ATMEGA640 datasheet(212/449 Pages) ATMEL

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ATmega640/1280/1281/2560/2561

2549K–AVR–01/07

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 Figure 83.

The USART Baud Rate Register (UBRRn) and the down-counter connected to it func-

tion as a programmable prescaler or baud rate generator. The down-counter, running at

system clock (f

osc), is loaded with the UBRRn value each time the counter has counted

down to zero or when the UBRRLn Register is written. A clock is generated each time

the counter reaches zero. This clock is the baud rate generator clock output (=

osc/(UBRRn+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 UMSELn,

U2Xn and DDR_XCKn bits.

Table 101 contains equations for calculating the baud rate (in bits per second) and for

calculating the UBRRn value for each mode of operation using an internally generated

clock source.

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)

OSC

System Oscillator clock frequency

UBRRn Contents of the UBRRHn and UBRRLn Registers, (0-4095)

Table 101. Equations for Calculating Baud Rate Register Setting

Operating Mode

Equation for Calculating

Baud Rate(1)

Equation for Calculating

UBRR Value

Asynchronous Normal

mode (U2Xn = 0)

Asynchronous Double

Speed mode (U2Xn =

Synchronous Master

mode

BAUD

OSC

16 UBRRn

()

------------------------------------------

UBRRn

OSC

16BAUD

------------------------

–

BAUD

OSC

8 UBRRn

()

---------------------------------------

UBRRn

OSC

8BAUD

--------------------

–

BAUD

OSC

2 UBRRn

()

---------------------------------------

UBRRn

OSC

2BAUD

--------------------

–

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