pc87591l-n05 National Semiconductor Corporation, pc87591l-n05 Datasheet - Page 159

• Manufacturer Part Number: pc87591l-n05
• Description: Lpc Mobile Embedded Controller
• Manufacturer: National Semiconductor Corporation

Revision 1.2
4.0 Embedded Controller Modules
4.9.5
Calculating the Baud Rate in Asynchronous Mode
The equation for calculating the baud rate is:
BR = SYS_CLK/(16xNxP)
where:
Assuming a system clock of 5 MHz and a desired baud rate of 9600, the NxP term, according to the equation above, is:
The NxP term is then divided by each pre-scaler factor in Table 20 on page 152 to obtain a value closest to an integer. The
factor for this example is 6.5:
The baud rate register is programed with a baud rate divisor of 4 (N = baud rate divisor +1). This produces a baud clock of:
Note that the percent error is much lower than would be possible without the non-integer pre-scaler factor. Refer to the table
below for more examples.
Calculating the Baud Rate in Synchronous Mode
The equation for calculating the baud rate is:
where:
The same procedure is used for determining the values of N and P, as in Asynchronous mode. However, non-integer pre-
scale values are not allowed.
• BR is the baud rate
• SYS_CLK is the system clock
• N is the value of the baud rate divisor + 1
• P is the pre-scaler divide factor selected by the value in the PSR register
• BR is the baud rate
• SYS_CLK is the system clock
• N is the value of the baud rate divisor + 1
• P is the pre-scaler divide factor selected by the value in the PSR register
NxP = (5x10
N = 32.552/6.5 = 5.008 (N = 5)
BR = (5x10
% error = (9615.385-9600)/9600 = 0.16
BR = SYS_CLK/(2xNxP)
Usage Hints
6
System Clock
)/(16x5x6.5) = 9615.385
6
)/(16x9600) = 32.552
10 MHz
20 MHz
4 MHz
5 MHz
Baud Rate
Desired
19200
19200
9600
9600
(Continued)
N
2
5
5
5
159
6.5
6.5
13
13
P
Actual Baud Rate
19230.769
19230.769
9615.385
9615.385
Percent Error
0.16
0.16
0.16
0.16
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