;***********
; author: Chris Hodapp
; version: 1.0
; filename: interrupt-wtfasm
; chip: PIC16F887
; clock freq: 4 MHz
; **********

list P = 16F887
#include <p16F887.inc>
__CONFIG    _CONFIG1, _LVP_OFF & _FCMEN_OFF & _IESO_OFF & _BOR_OFF & _CPD_OFF & _CP_OFF & _MCLRE_OFF & _PWRTE_ON & _WDT_OFF & _INTRC_OSC_NOCLKOUT
__CONFIG    _CONFIG2, _WRT_OFF & _BOR21V

; *************
; Declarations
; *************
	cblock 0x70     ; unbanked RAM
W_Save
STATUS_Save
	endc

org 0		; reset vector
	goto Start
org 4		; ISR
	goto ISR

;*************
; Subroutines
;*************
Init:
	bsf		STATUS,RP0  ; select Register Bank 1

	; Set b'10001000' instead of b'10000000' to enable PORTB change interrupt;
	; this also completely breaks the program.
	movlw	b'10000000'	; global interrupts
	movwf	INTCON
	movlw	b'00000001' ; set PORTB pin 0 to interrupt-on-change
	movwf	IOCB

	movlw	b'00000000' ; PORTA I/O configuration
	movwf	TRISA
	movlw	b'00000001' ; PORTB "
	movwf	TRISB
	movlw	b'00000000' ; PORTC "
	movwf	TRISC
	movlw	b'00000000' ; PORTD "
	movwf	TRISD
	movlw	b'00000000' ; PORTE "
	movwf	TRISE


	bsf		STATUS,RP1	; select Register Bank 3
	movlw	0x00
	movwf	ANSELH		; PortB pins are digital

	bcf		STATUS,RP1	; back to Register Bank 0
	bcf		STATUS,RP0

	retlw	0

ISR:
	movwf     W_Save              ; Save context
	movf      STATUS,w
	movwf     STATUS_Save
	
	btfsc INTCON,RBIF	; is this an interrupt from PORTB changing?
	goto portb_int
	goto other_int

portb_int:	
	bsf		PORTD,6		; LED#7: Interrupt was thrown from PORTB changing
other_int:
	bsf		PORTD,7		; LED#8: Some unexpected interrupt was thrown

leave_int:
	movf      STATUS_Save,w       ; Restore context
	movwf     STATUS
	swapf     W_Save,f
	swapf     W_Save,w
	retfie



;***************
; Program Start
;***************
Start:
	call	Init
	clrf	PORTD
	bsf 	PORTD,0		; LED#1: Init subroutine ran successfully
Main:

	btfsc	PORTB,0
	goto L2off
	bsf		PORTD,1		; LED#2: Pushbutton is pressed
	goto L2done
L2off:
	bcf		PORTD,1
L2done:

	btfsc	INTCON,RBIF
	goto L3off
	bsf		PORTD,2		; LED#3: Interrupt flag is set
	goto L3done
L3off:
	bcf		PORTD,2
L3done:

	bsf 	PORTD,3		; LED#4: Main loop has run at least once
	goto Main
	
end