Maths Routine Problem???

Hi all,

I managed to get my interrupts working, and i have figured out a way to calculate the heart rate which is:

z = 60/(0.39*R7) = 153.84/R7 = 154/R7

But when i tried this formula, the LCD only display 77 BPM and 154 BPM. It seems that the overflow counter, R7, only count 1 - 2.

This is my code:

;***************************************
;-----------INTERRUPT CONFIGURATIONS-----------
;***************************************
SETB EA          ; Enable Global Interrupts
SETB ET0         ; Enable Timer 0 Interrupt
SETB EX0         ; Enable External Interrupt 0
SETB PX0         ; External Interrupt 0 High Priority

;***************************************
;------------TIMER 0 CONFIGURATIONS------------
;***************************************
TIMER_0 EQU 65535
SETB IT0          ; External Interrupt 0 detect falling-edge
MOV TMOD,#01H   ; Set Timer 0 to Mode 1
MOV A,CKCON     ; Move CKCON to Accumualtor
CLR ACC.3       ; Set system clock to divided by 12
MOV 70H,#00H    ; Move 0 to 70H
MOV R7,#02H     ; Move 4 to R7
MOV 62H,#01H    ; Move 1 to 62H
MOV TH0,#HIGH TIMER_0 ; Set High Byte of Timer 0
MOV TL0,#LOW TIMER_0  ; Set Low Byte of Timer 0

;**********************
;-Wait Until 4th Pulse-
;**********************
WAIT:   CJNE R7,#00H,WAIT  ; Compare and jump to subroutine "WAIT" if R7 != 0
CLR TR0 ; Stop Timer 0

;******
;-Math-
;******
MATH:   MOV A,#9AH      ; Move 154 to A
        MOV B,70H
        DIV AB
        MOV R6,A

;**********************
;-External Interrupt 0-
;**********************
EX_INT0:        DJNZ 62H,COUNT  ; Decrement 62H and jump if != 0
        SETB TR0        ; Start Timer 0

COUNT:  DEC R7          ; Decrement R7
        RETI            ; Return from Interrupt

;***********************
;-Timer 0 Overflow Flag-
;***********************
T0_INT: INC 70H                 ; Overflow Counter
             MOV TH0,#HIGH TIMER_0      ; Reloads Timer 0 with High Byte
        MOV TL0,#LOW TIMER_0    ; Reloads Timer 0 with Low Byte
        RETI                    ; Return from Interrupt

Any advice would be greatly apprectiated.