Acorn Atom Reworked

Hi everyone, I’ve been working on a replica Acorn Atom on and off over a year now and finally have a working prototype I’m happy with.

I’ve had a lot of issues with the video out. I just wanted a black and white output, the same as the original Atom, but have had problems with colour interference. After modifying the video clock circuit, adding extra smoothing I have a black and white output that seems to work on most displays.

I’ve made some improvements to the original design, while still using parts that were available in the 1980’s.

ACORN ATOM REWORKED PCB updates

1. Reset circuit added
2. 2114 RAM replaced with 64256 / 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
6. Noise killer circuit added
7. 8304 chips replaced with 74LS245’s
8. 81LS95 chips replaced with 74LS244’s
9. Separate keyboard using Outemu switches (Cherry MX)
10. Configurable  CPU clock. 1, 2 or 4Mhz
11. #B400 chip select added to expansion port
12. Video out connector added
13. CPU clock circuit improved

I am currently working on a 3D printed case. It will hopefully be in four parts and glued together.

I’ve used white keys on the keyboard as it’s easier to print labels for the non standard keys.

Some earlier versions.

Early version with disk interface.

Acorn Atom Reworked Issue 3.8 Part List

Ribbon cable & connectors

Keyboard

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

Some labels for the keyboard I made

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 ROM, RAM 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
LK2 Fitted
LK3 Not Fitted
LK4 Left
LK5 Fitted
LK6 Right
LK7 Left
LK8 Down
LK9 Fitted
LK10 Left
LK11 Right
LK12 Right
LK13 Fitted
LK14 CE-1G

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)
#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)
#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
Noise Killer – Hoglet67
Atommc2 – Charlie Robson – (Sir Morris)