at91sam9263 ATMEL Corporation, at91sam9263 Datasheet - Page 514

• Manufacturer Part Number: at91sam9263
• Description: At91 Arm Thumb Microcontrollers
• Manufacturer: ATMEL Corporation

Figure 33-4. Fractional Baud Rate Generator
33.6.1.4
33.6.1.5
6249D–ATARM–20-Dec-07
SCK
Reserved
MCK/DIV
MCK
Baud Rate in Synchronous Mode
Baud Rate in ISO 7816 Mode
USCLKS
0
1
2
3
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.
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)
BaudRate
16-bit Counter
CD
B
Modulus
=
Control
=
FP
SelectedClock
------------------------------------- -
Di
----- -
Fi
×
f
CD
glitch-free
USCLKS = 3
logic
FP
SYNC
0
CD
>1
1
0
AT91SAM9263 Preliminary
1
0
OVER
Sampling
Divider
FIDI
0
1
SYNC
SCK
Baud Rate
Sampling
Clock
Clock
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