at91rm3400 ATMEL Corporation, at91rm3400 Datasheet - Page 272

• Manufacturer Part Number: at91rm3400
• Description: Atmel Advanced At91 Arm Microcontroller
• Manufacturer: ATMEL Corporation

Figure 92. Baud Rate Generator
Baud Rate in
Asynchronous Mode
Baud Rate Calculation
Example
272
AT91RM3400
SCK
Reserved
MCK/DIV
MCK
USCLKS
0
1
2
3
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 pos-
sible clock and that OVER is programmed at 1.
Table 44 shows calculations of CD to obtain a baud rate at 38400 bauds for different source
clock frequencies. This table also shows the actual resulting baud rate and the error.
Table 44. Baud Rate Example (OVER = 0)
Baudrate
Source Clock
12 000 000
12 288 000
14 318 180
14 745 600
3 686 400
4 915 200
5 000 000
7 372 800
8 000 000
MHz
=
16-bit Counter
--------------------------------------------
8 2 Over
SelectedClock
CD
–
Baud Rate
Expected
38 400
38 400
38 400
38 400
38 400
38 400
38 400
38 400
38 400
USCLKS = 3
Bit/s
CD
0
SYNC
CD
>1
Calculation
1
0
Result
12.00
13.02
19.53
20.00
23.30
24.00
6.00
8.00
8.14
1
0
OVER
CD
12
13
20
20
23
24
Sampling
6
8
8
Divider
FIDI
Baud Rate
38 400.00
38 400.00
39 062.50
38 400.00
38 461.54
37 500.00
38 400.00
38 908.10
38 400.00
Actual
0
1
Bit/s
SYNC
SCK
Baud Rate
Sampling
Clock
Clock
1790A–ATARM–11/03
0.00%
0.00%
1.70%
0.00%
0.16%
2.40%
0.00%
1.31%
0.00%
Error