PIC16F720-ESLASHML datasheet(129/244 Pages) MICROCHIP

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 2010 Microchip Technology Inc.

DS41430A-page 129

PIC16F/LF720/721

16.2

AUSART Baud Rate Generator

(BRG)

The Baud Rate Generator (BRG) is an 8-bit timer that

is dedicated to the support of both the asynchronous

and synchronous AUSART operation.

The SPBRG register determines the period of the free

running baud rate timer. In Asynchronous mode the

multiplier of the baud rate period is determined by the

BRGH bit of the TXSTA register. In Synchronous mode,

the BRGH bit is ignored.

Table 16-3 contains the formulas for determining the

baud rate. Example 16-1 provides a sample calculation

for determining the baud rate and baud rate error.

Typical baud rates and error values for various

asynchronous modes have been computed for your

convenience and are shown in Table 16-5. It may be

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

reduce the baud rate error.

Writing a new value to the SPBRG register causes the

BRG timer to be reset (or cleared). This ensures that

the BRG does not wait for a timer overflow before

outputting the new baud rate.

EXAMPLE 16-1:

CALCULATING BAUD

RATE ERROR

For a device with FOSC of 16 MHz, desired baud rate of

9600, and Asynchronous mode with SYNC = 0 and BRGH

= 0 (as seen in Table 16-5):

Solving for SPBRG:

SPBRG

FOSC

64 Desired Baud Rate



---------------------------------------------------------





–

Desired Baud Rate

FOSC

64 SPBRG



---------------------------------------

16000000

64 9600



------------------------





–

25.042



Actual Baud Rate

16000000

64 25



---------------------------

9615

Error

Actua l Baud Rate

Desired Baud Rate

–

Desired Baud Rate

--------------------------------------------------------------------------------------------------





100

9615

9600

–

9600

------------------------------





100

0.16%

TABLE 16-3:

BAUD RATE FORMULAS

Configuration Bits

AUSART Mode

Baud Rate Formula

SYNC

BRGH

Asynchronous

FOSC/[64 (n+1)]

Asynchronous

FOSC/[16 (n+1)]

Synchronous

FOSC/[4 (n+1)]

Legend:

= Don’t care, n = value of SPBRG register

TABLE 16-4:

REGISTERS ASSOCIATED WITH THE BAUD RATE GENERATOR

Name

Bit 7Bit 6Bit 5Bit 4Bit 3Bit 2Bit 1Bit 0

Value on

POR, BOR

Value on

all other

Resets

RCSTA

SPEN

RX9

SREN

CREN

ADDEN

FERR

OERR

RX9D

0000 000x

SPBRG

BRG7

BRG6

BRG5

BRG4

BRG3

BRG2

BRG1

BRG0

0000 0000

TXSTA

CSRC

TX9

TXEN

SYNC

—BRGH

TRMT

TX9D

0000 -010

Legend:

= unknown, - = unimplemented read as ‘0’. Shaded cells are not used for the Baud Rate Generator.

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