; Example of reading the dipswitch
;
; The Raven board interfaces to the dip switch (SW_DIP08) through a 74LS165 8-bit 
; Parallel-in/Serial-out Shift Register.  This device has only two control inputs from the
; PIC and one output.  The inputs are:
;      Clock-In      RC3
;      Load/Shift    Y14 (the demultiplexor)
;
; And the output is the bits to RB3, MSB first.
;
; Once the 74LS165 is loaded, it waits for pulses on the clock input pin and then outputs
; one bit at a time.  

; When Y14 is brought low and then released, it initiates the load operation that latches the
; current state of the dip switch into the 74LS165. RC3 is used to create this clock pulse by 
; setting it to 1 and then zero under program control. There is a possible lag between setting 
; the load and the bits being available, so if early bits are missed, a short delay MAY be 
; necessary - probably on the order of a couple of microseconds (I haven't seen this happen).

        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 stores the bit pattern in Dippy and then outputs
; to the led_bar.
;

        ORG       0x50
Start

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

        bsf       STATUS, RP0
        ****      TRISA 
        ***       TRISB,3
        ***       TRISC,3
        ****      PORTD              ; to use the led_bar for output
        bcf       STATUS, RP0

; Set up the clocking.  RC3 is the clock output pin for the SPI interface and SSPCON is the 
; register that controls the SPI. Since that might interfere with the use of RC3 set SSPCON to 
; all zeros.  Also, clear the clock bit on port C which sets the outgoing signal to low.  Low
; causes the dip switch value to be loaded and then we can read the bits.

        ****      SSPCON
        ***       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

        *****     *****
        *****     *****


loop1   
        call      ReadDipBit
        decfsz    Counter,F
        goto      loop1

; Put the value into the led_bar

        comf      *****
        call      DispLedBar

        call      Forever

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    ***
        bcf    ***
        btfsc  ***
        bsf    ****

; toggle the clock to generate a pulse that reads the next bit.  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 

DispLedBar

; Enable the led bar and put the value in W there.
; You can do this one

        return

; Wait forever
Forever
waitforever
        nop
        goto waitforever       
        return

        END