Gameboy as a USB Controller (Page 1 of 3)

Main:
    di ; disable interrupts
    ld    hl,$c200 ; we want a stack
    ld    sp,hl

ScreenOff:    
    ldh    a,[rLY]        ;load a with Current Y co-ordinate (144-153 represents VBLANK)
    cp    144        ;copmare with 144
    jr    nz,ScreenOff    ;not equal? then jmp to wait
    ld    a,0        ;equal? then load a with 0
    ldh    [rLCDC],a    ;load a into LCDC - Display off, BG off, Spr off etc

MainLoop:    
    call JoyPadRead
    call TRX_Byte
jp MainLoop

TRX_Byte:
    ld  [$FF01],a
    ld   a,$80
    ld  [$FF02],a
ByteNotSent:
    ld a,[$FF02]
    cp $FE
    jr z, ByteNotSent
    ld a,[$FF01]
    ret

JoyPadRead:
        ; Routine finding which buttons were pressed since the last check
        LD A,$20    ; Set 0 at the output line P14
        LD [$FF00],A ; 
        LD A,[$FF00] ; Read JOYPAD several times to accomodate the noise
        LD A,[$FF00] ;
        LD A,[$FF00] ;
        LD A,[$FF00] ;
        CPL          ; Bits 0-3 are now 1s if corresponding buttons pressed
        AND $0F      ; Extract lower 4 bits carrying button status...
        SWAP A       ; ...and move them into upper four bits
        LD B,A       ; At this point: B = START.SELECT.B.A.x.x.x.x
        LD A,$10     ; Set 0 at the output line P15
        LD [$FF00],A ;
        LD A,[$FF00] ; Read JOYPAD several times to accomodate the noise
        LD A,[$FF00] ;
        LD A,[$FF00];
        LD A,[$FF00] ;
        LD A,[$FF00] ;
        LD A,[$FF00] ;
        CPL          ; Bits 0-3 are now 1s if corresponding buttons pressed
        AND $0F      ; Extract lower 4 bits carrying buttons' status...
        OR B         ; ...and combine them with 4 other button status bits
        LD D,A       ; At this point: D = START.SELECT.B.A.DOWN.UP.LEFT.RIGHT
    
        LD A,$30     ; Set 1s at both P14 and P15 lines
        LD [$FF00],A ; [probably to reset the circuitry]
        ld A,D
        ret