Acorn Atom reworked

This is my version of the Acorn Atom computer, with a few improvements.
I’ve tried to make it easy to build by only using through hole components which were mostly available in the 1970’s.
This version has 32K RAM, 4K RAM at #A000, Screen noise killer circuit etc
I’ve also designed a case which can be 3D printed.

ACORN ATOM REWORKED PCB updates

1. Reset circuit added
2. 2114 RAM replaced with 62256 / 6264 chips
3. 4K RAM switchable at #A000
4. RAM from #0000 to #7FFF or #0000 to #03FF & #2000 to #7FFF
5. 27512 ROM used giving two OS settings (ATOMMC2 or floppy disk)
6. Screen noise killer circuit added for flicker free graphics
7. Obsolete buffer chips replaced with 74 series
8. Separate keyboard using Outemu switches (Cherry MX)
9. Configurable  CPU clock. 1, 2 or 4Mhz
10. #B400 chip select added to expansion port
11. Video out connector added
12. CPU clock circuit modified

Building the Atom

Parts List

NOTES
IC26 must be HCU version for CPU clock to run
IC9 must be LS version for Video clock to run
IC15 works best with a 74LS14 or 7414 (74HC14 gives noise)
All other 74 series should be OK with LS, HC etc
6502 – Cheap replica’s fail BCD tests and can have timing issues! Genuine old stock work fine

Speaker 16 ohm 0.5W 40mm

EXPANSION CONNECTOR
Farnell 1096906 (Harting 09 02 264 6824) for internal use, female
Farnell 1106779 (Harting 09 72 164 6903) for internal use, male
Male right angle (no part code yet!)

Main PCB

Keyboard

Top

Bottom

Keyboard Assembled

Assembled with Stickers

Outemu switches, keycaps and spacebar stabilizer from Aliexpress
Spacebar is 6.25U
I used the black switches as these give a nice, non clicky feel.
Some keycaps require a sticker for the correct keyboard designation

189 Key PBT Keycap Double-shot Green White ISA Keycaps Kit Backlit Key Cap Cherry MX For Wireless Mechanical Gaming Keyboards

Space bar stabiliser parts. Only the long bar is required, you have to buy the whole kit.

Customization PCB Satellite Axis For Cherry Mechanical Keyboard OEM Plate mounted Black 6.25u 2u Stabilizers Modifier Keys

Keyboard Linking Ribbon

Link Settings
LK1 EPROM #E000 enable left
LK2 Enable IRQ for IC1
LK3 PL8 IRQ Enable
LK4 CPU, left 1MHz, right 2MHz, up 4MHz 
LK5 4MHz clock to IC44
LK6  Left RAM at #0000 – #03FF & #2000 – #7FFF, Right RAM at #0000 – #7FFF
LK7 System ROM left low (#0000 – #7FFF), right high (#8000 – #FFFF)
Mapped to;
#A000 – #AFFF (if selected, Extension ROM)
#C000 – #CFFF BASIC
#D000 – #DFFF Floating Point
#E000 – #EFFF Atommc2 or Floppy disk
#F000 – #FFFF Kernal

LK8 #A000 RAM/ROM select.
ROM selected will enable data at #A000 – #AFFF in system ROM
RAM selected will enable IC12 #A000 – #AFFF
LK9 #A000 RAM enable, required if LK8 is set to RAM

LK10 Expansion port data bus select
Left #0400 – #1FFF, right #0400 – #1FFF and #E000 – #EFFF

Noise Killer
LK11 RAM_OE
LK12 RAM_WE
LK13 LOAD
LK14 VDG delay

Default settings
LK1 Left (Enable #E000 EPROM)
LK2 Fitted (6522 IRQ)
LK3 Not Fitted (PL8 interrupt)
LK4 Left (CPU 1Mhz)
LK5 Fitted (4Mhz clock)
LK6 Right (enable RAM from 0 – #7FFF)
LK7 Left (Atommc2)
LK8 Down (RAM enable #A000)
LK9 Fitted (RAM select #A000)
LK10 Left (Memory hole @ #0400-#1FFF)
LK11 Right (Noise killer)
LK12 Left (Noise killer)
LK13 Fitted (Noise killer)
LK14 CE-VDG (Noise killer)

System ROM W27C512 Winbond 64K

Kernal ROM mapping
Lower setting    Maps to;
#0000 – #1FFF #8000 – #9FFF NOT USED (Video RAM)
#2000 – #2FFF #A000 – #AFFF Willow 2 Extension ROM (LINK #A000 to execute)
#3000 – #3FFF #B000 – #BFFF NOT USED (I/O)
#4000 – #4FFF #C000 – #CFFF BASIC ROM
#5000 – #5FFF #D000 – #DFFF Floating point ROM
#6000 – #6fff #E000 – #EFFF ATOMMC2 V2.9
#7000 – #7FFF #F000 – #FFFF Kernal ROM

Upper setting    Maps to;
#8000 – #8FFF #8000 – #9FFF NOT USED (Video RAM)
#A000 – #AFFF #A000 – #AFFF AEK Monitor Extension ROM (LINK #A7C7 to execute)
#B000 – #BFFF #B000 – #BFFF NOT USED (I/O)
#C000 – #CFFF #C000 – #CFFF BASIC ROM
#D000 – D5FFF #D000 – #DFFF Floating point ROM
#E000 – #Efff #E000 – #EFFF DOS
#F000 – #FFFF #F000 – #FFFF Kernal ROM

CREDITS
Screen Noise Killer –  Alan Knowles / Hoglet67
Atommc2 – Charlie Robson – (Sir Morris)

Case

Printed using Bambu Lab X1 Carbon

Useful Info

Original Atom draws 1.8 Amps with all chips fitted.

My version draws 0.6 Amps with all chips fitted.

I normally use a 5V 1 Amp power supply, which I’ve had running for hours with no issues.

Colour test with PI based RGB to HDMI board

I also have a working floppy disk interface

MC2 drive

I made my own version that sits neatly on the main PCB.