ATmega640/1280/1281/2560/2561
• 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 UBRRH is written.
• Bit 11:0 – UBRR11:0: USART Baud Rate Register
This is a 12-bit register which contains the USART baud rate. The UBRRH contains the
four most significant bits, and the UBRRL contains the eight least significant bits of the
USART baud rate. Ongoing transmissions by the Transmitter and Receiver will be cor-
rupted if the baud rate is changed. Writing UBRRL will trigger an immediate update of
the baud rate prescaler.
Examples of Baud Rate
Setting
For standard crystal and resonator frequencies, the most commonly used baud rates for
asynchronous operation can be generated by using the UBRR settings in Table 106 to
Table 109. UBRR values which yield an actual baud rate differing less than 0.5ꢀ from
the target baud rate, are bold in the table. Higher error ratings are acceptable, but the
Receiver will have less noise resistance when the error ratings are high, especially for
large serial frames (see “Asynchronous Operational Range” on page 220). The error
values are calculated using the following equation:
BaudRateClosest Match
⎛
⎞
Error[ꢀ] = ------------------------------------------------------- – 1 • 100ꢀ
⎝
⎠
BaudRate
Table 106. Examples of UBRRn Settings for Commonly Used Oscillator Frequencies
fosc = 1.0000 MHz fosc = 1.8432 MHz
U2Xn = 0 U2Xn = 1 U2Xn = 0 U2Xn = 1
UBRR UBRR UBRR UBRR
fosc = 2.0000 MHz
U2Xn = 0 U2Xn = 1
UBRR UBRR
Baud
Rate
(bps)
Error
0.2ꢀ
0.2ꢀ
-7.0ꢀ
8.5ꢀ
8.5ꢀ
8.5ꢀ
-18.6ꢀ
8.5ꢀ
–
Error
0.2ꢀ
0.2ꢀ
0.2ꢀ
-3.5ꢀ
-7.0ꢀ
8.5ꢀ
8.5ꢀ
8.5ꢀ
-18.6ꢀ
8.5ꢀ
–
Error
0.0ꢀ
0.0ꢀ
0.0ꢀ
0.0ꢀ
0.0ꢀ
0.0ꢀ
0.0ꢀ
0.0ꢀ
-25.0ꢀ
0.0ꢀ
–
Error
0.0ꢀ
0.0ꢀ
0.0ꢀ
0.0ꢀ
0.0ꢀ
0.0ꢀ
0.0ꢀ
0.0ꢀ
0.0ꢀ
0.0ꢀ
0.0ꢀ
–
Error
0.2ꢀ
0.2ꢀ
0.2ꢀ
-3.5ꢀ
-7.0ꢀ
8.5ꢀ
8.5ꢀ
8.5ꢀ
-18.6ꢀ
8.5ꢀ
–
Error
0.2ꢀ
0.2ꢀ
0.2ꢀ
2.1ꢀ
0.2ꢀ
-3.5ꢀ
-7.0ꢀ
8.5ꢀ
8.5ꢀ
8.5ꢀ
–
2400
25
12
6
51
25
12
8
47
23
11
7
95
47
23
15
11
7
51
25
12
8
103
51
25
16
12
8
4800
9600
14.4k
19.2k
28.8k
38.4k
57.6k
76.8k
115.2k
230.4k
250k
3
2
6
5
6
1
3
3
3
1
2
2
5
2
6
0
1
1
3
1
3
–
1
1
2
1
2
–
–
0
0
1
0
1
–
–
–
–
0
–
–
–
–
–
–
–
–
–
–
–
0
0.0ꢀ
Max. (1)
62.5 kbps
UBRR = 0, Error = 0.0ꢀ
125 kbps
115.2 kbps
230.4 kbps
125 kbps
250 kbps
1.
227
2549A–AVR–03/05
ATMEL [ ATMEL ]
