Simple Square wave generation in Keil using timer

org 000h
MOV A,#00H
MOV TMOD,#10H
WAIT:MOV TH1,#0FCH
MOV TL1,#66H
SETB TR1
CPL A
MOV P1,A
BACK:JNB TF1,BACK
CLR TR1
CLR TF1
SJMP WAIT
END