Datasheet
891
SAM4E [DATASHEET]
Atmel-11157D-ATARM-SAM4E16-SAM4E8-Datasheet_12-Jun-14
Baud Rate Calculation Example
Table 38-5 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%.
38.7.1.2 Fractional Baud Rate in Asynchronous Mode
The baud rate generator is subject to the following limitation: the output frequency changes only by 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 38-5. 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 f
peripheral clock
CD 16×⁄=
Error 1
ExpectedBaudRate
ActualBaudRate
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