PC87591L datasheet(156/437 Pages) NSC

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Embedded Controller Modules (Continued)

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Revision 1.07

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:

BR = SYS_CLK/(16 x N x P)

where:

●

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

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:

●

Receive Buffer Full

●

Receive Error

●

Transmit Buffer Empty

Table 21. Pre-Scaler Factors

Pre-Scaler Select

Pre-Scaler Factor

Pre-Scaler Select

Pre-Scaler Factor

00000

NO CLOCK

10000

8.5

00001

10001

00010

1.5

10010

9.5

00011

10011

00100

2.5

10100

10.5

00101

10101

00110

3.5

10110

11.5

00111

10111

01000

4.5

11000

12.5

01001

11001

01010

5.5

11010

13.5

01011

11011

01100

6.5

11100

14.5

01101

11101

01110

7.5

11110

15.5

01111

11111

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