at91cap7e ATMEL Corporation, at91cap7e Datasheet - Page 348

• Manufacturer Part Number: at91cap7e
• Description: Customizable Microcontroller
• Manufacturer: ATMEL Corporation

Figure 29-3. Baud Rate Generator
29.6.1.1
348
AT91CAP7E
Baud Rate in Asynchronous Mode
SCK
Reserved
MCK/DIV
MCK
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.
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.
Baud Rate Calculation Example
Table 29-2
clock frequencies. This table also shows the actual resulting baud rate and the error.
USCLKS
0
1
2
3
Baudrate
shows calculations of CD to obtain a baud rate at 38400 bauds for different source
16-bit Counter
=
--------------------------------------------
(
8 2 Over
SelectedClock
CD
(
–
USCLKS = 3
)CD
0
SYNC
)
CD
>1
1
0
1
0
OVER
Sampling
Divider
FIDI
0
1
SYNC
SCK
Baud Rate
Sampling
Clock
Clock
8549A–CAP–10/08