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Sheet目录3841

99页 100页 101页 102页 [103页]104页 105页 106页 107页

2009 Microchip Technology Inc.

DS39636D-page 197

PIC18F2X1X/4X1X

17.1

Baud Rate Generator (BRG)

The BRG is a dedicated 8-bit or 16-bit generator that

supports both the Asynchronous and Synchronous

modes of the EUSART. By default, the BRG operates

in 8-Bit mode; setting the BRG16 bit (BAUDCON<3>)

selects 16-Bit mode.

The SPBRGH:SPBRG register pair controls the period

of a free running timer. In Asynchronous mode, bits

BRGH (TXSTA<2>) and BRG16 (BAUDCON<3>) also

control the baud rate. In Synchronous mode, BRGH is

ignored. Table 17-1 shows the formula for computation

of the baud rate for different EUSART modes which

only apply in Master mode (internally generated clock).

Given the desired baud rate and FOSC, the nearest

integer value for the SPBRGH:SPBRG registers can be

calculated using the formulas in Table 17-1. From this,

the error in baud rate can be determined. An example

calculation is shown in Example 17-1. Typical baud

rates and error values for the various Asynchronous

modes are shown in Table 17-2. It may be

advantageous to use the high baud rate (BRGH = 1) or

the 16-bit BRG to reduce the baud rate error, or

achieve a slow baud rate for a fast oscillator frequency.

Writing a new value to the SPBRGH:SPBRG registers

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

OPERATION IN POWER-MANAGED

MODES

The device clock is used to generate the desired baud

rate. When one of the power-managed modes is

entered, the new clock source may be operating at a

different frequency. This may require an adjustment to

the value in the SPBRG register pair.

17.1.2

SAMPLING

The data on the RX 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.

TABLE 17-1:

BAUD RATE FORMULAS

EXAMPLE 17-1:

CALCULATING BAUD RATE ERROR

TABLE 17-2:

REGISTERS ASSOCIATED WITH BAUD RATE GENERATOR

Configuration Bits

BRG/EUSART Mode

Baud Rate Formula

SYNC

BRG16

BRGH

8-bit/Asynchronous

FOSC/[64 (n + 1)]

8-bit/Asynchronous

FOSC/[16 (n + 1)]

16-bit/Asynchronous

FOSC/[4 (n + 1)]

8-bit/Synchronous

16-bit/Synchronous

Legend: x = Don’t care, n = value of SPBRGH:SPBRG register pair

Name

Bit 7

Bit 6

Bit 5

Bit 4

Bit 3

Bit 2

Bit 1

Bit 0

Reset Values

on page

TXSTA

CSRC

TX9

TXEN

SYNC

SENDB

BRGH

TRMT

TX9D

RCSTA

SPEN

RX9

SREN

CREN

ADDEN

FERR

OERR

RX9D

BAUDCON ABDOVF

RCIDL

—

SCKP

BRG16

—

WUE

ABDEN

SPBRGH

EUSART Baud Rate Generator Register, High Byte

SPBRG

EUSART Baud Rate Generator Register, Low Byte

Legend: — = unimplemented, read as ‘0’. Shaded cells are not used by the BRG.

For a device with FOSC of 16 MHz, desired baud rate of 9600, Asynchronous mode, 8-bit BRG:

Desired Baud Rate

= FOSC/(64 ([SPBRGH:SPBRG] + 1))

Solving for SPBRGH:SPBRG:

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%

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