Atmel ARM-Based Evaluation Kit AT91SAM9N12-EK AT91SAM9N12-EK Data Sheet
Product codes
AT91SAM9N12-EK
759
SAM9N12/SAM9CN11/SAM9CN12 [DATASHEET]
11063K–ATARM–05-Nov-13
Baud Rate Calculation Example
shows calculations of CD to obtain a baud rate at 38,400 bit/s 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%.
40.7.1.2 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 fraction of the
reference source clock. This fractional part is programmed with the FP field in the 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:
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 fraction of the
reference source clock. This fractional part is programmed with the FP field in the 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 40-4. Baud Rate Example (OVER = 0)
Source Clock
(MHz)
Expected Baud Rate
(Bit/s)
Calculation Result
CD
Actual Baud Rate
(Bit/s)
Error
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
---------------------------------------------------
–
=
Baudrate
SelectedClock
8 2
Over
–
(
) CD
FP
8
-------
+
-----------------------------------------------------------------
=