DOC7707 datasheet(149/307 Pages) ETC1

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7707F–AVR–11/10

AT90USB82/162

Table 17-1 contains equations for calculating the baud rate (in bits per second) and for calculat-

ing 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)

Some examples of UBRRn values for some system clock frequencies are found in Table 17-9 on

page 170.

17.2.2

Double Speed Operation (U2Xn)

The transfer rate can be doubled by setting the U2Xn bit in UCSRnA. Setting this bit only has

effect for the asynchronous operation. Set this bit to zero when using synchronous operation.

Setting this bit will reduce the divisor of the baud rate divider from 16 to 8, effectively doubling

the transfer rate for asynchronous communication. Note however that the Receiver will in this

case only use half the number of samples (reduced from 16 to 8) for data sampling and clock

recovery, and therefore a more accurate baud rate setting and system clock are required when

this mode is used. For the Transmitter, there are no downsides.

Table 17-1.

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 = 1)

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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