Datasheet
308
AT32UC3A
has no effect. The external clock frequency must be at least 4.5 times lower than the system
clock.
When either the external clock CLK or the internal clock divided (CLK_USART/DIV) is selected,
the value programmed in CD must be even if the user has to ensure a 50:50 mark/space ratio on
the CLK pin. If the internal clock CLK_USART is selected, the Baud Rate Generator ensures a
50:50 duty cycle on the CLK pin, even if the value programmed in CD is odd.
26.7.1.5 Baud Rate in ISO 7816 Mode
The ISO7816 specification defines the bit rate with the following formula:
where:
•B is the bit rate
•Di is the bit-rate adjustment factor
•Fi is the clock frequency division factor
•f is the ISO7816 clock frequency (Hz)
Di is a binary value encoded on a 4-bit field, named DI, as represented in Table 26-3 on page
308.
Fi is a binary value encoded on a 4-bit field, named FI, as represented in Table 26-4 on page
308.
Table 26-5 on page 308 shows the resulting Fi/Di Ratio, which is the ratio between the ISO7816
clock and the baud rate clock.
If the USART is configured in ISO7816 Mode, the clock selected by the USCLKS field in the
Mode Register (MR) is first divided by the value programmed in the field CD in the Baud Rate
B
Di
Fi
-------
f×=
Table 26-3. Binary and Decimal Values for Di
DI field 0001 0010 0011 0100 0101 0110 1000 1001
Di (decimal) 1 2 4 8 16 32 12 20
Table 26-4. Binary and Decimal Values for Fi
FI field 0000 0001 0010 0011 0100 0101 0110 1001 1010 1011 1100 1101
Fi (decimal 372 372 558 744 1116 1488 1860 512 768 1024 1536 2048
Table 26-5. Possible Values for the Fi/Di Ratio
Fi/Di 372 558 774 1116 1488 1806 512 768 1024 1536 2048
1 372 558 744 1116 1488 1860 512 768 1024 1536 2048
2 186 279 372 558 744 930 256 384 512 768 1024
4 93 139.5 186 279 372 465 128 192 256 384 512
8 46.5 69.75 93 139.5 186 232.5 64 96 128 192 256
16 23.25 34.87 46.5 69.75 93 116.2 32 48 64 96 128
32 11.62 17.43 23.25 34.87 46.5 58.13 16 24 32 48 64
12 31 46.5 62 93 124 155 42.66 64 85.33 128 170.6
20 18.6 27.9 37.2 55.8 74.4 93 25.6 38.4 51.2 76.8 102.4
32058K
AVR32-01/12