PIC18F4539-E/PT【PDF 80/322 页】IC PIC MCU FLASH 12KX16 44TQFP

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PIC18FXX39

DS30485A-page 168

Preliminary

2002 Microchip Technology Inc.

17.1

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 17-1 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

integer value for the SPBRG register can be calculated

using the formula in Table 17-1. From this, the error in

baud rate can be determined.

Example 17-1 shows the calculation of the baud rate

error for the following conditions:

FOSC = 16 MHz

Desired Baud Rate = 9600

BRGH = 0

SYNC = 0

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 overflow before

outputting the new baud rate.

17.1.1

SAMPLING

The data on the RC7/RX/DT pin is sampled three times

by a majority detect circuit to determine if a high or a

low level is present at the RX pin.

EXAMPLE 17-1:

CALCULATING BAUD RATE ERROR

TABLE 17-1:

BAUD RATE FORMULA

TABLE 17-2:

REGISTERS ASSOCIATED WITH BAUD RATE GENERATOR

Desired Baud Rate

= FOSC / (64 (X + 1))

Solving for X:

= ( (FOSC / Desired Baud Rate) / 64 ) – 1

= ((16000000 / 9600) / 64) – 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%

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))

N/A

Legend: X = value in SPBRG (0 to 255)

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

TXSTA

CSRC

TX9

TXEN

SYNC

—BRGH

TRMT

TX9D

0000 -010

RCSTA

SPEN

RX9

SREN

CREN

ADDEN

FERR

OERR

RX9D

0000 -00x

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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