12 ubrr1l - usart1 baud rate register low byte, 11 examples of baud rate setting, Atmega128rfa1 – Rainbow Electronics ATmega128RFA1 User Manual
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8266A-MCU Wireless-12/09
ATmega128RFA1
•
Bit 3:0 – UBRR11:8 - USART Baud Rate Register
These bits represent bits [11:8] of the Baud Rate Register. Sample values for
commonly used clock frequencies can be found in section "Examples of Baud Rate
Setting".
23.10.12 UBRR1L – USART1 Baud Rate Register Low Byte
Bit
7
6
5
4
3
2
1
0
NA ($CC)
UBRR7
UBRR6
UBRR5
UBRR4
UBRR3
UBRR2
UBRR1
UBRR0
UBRR1L
Read/Write
RW
RW
RW
RW
RW
RW
RW
RW
Initial Value
0
0
0
0
0
0
0
0
UBRR1 is a 12-bit register which contains the USART baud rate. The UBRR1H
contains the four most significant bits, and the UBRR1L 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 UBRR1L will trigger an
immediate update of the baud rate prescaler.
•
Bit 7:0 – UBRR7:0 - USART Baud Rate Register
These bits represent bits [7:0] of the Baud Rate Register. Sample values for commonly
used clock frequencies can be found in section "Examples of Baud Rate Setting".
23.11 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
. 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
). The error values are calculated using the following equation:
[ ]
%
100
1
%
⋅
−
=
BaudRate
BaudRate
Error
Match
Closest
Table 23-14. Examples of UBRRn Settings for Commonly Used Oscillator Frequencies
f
OSC
= 1.8432 MHz
f
OSC
= 2.0000 MHz
f
OSC
= 3.6864 MHz
U2Xn = 0
U2Xn = 1
U2Xn = 0
U2Xn = 1
U2Xn = 0
U2Xn = 1
Baud
Rate
(bps)
UBRR
Error
UBRR
Error
UBRR
Error
UBRR
Error
UBRR
Error
UBRR
Error
2400
47
0.0%
95
0.0%
51
0.2%
103
0.2%
95
0.0%
191
0.0%
4800
23
0.0%
47
0.0%
25
0.2%
51
0.2%
47
0.0%
95
0.0%
9600
11
0.0%
23
0.0%
12
0.2%
25
0.2%
23
0.0%
47
0.0%
14.4k
7
0.0%
15
0.0%
8
-3.5%
16
2.1%
15
0.0%
31
0.0%
19.2k
5
0.0%
11
0.0%
6
-7.0%
12
0.2%
11
0.0%
23
0.0%
28.8k
3
0.0%
7
0.0%
3
8.5%
8
-3.5%
7
0.0%
15
0.0%
38.4k
2
0.0%
5
0.0%
2
8.5%
6
-7.0%
5
0.0%
11
0.0%