45.6.1.4 Baud Rate in ISO 7816 Mode

The ISO7816 specification defines the bit rate with the following formula:

B = \frac{Di}{Fi} \times f

$B = \frac{Di}{Fi} \times f$

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 45-3.

Table 45-3. 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

Fi is a binary value encoded on a 4-bit field, named FI, as represented in Table 45-4.

Table 45-4. 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 45-5 shows the resulting Fi/Di ratio, which is the ratio between the ISO7816 clock and the baud rate clock.

Table 45-5. Possible Values for the Fi/Di Ratio
Fi/Di	372	558	744	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

If the USART is configured in ISO7816 mode, the clock selected by US_MR.USCLKS is first divided by the value programmed in US_BRGR.CD. The resulting clock can be provided to the SCK pin to feed the smart card clock inputs. This means that the US_MR.CLKO bit can be written to ‘1’.

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 noninteger values of the Fi/Di ratio are not supported and the user must program FI_DI_RATIO to a value as close as possible to the expected value.

FI_DI_RATIO 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).

The following figure shows the relation between the Elementary Time Unit, corresponding to a bit time, and the ISO 7816 clock.