PIC17C752-08SLASHCL datasheet(118/328 Pages) MICROCHIP

118 / 328 page

PIC17C7XX

DS30289A-page 118

14.1

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. Table 14-2 shows

the formula for computation of the baud rate for differ-

ent USART modes. These only apply when the USART

is in synchronous master mode (internal clock) and

asynchronous mode.

Given the desired baud rate and Fosc, the nearest inte-

ger value between 0 and 255 can be calculated using

the formula below. The error in baud rate can then be

determined.

TABLE 14-2:

BAUD RATE FORMULA

Example 14-1 shows the calculation of the baud rate

error for the following conditions:

FOSC = 16 MHz

Desired Baud Rate = 9600

SYNC = 0

EXAMPLE 14-1: CALCULATING BAUD

RATE ERROR

SYNC

Mode

Baud Rate

Asynchronous

Synchronous

FOSC/(64(X+1))

FOSC/(4(X+1))

X = value in SPBRG (0 to 255)

Desired Baud rate=Fosc / (64 (X + 1))

9600 =

16000000 /(64 (X + 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%

Writing a new value to the SPBRG, 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.

14.1.1

EFFECTS OF RESET

After any device reset the SPBRG register is cleared.

The SPBRG register will need to be loaded with the

desired value after each reset.

TABLE 14-3:

REGISTERS ASSOCIATED WITH BAUD RATE GENERATOR

Address

Name

Bit 7

Bit 6

Bit 5

Bit 4

Bit 3

Bit 2

Bit 1

Bit 0

Value on

POR,

BOR

MCLR, WDT

13h, Bank 0

RCSTA1

SPEN

RX9

SREN

CREN

—

FERR

OERR

RX9D

0000 -00x

0000 -00u

15h, Bank 0

TXSTA1

CSRC

TX9

TXEN

SYNC

—

TRMT

TX9D

0000 --1x

0000 --1u

17h, Bank 0

SPBRG1

Baud rate generator register

0000 0000

13h, Bank 4

RCSTA2

SPEN

RX9

SREN

CREN

—

FERR

OERR

RX9D

0000 -00x

0000 -00u

15h, Bank 4

TXSTA2

CSRC

TX9

TXEN

SYNC

—

TRMT

TX9D

0000 --1x

0000 --1u

17h, Bank 4

SPBRG2

Baud rate generator register

0000 0000

Legend:

= unknown, u = unchanged, - = unimplemented read as a '0', shaded cells are not used by the Baud Rate Generator.

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