Atmel SAM9X25, Manual

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805
11054A–ATARM–27-Jul-11
SAM9X25
805
11054A–ATARM–27-Jul-11
SAM9X25
system clock. In synchronous mode master (USCLKS = 0 or 1, CLK0 set to 1), the receive part
limits the SCK maximum frequency to MCK/4.5 in USART mode, or MCK/6 in SPI mode.
When either the external clock SCK or the internal clock divided (MCK/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
SCK pin. If the internal clock MCK is selected, the Baud Rate Generator ensures a 50:50 duty
cycle on the SCK pin, even if the value programmed in CD is odd.
39.7.1.4 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 39-6 .
Fi is a binary value encoded on a 4-bit field, named FI, as represented in Table 39-7 .
Table 39-8 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 (US_MR) is first divided by the value programmed in the field CD in the Baud
B
Di
Fi
------
f ×
=
Table 39-6. 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 39-7. 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 39-8. 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