
171
8154B–AVR–07/09
ATmega16A
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 8 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.
19.11 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 UBRR settings in
Table 19-9. 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
Error[%]
BaudRate
Closest Match
BaudRate
-------------------------------------------------------- 1
–
100%
=
Table 19-9.
Examples of UBRR Settings for Commonly Used Oscillator Frequencies
Baud
Rate
(bps)
f
osc = 1.0000 MHz
f
osc = 1.8432 MHz
f
osc = 2.0000 MHz
U2X = 0
U2X = 1
U2X = 0
U2X = 1
U2X = 0
U2X = 1
UBRR
Error
UBRR
Error
UBRR
Error
UBRR
Error
UBRR
Error
UBRR
Error
2400
250.2%510.2%470.0%
95
0.0%510.2%
103
0.2%
4800
120.2%250.2%230.0%
47
0.0%250.2%510.2%
9600
6
-7.0%
12
0.2%
11
0.0%
23
0.0%
12
0.2%
25
0.2%
14.4k
3
8.5%
8
-3.5%
7
0.0%
15
0.0%
8
-3.5%
16
2.1%
19.2k
2
8.5%
6
-7.0%
5
0.0%
11
0.0%
6
-7.0%
12
0.2%
28.8k
1
8.5%
3
8.5%
3
0.0%
7
0.0%
3
8.5%
8
-3.5%
38.4k
1
-18.6%
2
8.5%
2
0.0%
5
0.0%
2
8.5%
6
-7.0%
57.6k
0
8.5%
1
8.5%
1
0.0%
3
0.0%
1
8.5%
3
8.5%
76.8k
–
1
-18.6%
1
-25.0%
2
0.0%
1
-18.6%
2
8.5%
115.2k
–
0
8.5%
0
0.0%
1
0.0%
0
8.5%
1
8.5%
230.4k
–
–––
–0
0.0%
–––
–
250k
–
–––
––0
0.0%
Max (1)
62.5 kbps
125 kbps
115.2 kbps
230.4 kbps
125 kbps
250 kbps
1.
UBRR = 0, Error = 0.0%