;*********************************************************************
;                                                                    *
;     	Tutorial 4 Timer 0 External Counter with variable Prescalor  *    
;                                                                    *
;*********************************************************************
;*	TARGET PIC16F877
;*********************************************************************
; An external clock pulse increments Timer0 in counter mode. T
; The current Timer0 prescalor is shown on PortB and the current 
; Timer value is displayed on PORTC.
;*********************************************************************
;*	@version 	0.1, 27 September 2001
;*	@author		James Caska
;*********************************************************************

	list      p=16F877        
	#include <p16F877.inc>               


	ORG	.0
	GOTO	INIT

	ORG	.4
INTERRUPTHANDLER
	
		
	BSF	STATUS,RP0
	BCF	OPTION_REG,PSA	
	INCF	OPTION_REG,F
	BCF	OPTION_REG,PSA	;Increment the Prescalor
	MOVF	OPTION_REG,W
	BCF	STATUS,RP0
	CALL	LOOKUPSCALOR
	MOVWF	PORTC	

	BCF	INTCON,T0IF
	RETFIE

LOOKUPSCALOR
	ANDLW	.7
	ADDWF	PCL,F
	RETLW	.2
	RETLW	.4
	RETLW	.8
	RETLW	.16
	RETLW	.32
	RETLW	.64
	RETLW	.128
	RETLW	.255	;* Is really 256 

INIT
	;==========================================
	;Autogenerated configuration code PIC16F877
	;Config Wiz V0.1, www.virtualbreadboard.com
	;==========================================
	;======Begin PORT Configuration
	BSF STATUS, RP0	;PORT TRIS Registers are on page 1
	;======PORT A
	MOVLW   .63	;PORTA Configuration Bits
	MOVWF   TRISA
	;======PORT B
	MOVLW   .0	;PORTB Configuration Bits
	MOVWF   TRISB
	;======PORT C
	MOVLW   .0	;PORTC Configuration Bits
	MOVWF   TRISC
	BCF STATUS, RP0	;Restore Page0



	;==========================================
	;Autogenerated configuration code PIC16F877
	;Config Wiz V0.1, www.virtualbreadboard.com
	;==========================================
	;======Begin Timer0 Configuration
	BSF STATUS, RP0	;Timer 0 config register on page 1
	;======Timer0 Timer Mode, uses Internal clock
	BSF OPTION_REG,PS0	;Create so when we INC the first time in the interrupt goes to zero
	BSF OPTION_REG,PS1
	BSF OPTION_REG,PS2
	BCF OPTION_REG,T0CS	;T0CS: TMR0 Clock Source Select bit: 0 = Internal instruction cycle clock (CLKOUT)
	BSF OPTION_REG,PSA	;PSA: Prescaler Assignment bit:1 = Prescaler is assigned to the WDT
	BCF STATUS, RP0	;Restore page 1
	;======Timer0 Interrupt Configuration
	BCF INTCON, T0IF	;Timer 0 Clear Timer0 Interrupt flag
	BSF INTCON, T0IE	;Enabled Timer 0 Interrupt Mask
	;======End Timer0 Configuration

	BSF INTCON, GIE		;Enable Interrupts

	
	MOVLW	.1		;Note initially there is no prescalor 1:1
	MOVWF	PORTC
MAIN
	
	MOVF	TMR0,W
	MOVWF	PORTB
	GOTO	MAIN

	
	END