I have connected the output from a signal generator to the P3.2 (INT0) of the 8051 (AT89S51).

I use R1 and R2 to store TH0 and TL0 respectively, and I can convert this 16-bit hex value into bcd value (stored in R3, R4, R5, R6 and R7 ) using the math subroutine, Hex2BCD. The LCD will show this value and I can do a calculation, 1/(bcd value * 1e-6) manually to compare it with the frequency shown in the signal generation. i.e. 20000 => 50Hz
I don't why the value is only accurate at the first time (round) and after this, the value becomes almost half of the correct one and it's not stable. The following is my program (main + ISR)

RAM_ADDR EQU 40H

ORG 0000H
LJMP MAIN

ORG 0003H ;INT0
LJMP ISR_ET

;--------------MAIN PROGRAM----------------
ORG 30H
MAIN:
FLAG BIT PSW.5
FLAG2 BIT PSW.1

ACALL LCD_init

MOV TMOD, #01H ;timer 0, mode 1

SETB TCON.0 ;falling edge-triggered external interrupt 0
MOV IE, #80H ;enable all interrupts

HERE:
CLR FLAG ;set when the timer is stopped
CLR FLAG2 ;set when the timer is already started

MOV TH0, #0
MOV TL0, #0

SETB IE.0 ;enable external interrupt 0
HERE3: JNB FLAG, HERE3 ;jump if FLAG=0
CLR IE.0 ;disable external interrupt 0
CLR TF0 ;clear timer 0 flag
MOV R1, TH0 ;store TH0
MOV R2, TL0 ;store TL0

ACALL Hex2BCD ;R1(H), R2(L)
;result in R7(M) to R3(L)
MOV R0, #RAM_ADDR

MOV @R0, 07H
INC R0
MOV @R0, 06H
INC R0
MOV @R0, 05H
INC R0
MOV @R0, 04H
INC R0
MOV @R0, 03H

MOV R0, #RAM_ADDR
MOV R2, #5
AGAIN:
MOV A, @R0
ORL A, #30H ;make it an ASCII code
ACALL DATA_DISPLAY
INC R0
DJNZ R2, AGAIN

ACALL DELAY
ACALL DELAY
ACALL DELAY
ACALL DELAY
ACALL DELAY

MOV A, #02H ;cursor: line 1, pos. 1
ACALL COMMAND

SJMP HERE

ISR_ET:
JB FLAG2, HERE4 ;jump if FLAG2=1
SETB TR0 ;start timer 0
SETB FLAG2
RETI
HERE4:
CLR TR0 ;stop timer 0
CLR FLAG2
SETB FLAG
RETI

The strange thing is when I remove those "ACALL DELAY" in the main program and the LCD routine, the result is very accurate and stable.
Does anyone know what's wrong with my program?
I want to show the correct one on the screen and hold it like one second then jump back and start next round.

Please help, thanks.
Please help. Thanks.