State Machines 1

#include "msp430.h"				; #define controlled include file
;; DEBOUNCE WITH INTERRUPTS
        NAME    main                    	; module name

        PUBLIC  main                    	; make the main label vissible
						; outside this module
        ORG     0FFFEh
        DC16    init                    	; set reset vector to 'init' label
        ;; Timer A0 interrupt vector
        ORG     0xFFF2
        DW      debounce

        RSEG    CSTACK                  	; pre-declaration of segment
        RSEG    CODE                    	; place program in 'CODE' segment
init:   MOV     #SFE(CSTACK), SP        	; set up stack
main:   NOP                             	; main program
        MOV.W   #WDTPW+WDTHOLD,&WDTCTL  	; Stop watchdog timer
        ;; Set up a timer that interrupts every 5ms
        MOV     #625, &TACCR0
        MOV     #CCIE, &TACCTL0
        MOV     #MC_3 | ID_3 | TASSEL_2 | TACLR, &TACTL
        ;; Set up the LED for output
        BIC.B   #0x41, &P1OUT			; Make sure the LED is off
        BIS.B   #0x41, &P1DIR			; Set the direction of pin 1 (LED) to output
        ;; Set up the button
        BIS.B   #8, &P1REN			; Connect button to input
        ;; Our debounce algorithm variables
        MOV     #3, R4				; Counter
        MOV     #0, R5				; Expected state of the button (initially not pressed)
						; If R5 is 0, the button should not be pressed
        EINT					; Enable all maskable interrupts
state1:	BIC.B	#1, &P1OUT			; Turn off LED
	CMP		#1, R5			; Check if button is pressed
	JEQ		state2			; If so, move on to the next statement
	JMP		state1			; If not, go back to the statement
state2:	CMP		#0, R5			; Check if the button is released
	JEQ		state3			; If so, move on to the next statement
	JMP		state2			; If not, go back to the statement
state3: BIS.B	#1, &P1OUT			; Turn on LED
	CMP		#1, R5			; Check if button is pressed
	JEQ		state4			; If so, move on to the next statement
	JMP		state3			; If not, go back to the statement
state4:	CMP		#0, R5			; Check if the button is released
	JEQ		state1			; If so, go back to the first statement
	JMP		state4			; If not, go back to the statement

debounce:
	;; Check if the button is in its expected state
        CMP     #0, R5
        JEQ     checkifoff
	;; Check if the button is on
        BIT.B   #8, P1IN			; Is the button pressed?
        JZ      done				; Yes, nothing to be done
        JMP     check
checkifoff:
        BIT.B   #8, &P1IN			; Is the button not pressed?
        JNZ     done				; Yes, nothing to be done
check:
	;; Button is not in the expected state
        DEC     R4				; Decrement counter
        JZ      accept				; Is the counter now zero? Accept the new state
        RETI					; Otherwise, we're done
accept: XOR     #1, R5				; Toggle the expected state of the button
done:   MOV     #3, R4				; Reset the counter
        RETI					; Return from interrupt

        JMP $                           	; jump to current location '$'
                                        	; (endless loop)
        END