MOV	TMOD,#10H		
AGAIN:	MOV	TL1,#1AH		
		MOV	TH1,#0FFH		
		SETB	TR1			
BACK:		JNB	TF1,BACK		
		CLR	TR1			
		CPL	P1.5			
		CLR	TF1			
		SJMP	AGAIN