Acorn Atom Reworked

Acorn Atom Replica, Acorn Atom Copy.

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

NameDesignatorFootprintQtyPart No.SupplierNotes
10nfC1CAP_2.54_CERAMIC1
22uFC2,C3,C4,C5,C8CAP-D3.0XF1.55
10nFC6CAP_2.54_CERAMIC1
47nFC7,C9CAP_2.54_CERAMIC2
33pFC10,C11CAP_2.54_CERAMIC2
1000uFC12,C38CAP-TH_BD10.0-P5.00-D0.6-FD2
100nFC13,C14,C15,C16,
C17,C18,C19,C20,
C21,C22,C23,C24,
C25,C26,C27,C28,
C29,C30,C31,C32,
C36,C37,C39
CAP_5MM_CERAMIC23
100pFC33,C34CAP_2.54_CERAMIC2
10uFC40CAP-D3.0XF1.51
6522IC1DIP401
74LS244IC2,IC3,IC27,IC28,
IC50
DIP205
74LS245IC4,IC29DIP202
74LS27IC5DIP141
74LS04IC6,IC45DIP142
62256IC10,IC12,IC32DIP283
74LS00IC8,IC11,IC48DIP143
74HCT573IC13DIP201
74LS08IC14DIP141
74LS14IC15DIP141
W27C512IC20DIP281Winbond
74LS30IC21DIP141
6502IC22DIP401
74LS138IC23DIP161
8255IC25DIP401
6847IC31DIP401
555IC33DIP81
74LS163AIC47DIP161
74LS139IC49DIP161
CONN_2x15KB1CONN_2X151
1MhL1AXIAL-6.5X2.3(1/4W)1
3mmLED1LED-3MM RED1
HDR-M-2.54_1x3LK1,LK4,LK6,LK7
,LK8,LK10,LK11,
LK12
HDR-M-2.54_1X38
HDR-M-2.54_1x2LK2,LK3,LK5,LK9,
LK13,SPK1
HDR-M-2.54_1X26
HDR-M-2.54_1x5LK14HDR-M-2.54_1X51
HDR-M-2.54_2x5PL4HDR-M-2.54_2X51
HEADER_26_PINPL5CONN_2X131
ATOM_EXPANSION_
64PIN
PL6/7HEADER_2X64_A1
HDR-M-2.54_2x10PL8HDR-M-2.54_2X101
BC547BQ1,Q2BC5472
4K7R1,R2,R13,R14,
R15,R16,R21,R26,
R28,R29,R39,R40,
R41
1/4W 5%13
470RR3,R4,R5,R121/4W 5%4
82RR61/4W 5%1
8K2R71/4W 5%1
3K3R81/4W 5%1
1KR9,R22,R301/4W 5%3
270RR10,R241/4W 5%2
3K9R111/4W 5%1
2.2KR171/4W 5%1
1MR19,R201/4W 5%2
10KR231/4W 5%1
1MR271/4W 5%1
4k7R31,R32,R361/4W 5%3
47kR33,R341/4W 5%2
470kR351/4W 5%1
10kR371/4W 5%1
UM1233 E36SK1UM12331
7_PIN_DIN_PCBSK27_PIN_DIN_PCB12989968Farnell
DC005-2.5MMSK3DC-IN-TH_DC-005-20A1259-6425RS
rca connectorSK4RCA CONNECTOR1189-3957RS
3.58MHzX1OSC-49S-11
4MhzX2OSC-49S-11
74LS02IC7DIP141
74HC04IC26DIP141
74LS393IC44DIP141
LM348IC46DIP81
NOTES
IC26 must be HC version for CPU clock to run
IC11 must be LS version for video clock to run

Ribbon cable & connectors

Keyboard

NameDesignatorFootprintQtyPart No.SupplierNotes
MX 1.0UC1CHERRY MX60Outemu
KB1KB1HDR-M-2.54_2x151
LEDLEDHDR-M-2.54_1X21Optional
4K7R17-R251/4W 5%9
2K2R401/4W 5%1Optional

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)

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