at91sam9g20-cu ATMEL Corporation, at91sam9g20-cu Datasheet - Page 463

• Manufacturer Part Number: at91sam9g20-cu
• Description: At91 Arm Thumb Microcontrollers
• Manufacturer: ATMEL Corporation

32.6.1.1
32.6.1.2
Table 32-2.
6384B–ATARM–15-Dec-08
Source Clock
3 686 400
4 915 200
5 000 000
7 372 800
MHz
Baud Rate in Asynchronous Mode
Baud Rate Calculation Example
Baud Rate Example (OVER = 0)
Expected Baud
If the external SCK clock is selected, the duration of the low and high levels of the signal pro-
vided on the SCK pin must be longer than a Master Clock (MCK) period. The frequency of the
signal provided on SCK must be at least 4.5 times lower than MCK.
Figure 32-3. Baud Rate Generator
If the USART is programmed to operate in asynchronous mode, the selected clock is first
divided by CD, which is field programmed in the Baud Rate Generator Register (US_BRGR).
The resulting clock is provided to the receiver as a sampling clock and then divided by 16 or 8,
depending on the programming of the OVER bit in US_MR.
If OVER is set to 1, the receiver sampling is 8 times higher than the baud rate clock. If OVER is
cleared, the sampling is performed at 16 times the baud rate clock.
The following formula performs the calculation of the Baud Rate.
This gives a maximum baud rate of MCK divided by 8, assuming that MCK is the highest possi-
ble clock and that OVER is programmed at 1.
Table 32-2
clock frequencies. This table also shows the actual resulting baud rate and the error.
SCK
38 400
38 400
38 400
38 400
Rate
Bit/s
Reserved
MCK/DIV
Baudrate
MCK
shows calculations of CD to obtain a baud rate at 38400 bauds for different source
USCLKS
=
0
1
2
3
Calculation Result
--------------------------------------------
(
8 2 Over
SelectedClock
(
–
12.00
6.00
8.00
8.14
16-bit Counter
)CD
CD
)
USCLKS = 3
CD
12
6
8
8
0
SYNC
AT91SAM9G20 Preliminary
CD
>1
1
0
Actual Baud Rate
1
0
38 400.00
38 400.00
39 062.50
38 400.00
Bit/s
OVER
Sampling
Divider
FIDI
0
1
SYNC
0.00%
0.00%
1.70%
0.00%
Error
SCK
Baud Rate
Sampling
Clock
Clock
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