24.3.1 Internal Clock Generation – The Baud Rate Generator

Internal clock generation is used for the asynchronous and the synchronous master modes of operation. The description in this section refers to the block diagram above.

The USART Baud Rate Register (UBRR) and the down-counter connected to it function as a programmable prescaler or baud rate generator. The down-counter, running at system clock (fosc), is loaded with the UBRR value each time the counter has counted down to zero or when the UBRRL Register is written. A clock is generated each time the counter reaches zero. This clock is the baud rate generator clock output (= fosc/(UBRR+1)). The Transmitter divides the baud rate generator clock output by 2, 8, or 16 depending on mode. The baud rate generator output is used directly by the Receiver’s clock and data recovery units. However, the recovery units use a state machine that uses 2, 8, or 16 states depending on mode set by the state of the UMSEL, U2X and DDR_XCK bits.

The table below contains equations for calculating the baud rate (in bits per second) and for calculating the UBRR value for each mode of operation using an internally generated clock source.

Table 24-1. Equations for Calculating Baud Rate Register Setting
Operating Mode	Equation for Calculating Baud Rate⁽¹⁾	Equation for Calculating UBRR Value
Asynchronous Normal mode (U2X = 0)	$BAUD = \frac{f_{OSC}}{16 (U B R R + 1)}$	$U B R R = \frac{f_{OSC}}{16 BAUD} - 1$
Asynchronous Double Speed mode (U2X = 1)	$BAUD = \frac{f_{OSC}}{8 (U B R R + 1)}$	$U B R R = \frac{f_{OSC}}{8 BAUD} - 1$
Synchronous Master mode	$BAUD = \frac{f_{OSC}}{2 (U B R R + 1)}$	$U B R R = \frac{f_{OSC}}{2 BAUD} - 1$

Note: 1. The baud rate is defined to be the transfer rate in bit per second (bps).

BAUD: Baud rate (in bits per second, bps).
f_OSC: System oscillator clock frequency.
UBRR: Contents of the UBRRH and UBRRL Registers, (0-4095).

Some examples of UBRR values for some system clock frequencies are found in Table 24-9.