Atmel Evaluation Kit AT91SAM9M10-G45-EK AT91SAM9M10-G45-EK Data Sheet
Product codes
AT91SAM9M10-G45-EK
476
SAM9M10 [DATASHEET]
6355F–ATARM–12-Mar-13
31.7.1.2
Baud Rate Calculation Example
shows calculations of CD to obtain a baud rate at 38400 bauds for different source clock frequencies.
This table also shows the actual resulting baud rate and the error.
The baud rate is calculated with the following formula:
The baud rate error is calculated with the following formula. It is not recommended to work with an error higher than
5%.
5%.
31.7.1.3
Fractional Baud Rate in Asynchronous Mode
The Baud Rate generator previously defined is subject to the following limitation: the output frequency changes by
only integer multiples of the reference frequency. An approach to this problem is to integrate a fractional N clock
generator that has a high resolution. The generator architecture is modified to obtain Baud Rate changes by a frac-
tion of the reference source clock. This fractional part is programmed with the FP field in the Baud Rate Generator
Register (US_BRGR). If FP is not 0, the fractional part is activated. The resolution is one eighth of the clock divider.
This feature is only available when using USART normal mode. The fractional Baud Rate is calculated using the
following formula:
only integer multiples of the reference frequency. An approach to this problem is to integrate a fractional N clock
generator that has a high resolution. The generator architecture is modified to obtain Baud Rate changes by a frac-
tion of the reference source clock. This fractional part is programmed with the FP field in the Baud Rate Generator
Register (US_BRGR). If FP is not 0, the fractional part is activated. The resolution is one eighth of the clock divider.
This feature is only available when using USART normal mode. The fractional Baud Rate is calculated using the
following formula:
Table 31-5.
Baud Rate Example (OVER = 0)
Source Clock
Expected Baud
Rate
Calculation Result
CD
Actual Baud Rate
Error
MHz
Bit/s
Bit/s
3 686 400
38 400
6.00
6
38 400.00
0.00%
4 915 200
38 400
8.00
8
38 400.00
0.00%
5 000 000
38 400
8.14
8
39 062.50
1.70%
7 372 800
38 400
12.00
12
38 400.00
0.00%
8 000 000
38 400
13.02
13
38 461.54
0.16%
12 000 000
38 400
19.53
20
37 500.00
2.40%
12 288 000
38 400
20.00
20
38 400.00
0.00%
14 318 180
38 400
23.30
23
38 908.10
1.31%
14 745 600
38 400
24.00
24
38 400.00
0.00%
18 432 000
38 400
30.00
30
38 400.00
0.00%
24 000 000
38 400
39.06
39
38 461.54
0.16%
24 576 000
38 400
40.00
40
38 400.00
0.00%
25 000 000
38 400
40.69
40
38 109.76
0.76%
32 000 000
38 400
52.08
52
38 461.54
0.16%
32 768 000
38 400
53.33
53
38 641.51
0.63%
33 000 000
38 400
53.71
54
38 194.44
0.54%
40 000 000
38 400
65.10
65
38 461.54
0.16%
50 000 000
38 400
81.38
81
38 580.25
0.47%
BaudRate
MCK CD
16
×
⁄
=
Error
1
ExpectedBaudRate
ActualBaudRate
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