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 corrupted if the baud rate is
changed. Writing UBRRL will trigger an immediate update of the baud rate prescaler.
22.10 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 UBRR settings in Table 22-9 to Table 22-12.
UBRR 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
resistance when the error ratings are high, especially for large serial frames (see “Asynchronous
Operational Range” on page 221). The error values are calculated using the following equation:
BaudRateClosest Match
⎛
⎞
Error[%] = ------------------------------------------------------- – 1 • 100%
⎝
⎠
BaudRate
Table 22-9. 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
0.2%
0.2%
-7.0%
8.5%
8.5%
8.5%
-18.6%
8.5%
–
Error
0.2%
0.2%
0.2%
-3.5%
-7.0%
8.5%
8.5%
8.5%
-18.6%
8.5%
–
Error
0.0%
0.0%
0.0%
0.0%
0.0%
0.0%
0.0%
0.0%
-25.0%
0.0%
–
Error
0.0%
0.0%
0.0%
0.0%
0.0%
0.0%
0.0%
0.0%
0.0%
0.0%
0.0%
–
Error
0.2%
0.2%
0.2%
-3.5%
-7.0%
8.5%
8.5%
8.5%
-18.6%
8.5%
–
Error
0.2%
0.2%
0.2%
2.1%
0.2%
-3.5%
-7.0%
8.5%
8.5%
8.5%
–
2400
4800
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
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
0.0%
Max. (1)
62.5 kbps
UBRR = 0, Error = 0.0%
125 kbps
115.2 kbps
230.4 kbps
125 kbps
250 kbps
1.
228
ATmega640/1280/1281/2560/2561
2549L–AVR–08/07
ATMEL [ ATMEL ]
