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155

ATmega8515(L)

2512A–AVR–04/02

USART Baud Rate Registers –

UBRRL and UBRRH

The UBRRH Registershares the same I/Olocation as the UCSRC Register. See the

“Accessing UBRRH/UCSRC Registers”onpage 149section which describeshow to

access thisregister.

• Bit 15 – URSEL: Register Select

Thisbit selectsbetween accessing the UBRRH or the UCSRC Register. Itisread as

zero when reading UBRRH.The URSEL must be zero when writing the UBRRH.

• Bit 14:12 – Reserved Bits

These bits are reservedforfutureuse. Forcompatibilitywithfuture devices, these bit

must be written to zero when UBRRH iswritten.

• Bit 11:0 – UBRR11:0: USART Baud Rate Register

This is a12-bit registerwhich contains the USART baudrate. The UBRRHcontains the

four most significant bits, and the UBRRLcontains theeight least significant bits of the

USART baudrate. Ongoing transmissionsbytheTransmitter and Receiverwill be cor-

rupted if the baudrate ischanged. Writing UBRRLwill trigger an immediate update of

the baudrate prescaler.

Examples of Baud Rate

Setting

Forstandard crystal andresonatorfrequencies, themost commonly usedbaudratesfor

asynchronous operation can be generatedbyusing the UBRR settings in Table 68.

UBRR valueswhich yield an actualbaudrate differing less than 0.5%from thetarget

baudrate, are bold in thetable. Higher errorratings areacceptable,but the receiverwill

have less noise resistance when theerrorratings are high, especially forlarge serial

frames(see “AsynchronousOperational Range” on page 146).Theerror values are cal-

culated using the following equation:

Bit 15 14 13 12 11 10 9 8

URSEL – – – UBRR[11:8] UBRRH

UBRR[7:0] UBRRL

76543 210

Read/Write R/W RRRR/W R/W R/W R/W

R/W R/W R/W R/W R/W R/W R/W R/W

Initial Value00000000

00000000

Error[%]

BaudRate

Closest Match

BaudRate

--------- ----------------- ------------ ------------------ 1–

100%•=

1 2 ... 150 151 152 153 154 155 156 157 158 159 160 ... 222 223

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