which timer should we use for serial communication?
This is just a continuation of your existing thread, isn't it?
http://www.keil.com/forum/docs/thread10570.asp
Don't keep starting new threads on the same subject - continue in the existing one!
Clearly, what you really need to do is some basic background study on the 8051 architecture - try these tutorials:
www.8052.com/tutorial.phtml
And, of course, the so-called "bible" for the 8051:
Chapter 1 - 80C51 Family Architecture: www.nxp.com/.../80C51_FAM_ARCH_1.pdf
Chapter 2 - 80C51 Family Programmer’s Guide and Instruction Set: www.nxp.com/.../80C51_FAM_PROG_GUIDE_1.pdf
Chapter 3 - 80C51 Family Hardware Description: www.nxp.com/.../80C51_FAM_HARDWARE_1.pdf
Here are some book lists: http://www.keil.com/books/8051books.asp www.8052.com/books.phtml
And, of course, there are loads of examples provided by Keil and the many 8051 device manufacturers...