Datasheet

 2017 Microchip Technology Inc. DS60001516A-page 451
SAM9G20
31.6.1.4 Baud Rate in Synchronous Mode
If the USART is programmed to operate in synchronous mode, the selected clock is simply divided by the field CD in US_BRGR.
In synchronous mode, if the external clock is selected (USCLKS = 3), the clock is provided directly by the signal on the USART SCK pin.
No division is active. The value written in US_BRGR has no effect. The external clock frequency must be at least 4.5 times lower than the
system clock.
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.
31.6.1.5 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 31-3.
Fi is a binary value encoded on a 4-bit field, named FI, as represented in Table 31 -4 .
Table 31-5 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.
Table 31-3: 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 31-4: 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 31-5: 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
BaudRate
SelectedClock
CD
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=
B
Di
Fi
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f×=