为什么数据传送过去后TI没有置1

ORG0000H
LJMPSTART
ORG0060H
START:MOV TMOD,#20H
MOV SCON,#0xC0
MOV PCON,#00H
MOV TH1,#0xfd
MOV TL1,#0Xfd
MOV R7,#8
SETBTR1
LOOP1:MOV A,#00H
MOV P1,A
MOV SBUF,A
LJMP WAIT1
LJMP DEL
MOV A,#0xFF
MOV P1,A
MOV SBUF,A
LJMP WAIT1
LJMP DEL
LOOP2:DJNZ R7,LOOP1
LOOP3:MOV A,#0xF0
MOV P1,A
LJMP DEL
MOV A,#0FH
MOV P1,A
LOOP4:DJNZ R7,LOOP3
DEL: MOV R5,#20
D1: MOV R6,#40
D2: MOV R7,#248
DJNZ R7,$
DJNZ R6,D2
DJNZ R5,D1
WAIT1:JNB TI,WAIT1
CLRTI
END