ATmega3290P datasheet(190/385 Pages) ATMEL

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190

8021E–AVR–07/09

ATmega329P/3290P

18.10.5

UBRRnL and UBRRnH – USART Baud Rate Registers n

• Bit 15:12 – Reserved Bits

These bits are reserved for future use. For compatibility with future devices, these bit must be

written to zero when UBRRnH is written.

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

This is a 12-bit register which contains the USART baud rate. The UBRRnH contains the four

most significant bits, and the UBRRnL contains the eight least significant bits of the USART

baud rate. Ongoing transmissions by the Transmitter and Receiver will be corrupted if the baud

rate is changed. Writing UBRRnL will trigger an immediate update of the baud rate prescaler.

18.11 Examples of Baud Rate Setting

For standard crystal and resonator frequencies, the most commonly used baud rates for asyn-

chronous operation can be generated by using the UBRRn settings in Table 18-3. UBRRn

values which yield an actual baud rate differing less than 0.5% from the target baud rate, are

bold in the table. Higher error ratings are acceptable, but the Receiver will have less noise resis-

tance when the error ratings are high, especially for large serial frames (see ”Asynchronous

Operational Range” on page 183). The error values are calculated using the following equation:

Figure 18-13. UCPOLn Bit Settings

UCPOLn

Transmitted Data Changed (Output

of TxD Pin)

Received Data Sampled (Input

on RxD Pin)

Rising XCK Edge

Falling XCK Edge

Rising XCK Edge

Bit

1514131211

–

UBRRn[11:8]

UBRRnH

UBRRn[7:0]

UBRRnL

543210

Read/Write

RRRR

R/W

Initial Value

000000

Error[%]

BaudRate

Closest Match

BaudRate

-------------------------------------------------------- 1

–

⎝⎠

⎛⎞

100%

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