PIC16F688-ISLASHML datasheet(97/204 Pages) MICROCHIP

97 / 204 page

DS41203E-page 95

PIC16F688

10.3

EUSART Baud Rate Generator

(BRG)

The Baud Rate Generator (BRG) is an 8-bit or 16-bit

timer that is dedicated to the support of both the

asynchronous and synchronous EUSART operation.

By default, the BRG operates in 8-bit mode. Setting the

BRG16 bit of the BAUDCTL register selects 16-bit

mode.

The SPBRGH, SPBRG register pair determines the

period of the free running baud rate timer. In

Asynchronous mode the multiplier of the baud rate

period is determined by both the BRGH bit of the TXSTA

Synchronous mode, the BRGH bit is ignored.

Table 10-3 contains the formulas for determining the

baud rate. Example 10-1 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 10-3. It may be

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

or the 16-bit BRG (BRG16 = 1) to reduce the baud rate

error. The 16-bit BRG mode is used to achieve slow

baud rates for fast oscillator frequencies.

Writing a new value to the SPBRGH, SPBRG register

pair 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.

If the system clock is changed during an active receive

operation, a receive error or data loss may result. To

avoid this problem, check the status of the RCIDL bit to

make sure that the receive operation is Idle before

changing the system clock.

EXAMPLE 10-1:

CALCULATING BAUD

RATE ERROR

TABLE 10-3:

BAUD RATE FORMULAS

TABLE 10-4:

REGISTERS ASSOCIATED WITH THE BAUD RATE GENERATOR

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

of 9600, Asynchronous mode, 8-bit BRG:

Solving for SPBRGH:SPBRG:

FOSC

Desired Baud Rate

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

--------------------------------------------- 1

–

Desired Baud Rate

FOSC

64 [SPBRGH:SPBRG]

()

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

16000000

9600

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

------------------------1

–

25.042

[]

Calculated Baud Rate

16000000

64 25

()

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

9615

Error

Calc. Baud Rate Desired Baud Rate

–

Desired Baud Rate

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

9615 9600

–

()

9600

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

0.16%

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:

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

Name

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

Value on

POR, BOR

Value on

all other

Resets

BAUDCTL

ABDOVF

RCIDL

—

SCKP

BRG16

—

WUE

ABDEN

01-0 0-00

RCSTA

SPEN

RX9

SREN

CREN

ADDEN

FERR

OERR

RX9D

0000 000x

SPBRG

BRG7

BRG6

BRG5

BRG4

BRG3

BRG2

BRG1

BRG0

0000 0000

SPBRGH

BRG15

BRG14

BRG13

BRG12

BRG11

BRG10

BRG9

BRG8

0000 0000

TXSTA

CSRC

TX9

TXEN

SYNC

SENDB

BRGH

TRMT

TX9D

0000 0010

Legend:

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

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