SAM7X256_14 Datasheet, PDF (297/662 Pages) ATMEL Corporation – ARM-based Flash MCU

Table 30-2. Baud Rate Example (OVER = 0) (Continued)
Expected Baud
Source Clock
Rate
Calculation Result
CD
18 432 000
38 400
30.00
30
24 000 000
38 400
39.06
39
24 576 000
38 400
40.00
40
25 000 000
38 400
40.69
40
32 000 000
38 400
52.08
52
32 768 000
38 400
53.33
53
33 000 000
38 400
53.71
54
40 000 000
38 400
65.10
65
50 000 000
38 400
81.38
81
Actual Baud Rate
38 400.00
38 461.54
38 400.00
38 109.76
38 461.54
38 641.51
38 194.44
38 461.54
38 580.25
Error
0.00%
0.16%
0.00%
0.76%
0.16%
0.63%
0.54%
0.16%
0.47%
The baud rate is calculated with the following formula:
BaudRate = MCK ⁄ CD × 16
The baud rate error is calculated with the following formula. It is not recommended to work with an error higher than 5%.
Error
=
1
–


E_----A-x_---pc_---et_--u-c_--a-t_--e_-l--Bd---B_{-a---ua----du---Rd---}R_-a---ta--e--t_--e_{-}
30.6.1.3 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 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:
Baudrate = --------------^S---^e---^l--^e---^c---^t--^e---^d----^C----^l--^o---^c---^k--------------
^{8(2 – Over)}^{CD + F---8-P---}
The modified architecture is presented below:
SAM7X Series [DATASHEET]
6120K–ATARM–11-Feb-14
297