Baud rate generator – Rainbow Electronics ATmega103L User Manual
Page 64
ATmega603/103
64
•
Bit 0 - TXB8: Transmit Data Bit 8
When CHR9 is set (one), TXB8 is the 9th data bit in the character to be transmitted.
Baud Rate Generator
The baud rate generator is a frequency divider which generates baud rates according to the following equation:
• BAUD = Baud Rate
• f
CK
= CPU Clock frequency
• UBRR = Contents of the UART Baud Rate register, UBRR (0 - 255)
For standard crystal frequencies, the most commonly used baud rates can be generated by using the UBRR settings in
Table 25. Observe that CPU clock frequency can be lower than the XTAL frequency if the XTAL divider is enabled. UBRR
values which yield an actual baud rate differing less than 2% from the target baud rate, are bolded in the table. However,
using baud rates that have more than 1% error is not recommended. High error ratings give less noise resistance.
Table 25. UBRR Settings at Various CPU Frequencies
BAUD
f
CK
16 UBRR
1
+
(
)
--------------------------------------
=
Baud Rate
1 MHz %Error
1.8432 MHz %Error
2 MHz %Error
2.4576 MHz %Error
2400 UBRR=
25
0.2 UBRR=
47
0.0 UBRR=
51
0.2 UBRR=
63
0.0
4800 UBRR=
12
0.2 UBRR=
23
0.0 UBRR=
25
0.2 UBRR=
31
0.0
9600 UBRR=
6
7.5 UBRR=
11
0.0 UBRR=
12
0.2 UBRR=
15
0.0
14400 UBRR=
3
7.8 UBRR=
7
0.0 UBRR=
8
3.7 UBRR=
10
3.1
19200 UBRR=
2
7.8 UBRR=
5
0.0 UBRR=
6
7.5 UBRR=
7
0.0
28800 UBRR=
1
7.8 UBRR=
3
0.0 UBRR=
3
7.8 UBRR=
4
6.3
38400 UBRR=
1
22.9 UBRR=
2
0.0 UBRR=
2
7.8 UBRR=
3
0.0
57600 UBRR=
0
7.8 UBRR=
1
0.0 UBRR=
1
7.8 UBRR=
2
12.5
76800 UBRR=
0
22.9 UBRR=
1
33.3 UBRR=
1
22.9 UBRR=
1
0.0
115200 UBRR=
0
84.3 UBRR=
0
0.0 UBRR=
0
7.8 UBRR=
0
25.0
Baud Rate 3.2768 MHz %Error
3.6864 MHz %Error
4 MHz %Error
4.608 MHz %Error
2400 UBRR=
84
0.4 UBRR=
95
0.0 UBRR=
103
0.2 UBRR=
119
0.0
4800 UBRR=
42
0.8 UBRR=
47
0.0 UBRR=
51
0.2 UBRR=
59
0.0
9600 UBRR=
20
1.6 UBRR=
23
0.0 UBRR=
25
0.2 UBRR=
29
0.0
14400 UBRR=
13
1.6 UBRR=
15
0.0 UBRR=
16
2.1 UBRR=
19
0.0
19200 UBRR=
10
3.1 UBRR=
11
0.0 UBRR=
12
0.2 UBRR=
14
0.0
28800 UBRR=
6
1.6 UBRR=
7
0.0 UBRR=
8
3.7 UBRR=
9
0.0
38400 UBRR=
4
6.3 UBRR=
5
0.0 UBRR=
6
7.5 UBRR=
7
6.7
57600 UBRR=
3
12.5 UBRR=
3
0.0 UBRR=
3
7.8 UBRR=
4
0.0
76800 UBRR=
2
12.5 UBRR=
2
0.0 UBRR=
2
7.8 UBRR=
3
6.7
115200 UBRR=
1
12.5 UBRR=
1
0.0 UBRR=
1
7.8 UBRR=
2
20.0
Baud Rate 7.3728 MHz %Error
8 MHz %Error
9.216 MHz %Error
11.059 MHz %Error
2400 UBRR=
191
0.0 UBRR=
207
0.2 UBRR=
239
0.0 UBRR=
287
-
4800 UBRR=
95
0.0 UBRR=
103
0.2 UBRR=
119
0.0 UBRR=
143
0.0
9600 UBRR=
47
0.0 UBRR=
51
0.2 UBRR=
59
0.0 UBRR=
71
0.0
14400 UBRR=
31
0.0 UBRR=
34
0.8 UBRR=
39
0.0 UBRR=
47
0.0
19200 UBRR=
23
0.0 UBRR=
25
0.2 UBRR=
29
0.0 UBRR=
35
0.0
28800 UBRR=
15
0.0 UBRR=
16
2.1 UBRR=
19
0.0 UBRR=
23
0.0
38400 UBRR=
11
0.0 UBRR=
12
0.2 UBRR=
14
0.0 UBRR=
17
0.0
57600 UBRR=
7
0.0 UBRR=
8
3.7 UBRR=
9
0.0 UBRR=
11
0.0
76800 UBRR=
5
0.0 UBRR=
6
7.5 UBRR=
7
6.7 UBRR=
8
0.0
115200 UBRR=
3
0.0 UBRR=
3
7.8 UBRR=
4
0.0 UBRR=
5
0.0