Datasheet
782
SAM9G25 [DATASHEET]
11032C–ATARM–25-Jan-13
39.7.1.4 Baud Rate in ISO 7816 Mode
The ISO7816 specification defines the bit rate with the following formula:
where:
z B is the bit rate
z Di is the bit-rate adjustment factor
z Fi is the clock frequency division factor
z 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 Rate Generator Register (US_BRGR). The resulting
clock can be provided to the SCK pin to feed the smart card clock inputs. This means that the CLKO bit can be set in
US_MR.
This clock is then divided by the value programmed in the FI_DI_RATIO field in the FI_DI_Ratio register (US_FIDI). This
is performed by the Sampling Divider, which performs a division by up to 2047 in ISO7816 Mode. The non-integer values
of the Fi/Di Ratio are not supported and the user must program the FI_DI_RATIO field to a value as close as possible to
the expected value.
The FI_DI_RATIO field resets to the value 0x174 (372 in decimal) and is the most common divider between the ISO7816
clock and the bit rate (Fi = 372, Di = 1).
Figure 39-5 shows the relation between the Elementary Time Unit, corresponding to a bit time, and the ISO 7816 clock.
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 163212 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