I am working with Msc1211y5, 24 bit A2D with 8051 microcontroller. In my program, I have enabled A2D and timer 0 interrupts. When I also enable serial port 0 interrutps(Both send and receive are enabled with a same flag) the speed of executing the program is reduced. This happeds also when we do not send any data to msc1211y5, and trasmitting data by msc1211y5 is done with "printf" command. It seems that TI_0 is set and program jump to serial port 0 ISR. Clearing TI_0 in ISR also cause the program not send any data to serial port and it seems the program stay in ISR. We use the Keil software to compile our programs. Could somebody please explain me such behavior?