; KBSCAN will scan the keyboard for incoming data for about
; 105uS and returns with A=0 if no data was received.
; It does not decode anything, the non-zero value in A if data
; is ready is ambiguous.  You must call KBGET or KBINPUT to
; get the keyboard data.
;
kbscan:         ldx   #$05              ; timer: x = (cycles - 40)/13   (105-40)/13=5
               lda   kbportddr         ;
               and   #$CF              ; set clk to input (change if port bits change)
               sta   kbportddr         ;
kbscan1:        lda   #clk              ;
               bit   kbportreg         ;
               beq   kbscan2           ; if clk goes low, data ready
               dex                     ; reduce timer
               bne   kbscan1           ; wait while clk is high
               jsr   kbdis             ; timed out, no data, disable receiver
               lda   #$00              ; set data not ready flag
               rts                     ; return
kbscan2:        jsr   kbdis             ; disable the receiver so other routines get it
; Three alternative exits if data is ready to be received: Either return or jmp to handler
;               rts                     ; return (A<>0, A=clk bit mask value from kbdis)
               jmp   kbinput           ; if key pressed, decode it with KBINPUT
;               jmp   KBGET             ; if key pressed, decode it with KBGET
;
;