PIC16C73A-04/SO【PDF 4/114 页】IC MCU OTP 4KX14 A/D PWM 28SOIC

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1997 Microchip Technology Inc.

DS30390E-page 101

PIC16C7X

12.1

USART Baud Rate Generator (BRG)

The BRG supports both the Asynchronous and Syn-

chronous modes of the USART. It is a dedicated 8-bit

baud rate generator. The SPBRG register controls the

period of a free running 8-bit timer. In asynchronous

mode bit BRGH (TXSTA<2>) also controls the baud

rate. In synchronous mode bit BRGH is ignored.

Table 12-1 shows the formula for computation of the

baud rate for different USART modes which only apply

in master mode (internal clock).

Given the desired baud rate and Fosc, the nearest inte-

ger value for the SPBRG register can be calculated

using the formula in Table 12-1. From this, the error in

baud rate can be determined.

Example 12-1 shows the calculation of the baud rate

error for the following conditions:

FOSC = 16 MHz

Desired Baud Rate = 9600

BRGH = 0

SYNC = 0

Applicable Devices

72 73 73A 74 74A 76 77

EXAMPLE 12-1: CALCULATING BAUD

RATE ERROR

It may be advantageous to use the high baud rate

(BRGH = 1) even for slower baud clocks. This is

because the FOSC/(16(X + 1)) equation can reduce the

baud rate error in some cases.

Writing a new value to the SPBRG register, causes the

BRG timer to be reset (or cleared), this ensures the

BRG does not wait for a timer overow before output-

ting the new baud rate.

Note:

For the PIC16C73/73A/74/74A, the asyn-

chronous high speed mode (BRGH = 1)

may experience a high rate of receive

errors. It is recommended that BRGH = 0.

If you desire a higher baud rate than

BRGH = 0 can support, refer to the device

errata for additional information, or use the

PIC16C76/77.

Desired Baud rate = Fosc / (64 (X + 1))

9600

16000000 /(64 (X + 1))

25.042 = 25

Calculated Baud Rate=16000000 / (64 (25 + 1))

9615

Error

(Calculated Baud Rate - Desired Baud Rate)

Desired Baud Rate

(9615 - 9600) / 9600

0.16%

TABLE 12-1:

BAUD RATE FORMULA

TABLE 12-2:

REGISTERS ASSOCIATED WITH BAUD RATE GENERATOR

SYNC

BRGH = 0 (Low Speed)

BRGH = 1 (High Speed)

(Asynchronous) Baud Rate = FOSC/(64(X+1))

(Synchronous) Baud Rate = FOSC/(4(X+1))

Baud Rate= FOSC/(16(X+1))

X = value in SPBRG (0 to 255)

Address

Name

Bit 7

Bit 6

Bit 5

Bit 4

Bit 3

Bit 2

Bit 1

Bit 0

Value on:

POR,

BOR

Value on all

other resets

98h

TXSTA

CSRC

TX9

TXEN

SYNC

—

BRGH

TRMT

TX9D

0000 -010

18h

RCSTA

SPEN

RX9

SREN

CREN

—

FERR

OERR RX9D 0000 -00x

0000 -00x

99h

SPBRG

Baud Rate Generator Register

0000 0000

Legend: x = unknown, - = unimplemented read as '0'. Shaded cells are not used by the BRG.

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