16F628 datasheet(73/168 Pages) MICROCHIP

73 / 168 page

 2002 Microchip Technology Inc.

Preliminary

DS40044A-page 71

PIC16F627A/628A/648A

12.1

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 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 F

OSC, 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:

OSC = 16 MHz

Desired Baud Rate = 9600

BRGH = 0

SYNC = 0

EXAMPLE 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 F

OSC/(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 output-

ting the new baud rate.

TABLE 12-1:

BAUD RATE FORMULA

TABLE 12-2:

REGISTERS ASSOCIATED WITH BAUD RATE GENERATOR

Desired Baud Rate

Fosc

x 1

()

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

9600

16000000

x 1

()

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

25.042

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 = F

OSC/(64(X+1))

(Synchronous) Baud Rate = F

OSC/(4(X+1))

Baud Rate= F

OSC/(16(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:

x = unknown, - = unimplemented read as '0'.

Shaded cells are not used by the BRG.

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