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142
8246B–AVR–09/11
ATtiny2313A/4313
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 corrupted if the baud rate is
changed. Writing UBRRL will trigger an immediate update of the baud rate prescaler.
14.11 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, see the following tables from
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
lated using the following equation:
Error[%]
BaudRate
Closest Match
BaudRate
--------------------------------------------------------
1
–
100%
=
Table 14-9.
Examples of UBRR Settings for Commonly Used Oscillator Frequencies
Baud
Rate
(bps)
fosc = 1.0000 MHz
fosc = 1.8432 MHz
fosc = 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%