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| type        = Home Computer
| type        = Home Computer
| release date = March 1980
| release date = March 1980
| discontinued = 1982
| discontinued = 1983
| price        = Kit: £120, Assembled: £170 (1980)
| price        = Kit: £120, Assembled: £170 (1980)
| cpu          = MOS Technology 6502 @ 1 MHz
| cpu          = MOS Technology 6502 @ 1 MHz
| memory      = 2 KB RAM (expandable to 12 KB), 8 KB ROM
| memory      = 2 KB RAM (expandable to 12 KB), 8 KB ROM (expandable to 12 KB)
| storage      = Cassette tape interface, optional floppy disk
| storage      = Cassette tape interface (CUTS, 300 baud), optional floppy disk
| display      = 256×192 monochrome graphics, 32×16 text mode
| display      = 256×192 monochrome graphics, 32×16 text mode (MC6847)
| sound        = Internal speaker (software-driven)
| sound        = 1 channel, internal speaker
| dimensions  = 381 mm × 241 mm × 64 mm
| dimensions  = 381 mm × 241 mm × 64 mm
| weight      = 1 kg
| os          = Acorn MOS with Atom BASIC
| os          = Acorn MOS with Atom BASIC
| predecessor  = [[Acorn System 3]]
| predecessor  = [[Acorn System 3]]
| successor    = [[BBC Micro Model A/B]]
| successor    = [[BBC Micro Model A/B]]
| codename    =
| model        = PCB 202,000
| model        =  
}}
}}


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.
The '''Acorn Atom''' was a home computer made by Acorn Computers Ltd from 1980 to 1983, when it was replaced by the [[Acorn Electron]] and the [[BBC Micro Model A/B|BBC Micro]] as Acorn's low-cost option.<ref name="wp">[https://en.wikipedia.org/wiki/Acorn_Atom "Acorn Atom"], Wikipedia. Source for the 1980&ndash;1983 production span, pricing, the nine MC6847 video modes (64×64 4-colour up to 256×192 monochrome), the 50&nbsp;Hz PAL colour-card requirement, the 300&nbsp;baud CUTS cassette interface, the dual on-board LM7805 regulators, and the memory map.</ref> It was a cut-down [[Acorn System 3]] built into a case with an integral keyboard and a cassette interface, sold in kit or assembled form. The Atom used the MOS Technology 6502 and was designed around an 8&nbsp;KB ROM holding the operating system and Acorn's own Atom BASIC, written by Sophie Wilson.
 
The minimum machine had 2&nbsp;KB of RAM and 8&nbsp;KB of ROM; the maximum specification had 12&nbsp;KB of each, plus an optional floating-point extension ROM. The case was designed by industrial designer Allen Boothroyd of Cambridge Product Design.<ref name="wp" />


== Architecture ==
== 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.
The Atom used a single double-sided PCB (part 202,000) with the 6502 running at 1&nbsp;MHz and memory-mapped I/O. The system ROM held the Machine Operating System and Atom BASIC; the BASIC interpreter included an in-line 6502 assembler, which was unusual for the period.<ref name="wp" />


=== Memory Organization ===
=== Memory map ===


{| class="wikitable styled-table" style="width:70%; text-align:center;"
{| class="wikitable styled-table" style="width:75%; text-align:left;"
|+'''Acorn Atom Memory Map'''
|+'''Acorn Atom memory map'''
! Address Range !! Size !! Function
! Address !! Contents
|-
| $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)
| $0000 || Block Zero RAM (1 KB: zero page, 6502 stack, OS and BASIC variables)
|-
|-
| $8400 – $84FF || 256 bytes || I/O space (6522 VIA)
| $0400 || Text / VDG screen RAM (low)
|-
|-
| $A000 – $AFFF || 4 KB || Extension ROM socket A2
| $0800 || VDG / CRT-controller area
|-
|-
| $B000 – $B7FF || 2 KB || Peripheral I/O (optional)
| $0A00 || Optional floppy-disc controller (FDC)
|-
|-
| $B800 – $BBFF || 1 KB || VDU controller (6847)
| $1000 || Peripheral / extension space
|-
|-
| $C000 – $CFFF || 4 KB || Atom BASIC interpreter ROM
| $2000 || Catalogue buffer
|-
|-
| $D000 – $DFFF || 4 KB || Extension ROM socket A1
| $2200 || Sequential file buffers
|-
|-
| $E000 – $EFFF || 4 KB || Optional floating point ROM
| $2800 || Floating-point variables (internal RAM, up to 5 KB)
|-
|-
| $F000 – $FFFF || 4 KB || Machine Operating System ROM
| $2900 || Extension text-space RAM
|}
 
== Hardware Specifications ==
 
=== Main Board Components ===
 
{| class="wikitable styled-table" style="width:90%; text-align:center;"
! Component !! Specification !! Function
|-
|-
| '''CPU''' || MOS 6502 @ 1 MHz || Central processor
| $3C00 || Off-board extension RAM
|-
|-
| '''VIA''' || MOS 6522 || Versatile Interface Adapter for I/O, timers, and keyboard scanning
| $8000 || Video and BASIC RAM (mode-dependent, up to 6 KB)
|-
|-
| '''Video Controller''' || MC6847 || Video Display Generator, 256×192 graphics, 32×16 text
| $A000 || Optional utility ROM
|-
|-
| '''RAM''' || 2114 static RAM (1K×4 bit) || 2 KB minimum, expandable to 12 KB
| $B000 || INS8255 PPI I/O device
|-
|-
| '''ROM''' || 2532 EPROMs || 8 KB system ROM (4 KB MOS + 4 KB BASIC)
| $B800 || Optional 6522 VIA (printer interface)
|-
|-
| '''Clock Generator''' || 4 MHz crystal oscillator || Divided to 1 MHz CPU clock, 3.58 MHz video clock
| $C000 || Atom BASIC interpreter ROM (4 KB)
|-
|-
| '''Power Regulator''' || 7805 || +5V regulation for logic circuits
| $D000 || Optional extension ROM
|-
|-
| '''Keyboard Decoder''' || 74LS138 || Row/column scanning via 6522 VIA
| $E000 || Optional DOS / floating-point ROM
|-
|-
| '''Address Decoding''' || 74LS138, 74LS00 || Memory and I/O chip select generation
| $F000 || MOS and assembler ROM (4 KB)
|}
|}


=== Power Supply ===
Source: Acorn Atom memory map.<ref name="wp" />


The Atom required an external power supply providing:
== Main board components ==
* '''+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.
The IC designators below are from the [https://chrisacorns.computinghistory.org.uk/docs/Acorn/Manuals/Acorn_AtomTechnicalManual.pdf Acorn Atom Technical Manual] parts list.<ref name="atomtm">[[Acorn Atom Technical Manual]] (Issue 2, October 1980), Acorn Computers &mdash; hosted on this wiki. Source for the IC complement (IC22 6502, IC31 6847, IC25 INS8255, IC26 7445, IC20 MM52164 ROM, IC42/43/51/52 2114 RAM, IC53/IC54 LM340T-5 regulators), the crystals (X1 3.58&nbsp;MHz, X2 4.00&nbsp;MHz), the capacitor list, and the 8&nbsp;V supply with the LK6/LK7 5&nbsp;V conversion.</ref>


== Video System ==
{| class="wikitable styled-table" style="width:95%; text-align:center;"
 
! Component !! Device !! Designator !! Function
The MC6847 Video Display Generator provided:
 
{| class="wikitable styled-table" style="width:70%; text-align:center;"
! Mode !! Resolution !! Memory Required !! Colours
|-
|-
| Text || 32×16 characters || 512 bytes || Black on green
| '''CPU''' || MOS 6502 @ 1 MHz || IC22 || Central processor
|-
|-
| Graphics 1 || 64×64 pixels || 1 KB || 4 colours
| '''Video''' || Motorola MC6847 VDG || IC31 || 256×192 graphics / 32×16 text
|-
|-
| Graphics 2 || 128×64 pixels || 1 KB || 4 colours
| '''Peripheral interface''' || INS8255 PPI || IC25 || Keyboard, cassette and loudspeaker I/O
|-
|-
| Graphics 3 || 128×96 pixels || 1.5 KB || 4 colours
| '''Keyboard row driver''' || 7445 BCD-to-decimal decoder || IC26 || Drives the keyboard matrix rows
|-
|-
| Graphics 4 || 128×96 pixels || 3 KB || 2 colours
| '''VIA (optional)''' || MOS 6522 || (B800) || Printer (Centronics) interface &mdash; fitted only on expanded machines
|-
| 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 ===
 
{| class="wikitable styled-table" style="width:90%; text-align:center;"
! Interface !! Connector !! Signals !! Purpose
|-
|-
| '''Cassette''' || 7-pin DIN || Audio in/out, motor control || 300 or 1200 baud data storage
| '''RAM''' || 2114 static RAM (1K×4) || IC42, IC43, IC51, IC52 (+ expansion) || 2 KB base, expandable to 12 KB
|-
|-
| '''Expansion Bus''' || 64-way edge connector || Full 6502 bus || External peripherals
| '''ROM''' || MM52164 mask ROM (base) || IC20 || 8 KB system ROM (MOS + BASIC)
|-
|-
| '''Printer''' || 26-way IDC header || 8-bit parallel + handshake || Centronics printer interface
| '''Clocks''' || 4.00 MHz + 3.58 MHz crystals || X2, X1 || CPU clock (÷ to 1 MHz) and MC6847 video timing
|-
|-
| '''User Port''' || 20-way IDC header || 8-bit bidirectional + control || General purpose I/O
| '''Cassette amp''' || LM358 op-amp || IC46 || CUTS read/write circuit
|-
|-
| '''Utility Socket''' || 16-pin DIL || Address/data/control subset || Small expansion cards
| '''Regulators''' || 2× LM340T-5 (7805) || IC53, IC54 || +5 V regulation, one per board half
|}
|}


=== 6522 VIA Configuration ===
== Video system ==


The 6522 Versatile Interface Adapter managed:
The MC6847 Video Display Generator provided text (32×16) and graphics up to 256×192 monochrome, across nine modes from 64×64 in four colours to 256×192 monochrome. The MC6847 is a 60&nbsp;Hz NTSC device, so on European 50&nbsp;Hz sets the standard machine produced a monochrome picture; an Acorn 50&nbsp;Hz PAL colour card (Acorn drawing 102,006-C) was available to add colour.<ref name="wp" /> Output was via a UHF modulator (channel 36); a composite-video output was available by modification.
* '''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 ==
== Keyboard, cassette and sound ==


The Atom featured a 60-key QWERTY keyboard with:
The keyboard, cassette interface and loudspeaker are handled by the '''INS8255 PPI (IC25)''', with the '''7445 (IC26)''' driving the keyboard-matrix rows; the optional 6522 VIA is used only for the printer interface on expanded machines.<ref name="atomtm" /> The full-travel QWERTY keyboard includes CTRL, SHIFT, REPT (repeat) and BREAK keys, with software debouncing in the MOS ROM.
* 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:
The cassette interface uses the CUTS variant of the Kansas City standard at '''300&nbsp;baud only''', built around the LM358 op-amp (IC46) and transistors Q1/Q2 (BC107). There is no internal cassette deck or motor relay. Commercial BBC Micro tapes (1200&nbsp;baud) cannot be loaded on the Atom.<ref name="wp" />
* 8 rows × 10 columns matrix
* Software debouncing in MOS ROM
* Auto-repeat functionality
* N-key rollover limited to 2 keys


== Expansion Options ==
== Power supply ==


=== Internal Expansion ===
The Atom takes '''8&nbsp;V DC (unregulated)''' on a 2.1&nbsp;mm jack (SK3) and regulates it on the board to '''+5&nbsp;V''' using two LM340T-5 (7805-type) regulators, IC53 and IC54, each feeding one half of the logic, with a heatsink fitted between them. There is no &minus;5&nbsp;V rail and no AC inside the machine.<ref name="atomtm" /> The Acorn adaptor is rated 8&nbsp;V at about 1.5&ndash;1.8&nbsp;A, which is marginal: a fully expanded Atom can draw up to 3&nbsp;A, and the two regulators run hot.<ref name="wp" /> The board can instead be powered from a 5&nbsp;V regulated supply by fitting links LK6 and LK7, which join the two power sections and bypass the regulators.<ref name="atomtm" />


The Atom motherboard included sockets for:
== Input/output and expansion ==
* '''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 ===
Standard ports: a 7-pin DIN cassette socket (SK2), the UHF output (SK1), and the 8&nbsp;V DC jack (SK3). The rear 64-way edge connector carries the full 6502 bus for Acorn Eurocard expansion. Expansion options fitted into the on-board sockets and via the bus included additional lower-text RAM (in 1&nbsp;KB steps to 5&nbsp;KB), video graphics RAM (to 6&nbsp;KB), the 4&nbsp;KB floating-point ROM, a 4&nbsp;KB utility ROM, the 6522 VIA and Centronics printer port, the Acorn Disc Pack, an Econet interface, the PAL colour card and a speech board.<ref name="atomtm" />


Via the rear expansion connector:
== Board revisions ==
* '''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 ==
The Atom main PCB carries the part number '''202,000''' on the silkscreen with an issue number. The Technical Manual documents Issue&nbsp;2 (October 1980); production boards are commonly seen up to Issue&nbsp;4.<ref name="chris">Whytehead, Chris. [https://chrisacorns.computinghistory.org.uk/Computers/Atom.html "Acorn Atom"], Chris's Acorns / The Centre for Computing History. Documents the 202,000 Issue&nbsp;4 board, the base IC complement, and the recommendation to use a regulated 5&nbsp;V supply for a fully expanded machine.</ref>


=== Machine Operating System (4 KB) ===
== Known hardware issues ==


Located at $F000-$FFFF, provided:
* The two LM7805 regulators (IC53, IC54) run hot; on a fully expanded machine the supply is marginal and thermal shutdown causes intermittent resets.<ref name="wp" />
* Hardware initialization routines
* 2114 static RAM is a common failure part, producing corrupted text/graphics or crashes.
* Keyboard scanning and decoding
* The rear edge-connector fingers are exposed and oxidise, causing expansion-bus faults.
* Cassette tape I/O routines
* Socketed ICs (ROM, RAM, MC6847, 8255) develop intermittent contacts and benefit from reseating.
* VDU driver routines
* Interrupt handlers
* Monitor commands (memory examine/modify)


=== Atom BASIC (4 KB) ===
For diagnosis and repair, see the maintenance, troubleshooting and capacitor pages below.


Located at $C000-$CFFF, featured:
== See also ==
* Integer arithmetic only (no floating point in base model)
* [[Acorn Atom General Maintenance]]
* Direct memory access via ! and ? operators
* [[Acorn Atom Troubleshooting Guide]]
* Built-in assembler for 6502 code
* [[Acorn Atom Capacitor Replacement Guide]]
* Graphics commands (DRAW, MOVE, PLOT)
* [[Acorn Electron]]
* String handling (12 character maximum)
* [[BBC Micro Model A/B]]
* 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:
== References ==
* '''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 ==
<references />
 
{| class="wikitable styled-table" style="width:80%; text-align:center;"
! 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
|}
 
== See Also ==
* [[Acorn Atom General Maintenance]]
* [[Acorn Atom Troubleshooting]]
* [[Acorn Atom Capacitor Replacement Guide]]


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