ATmega328P datasheet(170/345 Pages) ATMEL

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170

7810A–AVR–11/09

ATmega328P [Preliminary]

Table 19-1 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 UBRRnH and UBRRnL Registers, (0-4095)

Some examples of UBRRn values for some system clock frequencies are found in Table 19-9

(see page 192).

19.3.2

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 19-1.

Equations for Calculating Baud Rate Register Setting

Operating Mode

Equation for Calculating Baud

Rate(1)

Equation for Calculating

UBRRn Value

Asynchronous Normal mode

(U2Xn = 0)

Asynchronous Double Speed

mode (U2Xn = 1)

Synchronous Master mode

BAUD

fOSC

UBRRn 1

()

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

UBRRn

fOSC

BAUD

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

–

BAUD

fOSC

UBRRn 1

()

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

UBRRn

fOSC

BAUD

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

–

BAUD

fOSC

UBRRn 1

()

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

UBRRn

fOSC

BAUD

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

–

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