;keypad.asm

; Example of reading the keypad
;

        LIST P=16F877, R=DEC

        INCLUDE "C:\MPLAB\P16F877.inc"

; Definitions

;  __CONFIG _CP_OFF & _DEBUG_OFF & _WRT_ENABLE_OFF & _CPD_OFF & _LVP_OFF & _BODEN_OFF & _PWRTE_ON & _WDT_OFF & _HS_OSC 

; Define start of vars

ScratchPad  EQU   0x20

; Variables

KCode      EQU   ScratchPad+0     ; Value from keypad
Row        EQU   ScratchPad+1
Count      EQU   ScratchPad+2
Temp       EQU   ScratchPad+3
CurrD      EQU   ScratchPad+4


; Start of the Program

        ORG       3
        goto      Start

; This code reads the keypad and puts the code
; on the led_bar.
;

        ORG       0x50
Start

; Set up RC3 to be output, RB3 to be input, A4-A0 to be output.

        bsf       STATUS, RP0
        clrf      TRISA 
        movlw     b'11110000'
        movwf     TRISB
        clrf      TRISD
        bcf       STATUS, RP0


; A simple loop checks all of the rows or until something is returned

BigLoop
        movlw     b'10000000'           ; set the scan row to begin with
        movwf     Row
        movlw     4
        movwf     Count
        clrf      KCode                 ; nothing read

ScanLoop
        call      ScanKeyPadRow
        movlw     0xF0
        andwf     KCode,F                   ; if KCode is zero, nothing was read
        btfss     STATUS,Z

        goto      KeyRead

        bcf       STATUS,C               ; rotate the row indicator
        rrf       Row,F
        decfsz    Count,F
        goto      ScanLoop

; We've done the scan, KCode should contain any button pressed
; loop to continuously read the keypad.  If KCode is zero, there is nothing 
; there

KeyRead

      movf       KCode,W
      call       DispLedBar

      goto       BigLoop
 
ScanKeyPadRow

      movlw  0x1D                        ; enable keypad output
      movwf  PORTA
      movf   Row, W                      ; write the row number to the keypad
      movwf  PORTD

      fill (nop),10                      ; wait for scan to complete

; Read port B.  The high order 4 bits contain the 4 bits of the scan, 
; where each bit represents a different key in the row. Note that the 
; polarity is reversed - 1 = no and 0 = yes.

      movf   PORTB, W                    
      movwf  KCode
      movlw  0x10              ; Disable reading of keypad
      movwf  PORTA

      movlw  0xF0
      andwf  KCode,F
      movlw  0xF0              ; Reverse the reverse bit meaning
      xorwf  KCode,F
      return

; =======================================================================
; Routine to display a code on the led bar

DispLedBar

; Enable the led bar and put the value in W there.

        movwf     CurrD             ; save W
        movlw     0x12
        movwf     PORTA

        comf      CurrD,W
        movwf     PORTD

        return

; Wait forever
Forever
waitforever
        nop
        goto waitforever       
        return

        END