Dear Thinh Truong Nguyen,
As far as I know, you can not pas arguments to and get a return value from a interrupt service function. You are passing an argument to the interrput service routine [void quay_so(char i)interrupt 2]. This could be causing the problem.

Refer to the Franklin C Compiler Help on "Interrupt Functions", which states as follows:

"...........No function arguments may be specified for an interrupt function. The compiler emits an error message if an interrupt function is declared with any arguments.
Interrupt function declarations may not include a return value. They must be declared as void (see the above examples). The compiler emits an error message if any attempt is made to define a return value for the interrupt function. The implicit int return value, however, is ignored by the compiler................"

Bye and good luck

Jayanth Madhur