PIC16F627A datasheet(75/180 Pages) MICROCHIP

75 / 180 page

DS40044D-page 73

PIC16F627A/628A/648A

12.1

USART Baud Rate Generator

(BRG)

The BRG supports both the Asynchronous and

Synchronous 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

integer 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

EQUATION 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) and ensures the

BRG does not wait for a timer overflow before

outputting the new baud rate.

The data on the RB1/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.

TABLE 12-1:

BAUD RATE FORMULA

TABLE 12-2:

REGISTERS ASSOCIATED WITH BAUD RATE GENERATOR

Desired Baud Rate

Fosc

64 x

()

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

9600

16000000

64 x

()

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

x25.042

Calculated Baud Rate

16000000

64 25

()

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

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))

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

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

Legend:

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

Value on all

other Resets

98h

TXSTA

CSRC

TX9

TXEN

SYNC

—BRGH

TRMT

TX9D

0000 -010

18h

RCSTA

SPEN

RX9

SREN

CREN

ADEN

FERR

OERR

RX9D

0000 000x

99h

SPBRG

Baud Rate Generator Register

0000 0000

Legend:

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

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