188
7530J–AVR–03/12
Atmel ATmega48/88/168 Automotive
17.9.5
USART Baud Rate Registers – UBRRnL and UBRRnH
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.
17.10 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 UBRRn settings in
Table 17-9. 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
“AsynchronousBit
15
14
13121110
9
8
––––
UBRRn[11:8]
UBRRnH
UBRRn[7:0]
UBRRnL
76543210
Read/Write
RRRR
R/W
Initial Value
00000000
Error[%]
BaudRate
Closest Match
BaudRate
--------------------------------------------------------
1
–
100%
=
Table 17-9.
Examples of UBRRn Settings for Commonly Used Oscillator Frequencies
Baud
Rate
(bps)
fosc = 1.0000 MHz
fosc = 1.8432 MHz
fosc = 2.0000 MHz
U2Xn = 0U2Xn = 1U2Xn = 0U2Xn = 1U2Xn = 0U2Xn = 1
UBRR
nError
UBRR
nError
UBRR
nError
UBRR
nError
UBRR
nError
UBRR
nError
2400
250.2%510.2%470.0%950.0%510.2%
103
0.2%
4800
120.2%250.2%230.0%470.0%250.2%510.2%
9600
6
-7.0%
120.2%110.0%230.0%120.2%250.2%
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%