How to make a speed trap for a bullet ?

//the code measures time elapsed between two active high signals (IN1 and IN2)
//the elapsed time is shown on OUT_PORT.

#include <regx51.h>
#include "gpio.h"

#define OUT_PORT		P3
#define OUT_DDR			P3
#define OUT_PINs		0xff			//output pins on p2

#define IN_PORT			P2
#define IN_DDR			P2
#define IN1				(1<<0)			//first sensor, active high
#define IN2				(1<<1)			//2nd sensor, active high

void tmr0_init(void) {					//set up tmr0
	TMOD &= 0xf0;						//clear tmod's lower four bits
	TMOD |= 0x01;						//tmr0 as 16-bit timer, not gated
	TR0=0;								//stop the timer
}

void char_display(unsigned char val_char) {				//display tmr0's readings
	OUT_PORT = val_char;						//display tl0 only
}

void mcu_init(void) {					//reset mcu and set-up the pins
	IO_CLR(OUT_PORT, OUT_PINs);			//drive out_pins low
	IO_OUT(OUT_DDR, OUT_PINs);			//out_pins as output

	IO_IN(IN_DDR, IN1 | IN2);			//in1 and in2 as input
}

int main(void) {
	mcu_init();							//reset the mcu / pins
	tmr0_init();						//initialize the tmr

	while (1) {
		TH0=0, TL0=0;					//reset tmr counters
		while (!IO_GET(IN_PORT, IN1))
			continue;					//wait for IN1 to go high
		TR0=1;							//start tmr0
		while (!IO_GET(IN_PORT, IN2))
			continue;					//wait for IN2 to go high
		TR0=0;							//stop tmr0
		
		char_display(TH0);				//display th0
		char_display(TL0);				//display tl0
	}
}

//the code measures time elapsed between two active high signals (IN1 and IN2)
//the elapsed time is shown on OUT_PORT.

#include <regx51.h>
#include "gpio.h"

#define OUT_PORT		P2
#define OUT_DDR			P2
#define OUT_PINs		0xff			//output pins on p2

#define IN_PORT			P3
#define IN_DDR			P3
#define IN1				(1<<0)			//first sensor, active high
#define IN2				(1<<2)			//2nd sensor, active high, on int0

void tmr0_init(void) {					//set up tmr0
	TMOD &= 0xf0;						//clear tmod's lower four bits
	TMOD |= 0x01 | 0x08;				//tmr0 as 16-bit timer and gated
	TR0=0;								//stop the timer
}

void char_display(unsigned char val_char) {				//display tmr0's readings
	OUT_PORT = val_char;						//display tl0 only
}

void mcu_init(void) {					//reset mcu and set-up the pins
	IO_CLR(OUT_PORT, OUT_PINs);			//drive out_pins low
	IO_OUT(OUT_DDR, OUT_PINs);			//out_pins as output

	IO_IN(IN_DDR, IN1 | IN2);			//in1 and in2 as input
}

int main(void) {
	mcu_init();							//reset the mcu / pins
	tmr0_init();						//initialize the tmr

	while (1) {
		TH0=0, TL0=0;					//reset tmr counters
		while (!IO_GET(IN_PORT, IN1))
			continue;					//wait for IN1 to go high
		TR0=1;							//start tmr0
		while (IO_GET(IN_PORT, IN2))
			continue;					//wait for IN2 to go low
		TR0=0;							//stop tmr0
		
		char_display(TH0);				//display th0
		char_display(TL0);				//display tl0
	}
}