graphics mode)
$8200 – $83FF	512 bytes	Video RAM (text mode)
$8400 – $84FF	256 bytes	I/O space (6522 VIA)
$A000 – $AFFF	4 KB	Extension ROM socket A2
$B000 – $B7FF	2 KB	Peripheral I/O (optional)
$B800 – $BBFF	1 KB	VDU controller (6847)
$C000 – $CFFF	4 KB	Atom BASIC interpreter ROM
$D000 – $DFFF	4 KB	Extension ROM socket A1
$E000 – $EFFF	4 KB	Optional floating point ROM
$F000 – $FFFF	4 KB	Machine Operating System ROM

Hardware Specifications

Main Board Components

Component	Specification	Function
CPU	MOS 6502 @ 1 MHz	Central processor
VIA	MOS 6522	Versatile Interface Adapter for I/O, timers, and keyboard scanning
Video Controller	MC6847	Video Display Generator, 256×192 graphics, 32×16 text
RAM	2114 static RAM (1K×4 bit)	2 KB minimum, expandable to 12 KB
ROM	2532 EPROMs	8 KB system ROM (4 KB MOS + 4 KB BASIC)
Clock Generator	4 MHz crystal oscillator	Divided to 1 MHz CPU clock, 3.58 MHz video clock
Power Regulator	7805	+5V regulation for logic circuits
Keyboard Decoder	74LS138	Row/column scanning via 6522 VIA
Address Decoding	74LS138, 74LS00	Memory and I/O chip select generation

Power Supply

The Atom required an external power supply providing:

+5V DC @ 2A for logic circuits
-5V DC @ 100mA for video generation (MC6847)
Unregulated 8V AC @ 20VA for cassette motor control

Early units shipped with a linear transformer supply, while later models included a more efficient switched-mode unit.

Video System

The MC6847 Video Display Generator provided:

Mode	Resolution	Memory Required	Colours
Text	32×16 characters	512 bytes	Black on green
Graphics 1	64×64 pixels	1 KB	4 colours
Graphics 2	128×64 pixels	1 KB	4 colours
Graphics 3	128×96 pixels	1.5 KB	4 colours
Graphics 4	128×96 pixels	3 KB	2 colours
Graphics 5	128×192 pixels	3 KB	4 colours
Graphics 6	256×192 pixels	6 KB	Monochrome

Video output options:

UHF modulator for television connection (Channel 36)
Composite video output (1V peak-to-peak, 75Ω)
RGB output (optional modification)

Input/Output Interfaces

Standard Ports

Interface	Connector	Signals	Purpose
Cassette	7-pin DIN	Audio in/out, motor control	300 or 1200 baud data storage
Expansion Bus	64-way edge connector	Full 6502 bus	External peripherals
Printer	26-way IDC header	8-bit parallel + handshake	Centronics printer interface
User Port	20-way IDC header	8-bit bidirectional + control	General purpose I/O
Utility Socket	16-pin DIL	Address/data/control subset	Small expansion cards

6522 VIA Configuration

The 6522 Versatile Interface Adapter managed:

Port A: Printer data output (8 bits)
Port B bits 0-3: Keyboard row select
Port B bit 4: Cassette output
Port B bit 5: Loudspeaker output
Port B bit 6: Keyboard REPT key
Port B bit 7: Keyboard CTRL key
Timer 1: Cassette tape timing
Timer 2: Sound generation

Keyboard

The Atom featured a 60-key QWERTY keyboard with:

Full alphanumeric keys
CTRL, SHIFT, REPT (repeat) modifier keys
BREAK key (NMI interrupt)
LOCK key (caps lock)
COPY key (screen copy to printer)
Direct keyboard matrix scanning via 6522 VIA

Keyboard matrix organization:

8 rows × 10 columns matrix
Software debouncing in MOS ROM
Auto-repeat functionality
N-key rollover limited to 2 keys

Expansion Options

Internal Expansion

The Atom motherboard included sockets for:

Additional RAM: Up to 5 KB lower text RAM (Block Zero)
Video RAM: Up to 6 KB for high-resolution graphics
Upper text RAM: 3 KB expansion at $3000-$3BFF
Floating Point ROM: 4 KB scientific calculator functions
Utility ROMs: Three 4 KB ROM sockets for languages or utilities

External Expansion

Via the rear expansion connector:

Acorn Disk Pack: Dual 5.25" floppy drives with DOS
Econet Interface: Network adapter for educational environments
Colour Card: PAL colour encoder for MC6847
Atom Speechbox: SP0256 speech synthesizer
A/D Converter: 8-bit analog input card

ROM Software

Machine Operating System (4 KB)

Located at $F000-$FFFF, provided:

Hardware initialization routines
Keyboard scanning and decoding
Cassette tape I/O routines
VDU driver routines
Interrupt handlers
Monitor commands (memory examine/modify)

Atom BASIC (4 KB)

Located at $C000-$CFFF, featured:

Integer arithmetic only (no floating point in base model)
Direct memory access via ! and ? operators
Built-in assembler for 6502 code
Graphics commands (DRAW, MOVE, PLOT)
String handling (12 character maximum)
Array support (integer only)
Procedures with local variables

Optional Floating Point ROM (4 KB)

Located at $E000-$EFFF when installed:

5-byte BCD floating point format
9 significant digits, exponent ±99
Trigonometric functions
Logarithmic functions
Scientific notation support

Technical Innovations

The Atom introduced several design features:

Single-board construction reduced manufacturing costs
Memory-mapped I/O simplified hardware design
Built-in assembler in BASIC ROM
Direct video memory access from BASIC
Modular expansion system via edge connector

Component-Level Details

Clock Generation

4 MHz crystal oscillator (X1)
74LS163 counter divides by 4 for 1 MHz CPU clock
3.58 MHz NTSC subcarrier for MC6847 video timing
Phase relationships maintained for video stability

Address Decoding Logic

IC7 (74LS138) decodes A12-A15 for 4K blocks
IC8 (74LS138) further decodes I/O space
IC20 (74LS00) generates chip enables
Pull-up resistors on unused decoder outputs

Reset Circuit

RC network (R1=4.7kΩ, C1=10µF) provides power-on reset
Manual reset via BREAK key triggers NMI
74LS00 gate buffers and inverts reset signal
Reset pulse width approximately 47ms

Cassette Interface

Output: Square wave from 6522 PB4, AC-coupled via C15 (0.1µF)
Input: Comparator circuit using LM311 or similar
Motor control: Relay driver using BC108 transistor
Data encoding: Kansas City Standard (300 baud) or fast mode (1200 baud)

Known Hardware Issues

Common failure points requiring maintenance:

7805 voltage regulator overheating without adequate heatsink
MC6847 video chip thermal stress causing intermittent display
Keyboard matrix diodes failing causing phantom keypresses
2114 RAM chips prone to failure, especially in lower addresses
Edge connector oxidation causing expansion problems
Cassette relay contacts wearing from repeated use

Board Revisions

Revision	Changes	Issues Addressed
Issue 1	Original design	N/A
Issue 2	Improved ground plane	Reduced video interference
Issue 3	Modified reset circuit	More reliable power-on reset
Issue 4	Updated cassette interface	Better tape reliability
Issue 5	Final production version	Cost-reduced component selection