;on_off.asm
;
; =============================================================================
;
; Turn on one of the LED's and make it blink
; First example program
; harkin, 02/01
;
; ==============================================================================

        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
mS_0        EQU   ScratchPad+1     ; counters for the delay routine
mS_1        EQU   ScratchPad+2

; Variables
;    Start at some place in ram above the special registers (0x20 or higher)

ScratchPad EQU 0x20
Counter    EQU ScratchPad + 0  ; 8 bit counter

; Start of the Program

        ORG       3
        goto      Start

; start code at someplace higher than 0x04

        ORG       0x50
Start

; Since A is bi-directional, we need to specify that the lower five bits (A4-A0) of
; A are output by putting zeros in those bits of TRISA.  The upper three don't matter, but set 
; them to be output also .  TRISA is in bank 1, so switch to bank 1 and then back when 
; done. Similarly, port D controls the LED's, so also set PORTD to put output.

        bsf       STATUS, RP0
        movlw     b'00000000'
        movwf     TRISA 
        clrf      TRISD           ; A different way to clear all the bits.
        bcf       STATUS, RP0


; Port A, lines 0,1,2,3,4 control which output device is used.  See page 8
; in the Raven Board manual.  The three colored LEDs, are selected by setting
; A0-A3 to 0x11 (this includes the latch) or 0x01 if the input isn't latched.
; If you are only going to use one of the circuits encoded by the multiplexor, it
; doesn't matter how you set the latch.  However, if you are going to access multiple
; devices, you will need to set (one) the latch bit so that the changes are noted.  On the other
; hand, if you are using A for other things that might affect the value of A0-A3, you need
; to set the value of A with the latch cleared (zero), so that value will stay in the
; multiplexor until you once again raise the latch bit and change the value of A0-A3. The 
; most likely bug is to try to change from one device to another with the latch bit cleared.
; It remembers only the first value set at A0-A3 and won't change it until you set the bit
; and then possibly set it clear again.  To say it more plainly, if the latch bit is cleared,
; you can't change the input to A unless to set the latch bit.  You can either leave it set
; or you can clear it to latch the new input.

        movlw     0x11
        movwf     PORTA

; Now, the LED's can be controlled by setting RD0-RD2.  RD0 controls green, RD1 controls
; yellow and RD2 controlss red; zero is on and one is off. To turn on only the red LED,
; set PORTD to b'xxxxx011' or 0x03.

        movlw     0x07              ; this turns all leds off
        movwf     PORTD
loop
        bcf       PORTD,2           ; turn red on (011)

; OK, the red LED is on.  Now use a timer to make it blink
        
        movlw     255
        call      mS_Delay

        bsf       PORTD,2          ; turn it off (111)

        movlw     255
        call      mS_Delay

        goto      loop

; ----------------------------------------------------------------
; Routine to delay a specified number of milliseconds
; inputs - milliseconds to Delay (w), outputs - none
; This routine is designed to work for a 20 MHz clock, so
; each clock cycle takes 50 ns and each instruction takes 200 ns.
; Ignoring overhead in the loops, each inner loop is 250 x 17 nops, 
; or about .85 ms.  An input value of 255 generates a wait of 
; 216.75 ms or a little less than 0.25 sec.  Obviously, you could
; change the parameters to get some other timing.
; ------------------------------------------------------------------

mS_Delay  
        movwf  	mS_0
mS_loop_0	   ; outer delay loop
        movlw  	250
        movwf  	mS_1
mS_loop_1	   ; inner delay loop
        fill (nop), 17            ; This generates 17 nops
        decfsz 	mS_1
        goto      mS_loop_1
        clrwdt
        decfsz 	mS_0
        goto   	mS_loop_0
        return
      
        END