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RI working is good but TI not set for at serial interrupt using time why?
if(TI==1) { SBUF = 'A'; TI=0; }
Insufficient information - you need to show a minimum but complete example.
Are you sure that you've enabled the interrupt?
There are pleny of examples availablre; rg,
https://www.keil.com/download/list/c51.htm
https://www.keil.com/appnotes/list/c51.htm
https://www.keil.com/books/8051books.asp