Datasheet

366
SAM7S Series [DATASHEET]
6175M–ATARM–26-Oct-12
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%.
31.6.1.3 SAM7S512/156/128 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:
The modified architecture is presented below:
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%
Table 31-2. Baud Rate Example (OVER = 0) (Continued)
Source Clock
Expected Baud
Rate Calculation Result CD Actual Baud Rate Error
BaudRate MCK CD 16×⁄=
Error 1
ExpectedBaudRate
ActualBaudRate
-------------------------------------------------------- -
⎝⎠
⎛⎞
–=
Baudrate
SelectedClock
82 Over–()CD
FP
8
------- -+
⎝⎠
⎛⎞
⎝⎠
⎛⎞
------------------------------------------------------------------=