clr  TR0
 mov  TH0,#HIGH(-4800h)	; set for 20 ms
 mov  TL0,#LOW(-4800h)	;
 setb TR0
; push acc  ;if needed
; push psw  ;if needed
 djnz tcount,xx2
 mov  tcount,#250
xx1:
 jnb  ti,xx1
 mov  SBUF,#'C'
 clr  ti
xx2:
; pop  psw ;if needed
; pop  acc ;if needed
 reti