Example1:  Calculate the UBR value for 9600 Baud with a 4MHz crystal.

    Solution:

                UBR =  ( 4E6 / (9600 * 16) ) - 1

                UBR = ( 4,000,000 / 153,600) - 1

                UBR = 26.041 - 1

                UBR = 25.041

            Now What?

                    Truncate the answer down to 25 and use this value for UBR.

                            UBRR = 25;                // sets baud rate to 9600 with a 4MHz clock

                    *Since the answer to the equation is less than 255, the value of UBRRHI is zero.

                            UBRRHI = 0;

    Example2:  Have the compiler calculate the UBR value for 9600 Baud with a 4MHz crystal.

    Solution:       

                #define XTAL 4000000L

                #define BAUD 9600

                unsigned int UBR;

                UBR = XTAL / 16 / BAUD - 1;

                UBRR = UBR & 0xFF;

                UBRRHI = UBR >> 8;