SNES2Atari

What is it?

The SNES2Atari is an adapter that allows you to connect an SNES controller to the Atari 7800 console, Atari 2600 console, the Atari 8-bit computer line, and likely many other platforms that have a DB9 joystick port and can use Atari joysticks. Games must be specifically coded to use the SNES controller.

The design is quite minimal, and can be implemented with just an SNES extension cable, a 1kΩ resistor, and a serial DB9 breakout connector. Alternatively, a more robust adapter can be implemented with a minimal pcb, a SNES port, a 1kΩ resistor, and a serial DB9 connector.

The SNES2Atari circuit and driver was created AtariAge user Danjovic.

SNES2Atari Adapter Wiring

Note: when the 7800 console has the controller port in two-button mode, 5 volts is directly connected to pin 6. The 1kΩ resistor in snes2atari serves to protect the snes controller data output line when the controller port is in this state.

SNES2Atari Driver

 ;    ___ _  _ ___ ___      _     _
 ;   / __| \| | __/ __|  __| |_ _(_)_ _____ _ _
 ;   \__ \ .` | _|\__ \ / _` | '_| \ V / -_) '_|
 ;   |___/_|\_|___|___/ \__,_|_| |_|\_/\___|_|
 ;
 ; SNES controller on port 2
 ;      V2 - Detect SNES controller

 ;  Connections
 ;  Atari                SNES
 ;                      +---+
 ; VCC   (7)-+----------| o | VCC  
 ;  UP   (1)------------| o | CLOCK
 ; DOWN  (2)------------| o | LATCH
 ; FIRE  (6)----vvvv----| o | DATA
 ;             1k Ohm   +---+
 ;                      | o |
 ;                      | o |
 ; GND   (8)-+----------| o | GND
 ;                      '._.
 ; LEFT  (3)- n.c.
 ; RIGHT (4)- n.c.
 ; POT1  (5)- n.c.
 ; POT2  (9)- n.c.
 ;


 ; initialize port 2 and detect controller
 ; return zero flag activated if controller detected

INITPORT2:
   lda SWCHB  ; 7800 specific init...
   ora #$10   ; Skip this block if you're targeting the 2600
   sta SWCHB  ; PB4 High->DISABLE transistor Q5, enabling single button joystick

   lda #$03         ; 2 7800/2600 joystick port init...
   sta CTLSWA       ; 5 enable pins UP/DOWN to work as outputs
   sta SWCHA        ; 5 make pins clock (UP) and latch (DOWN) to go high
   rts              ; 6

 ; detect controller
 ; return carry set if fail
DETECT:
   jsr SAMPLESNES
   bcs END_DETECT
   lda LowDataByte  ; bits 16 15 14 13 12 11 10 09
   ora #$0f         ; bits 16 15 14 13 Hi Hi Hi Hi  all Hi's for SNES controller
   eor #$ff         ; shall be all Low's for SNES controller
   beq END_DETECT
   sec ; set carry to indicate no controller detected
END_DETECT:
   rts

 ; sample controller on port 2
 ; assume port already initialized
 ; At the end:     7     6    5    4    3     2     1    0
 ; LowDataByte:    1     1    1    1   RSH   LSH    X    A
 ; HighDataByte: RIGHT LEFT DOWN  UP  START SELECT  Y    B
 ; Carry Flag is the 17th bit
 ;
SAMPLESNES:         ; 6 (from jsr)
   lda #$0          ; 2 clock down, latch down
   sta SWCHA        ; 4 make both latch and clock down
   ldx #$01         ; 2 clock up, latch down
   ldy #16          ; 2 16 bits
   lda #$ff         ; 2 Initialize with no button pressed
   sta LowDataByte  ; 5 if necessary to gain a few microseconds
   sta HighDataByte ; 5 from latch/clk down till first bit sample
   lda #$0          ; 2
SMPSN0:          
   lda INPT5        ; 3 sample data
   rol              ; 2 
   stx SWCHA        ; 5 clock up
   ror LowDataByte  ; 5 take some time  
   ror HighDataByte ; 5 shifting controller data
   lda #$0          ; 2 and reloading accumulator
   sta SWCHA        ; 4 clock down 
   dey              ; 2 next bit
   bne SMPSN0       ; 2 repeat 
 
   stx SWCHA        ; 5 clock up, next edge  to read 17th bit
   lda INPT5        ; 3
   rol              ; 2 sample data, 17th bit on Carry
  
   lda #$03         ; 2 both clock and latch up
   sta SWCHA        ; 5 finish sampling
   rts              ; 6
 
 
 ; Total time take for sample, from jsr SAMPLE till return:
 ; 6+2+4+2+2+2+5+5+2 + 16* ([3+2]+5+5+5+[2]+4+2+2) + 5+[3+2]+2+5+6
 ; 30                + 16* (28+[2])                + 23
 ; 30                + 480                         + 23  = 533 cycles  =~447us at 1.19MHz
 
 ;clocks:
 ;____            ____        ____        ____       ___
 ;    \_______s__/    \___s__/    \___s__/    \___s....
 ;____
 ;    \4222552s32/5552\422s32/5552\422s32/5552\422s....
 ;
 ;
 ;from Latch down to read first bit:    4+2+2+2+5+5+2 = 22 -> 18.5us
 ;from clock rise to next sample: 3+2+5+5+5+[2]+4+2+2 = 30 -> 25.2us

Test Rom

Snes2atari_test_rom.zip can be used to test your snes2atari adapter, or your emulator snes2atari implementation.

Authorship

The SNES2Atari circuit design and driver was written by AtariAge user Danjovic. All code and circuit design was kindly provided by him under the "no rights reserved" CC0 license.