;lsd.asm

; Example of writing to the 7 segment leds.
;
; The Raven board interfaces to the 7-segment LED's or LSD's through the Y outputs of the
; U12 multiplexor and PortD.  The output of U12 controls the digit being written to
; with Y3 through Y8 controlling the digits from right to left, and RD0 through RD7
; controlling the segments in the following way:
;
;                 |-- RD0
;               |---|
;          RD5--|   |--- RD1
;          RD6--|---|
;          RD4--|   |-- RD2
;               |---| []-- RD7
;                 |-- RD3

        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

Dippy      EQU   ScratchPad+0     ; Value from dip switch
Temp       EQU   ScratchPad+1
Counter    EQU   ScratchPad+2


; Start of the Program

        ORG       3
        goto      Start

; This code reads the dip switch and sends the results to the rightmost digit in the display.
;

        ORG       0x50
Start

; Set  A4-A0 to be output and RD0-RD7 to be output.

        bsf       STATUS, RP0
        clrf      TRISA
        clrf      PORTD

        bsf       TRISB,3
        bcf       TRISC,3

        bcf       STATUS, RP0

; 
        clrf      SSPCON
        bcf       PORTC,3

; Now, we want to enable the 74LS165 and then disable it to create a low-going pulse.

        movlw     0x1E
        movwf     PORTA
        movlw     0x10
        movwf     PORTA

; Now read each bit, shifting it into Dippy

        movlw     8
        movwf     Counter

loop1   
        call      ReadDipBit
        decfsz    Counter,F
        goto      loop1

; Put the to LSD bit 5

        comf      Dippy,W
        call      DispLSD


loop2
; loop to do something else.  If you disable the segment enable, you will need to reenable
; it frequently enough to keep it from decaying to zero.  50 instructions is probably OK,
; 100 is probably too much.

        nop
        goto      loop2


ReadDipBit

; Rotate Dippy to make room for the next bit.  Clear bit zero and then test PORTB, bit 3 to 
; see its state.  If zero, we be done. If not, set bit 0 of Dippy

        rlf    Dippy,F
        bcf    Dippy,0
        btfsc  PORTB,3
        bsf    Dippy,0

; toggle the clock to generate a pulse that reads the next.  Setting the bit drives it low
; and clearing sets it high, giving a square wave, low-going pulse that causes the
; next bit to be shifted out.
  
        bsf    PORTC,3
        bcf    PORTC,3
        return 

DispLSD

; Enable LSD 5 and output to it

        movwf     Temp             ; save W
        movlw     0x13
        movwf     PORTA

        movf      Temp,W
        
        movwf     PORTD

        return

; Wait forever
Forever
waitforever
        nop
        goto waitforever       
        return

        END