BAUD Rate Generator

The USART Baud Rate Register (UBRRxx) and the down-counter connected to it function as a programmable prescaler or baud rate generator. The down-counter, running at system clock (f_osc), is loaded with the UBRRxx value each time the counter has counted down to zero or when the UBRRxx register is written. A clock is generated each time the counter reaches zero.