MOV	TMOD,#10H		
AGAIN:	MOV	TL1,#00		
		MOV	TH1,#0DCH		
		SETB	TR1			
BACK:		JNB	TF1,BACK		
		CLR	TR1			
		CPL	P2.3			
		CLR	TF1			
		SJMP	AGAIN