Rainbow Electronics AT91CAP9S250A - Manual (Page 531)

531
6264A–CAP–21-May-07
AT91CAP9S500A/AT91CAP9S250A
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%.
35.6.1.2 Fractional Baud Rate in Asynchronous Mode
The Baud Rate generator previously defined is subject to the following limitation: the output fre-
quency 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 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:
14 318 180 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%
Table 35-2. 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
8 2 Over –
( )
CD
FP
8
------- + ⎝ ⎠
⎛ ⎞
⎝ ⎠
⎛ ⎞
----------------------------------------------------------------- =