pc87591l National Semiconductor Corporation, pc87591l Datasheet - Page 156

• Manufacturer Part Number: pc87591l
• Description: Lpc Mobile Embedded Controllers
• Manufacturer: National Semiconductor Corporation

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Embedded Controller Modules
A pre-scaler factor of zero corresponds to NO CLOCK. The NO CLOCK condition is the USART Power-Down mode. In this
mode, the USART clock is turned off in order to reduce power consumption. The user should set the pre-scaler factor to zero
(NO CLOCK) before selecting a new baud rate. Altering the baud rate while the USART is in operation could cause incorrect
data to be received or transmitted. UPSR register must contain a value other than zero when an external clock is used at
USCLK.
In Asynchronous mode, the baud rate is calculated by:
where:
The divide by 16 operation is performed because in Asynchronous mode, the input frequency to the USART is 16 times the
baud rate. In Synchronous mode, the input clock to the USART is equal to the baud rate.
Interrupts
The USART is capable of generating interrupts on one of the following conditions:
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
Receive Buffer Full
Receive Error
Transmit Buffer Empty
BR = SYS_CLK/(16 x N x P)
Pre-Scaler Select
00000
00001
00010
00011
00100
00101
00110
00111
01000
01001
01010
01011
01100
01101
01110
01111
Pre-Scaler Factor
NO CLOCK
(Continued)
Table 21. Pre-Scaler Factors
1.5
2.5
3.5
4.5
5.5
6.5
7.5
1
2
3
4
5
6
7
8
156
Pre-Scaler Select
10000
10001
10010
10011
10100
10101
10110
10111
11000
11001
11010
11011
11100
11101
11110
11111
Pre-Scaler Factor
10.5
11.5
12.5
13.5
14.5
15.5
8.5
9.5
10
11
12
13
14
15
16
9
Revision 1.07