PIC16C65T-10I/P View Datasheet(PDF) - Microchip Technology

Part Name

Description

Manufacturer

PIC16C65T-10I/P

8-Bit CMOS Microcontrollers

Microchip Technology 

12.1 USART Baud Rate Generator (BRG)

Applicable Devices

61 62 62A R62 63 R63 64 64A R64 65 65A R65 66 67

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 Fosc, the nearest inte-

ger 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

PIC16C6X

EXAMPLE 12-1: CALCULATING BAUD

RATE ERROR

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

9600 = 16000000 /(64 (X + 1))

X = 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%

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.

Note:

For the PIC16C63/R63/65/65A/R65 the

asynchronous high speed mode

(BRGH = 1) may experience a high rate of

receive errors. It is recommended that

BRGH = 0. If you desire a higher baud rate

than BRGH = 0 can support, refer to the

device errata for additional information or

use the PIC16C66/67.

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 output-

ting the new baud rate.

TABLE 12-1: BAUD RATE FORMULA

SYNC

BRGH = 0 (Low Speed)

0

(Asynchronous) Baud Rate = FOSC/(64(X+1))

1

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

X = value in SPBRG (0 to 255)

BRGH = 1 (High Speed)

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

N/A

TABLE 12-2: 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

Value on

all other

Resets

98h

TXSTA

CSRC TX9 TXEN SYNC — BRGH TRMT TX9D

18h

RCSTA

SPEN RX9 SREN CREN — FERR OERR RX9D

99h

SPBRG Baud Rate Generator Register

Legend: x = unknown, - = unimplemented read as '0'. Shaded cells are not used by the BRG.

0000 -010 0000 -010

0000 -00x 0000 -00x

0000 0000 0000 0000

© 1997 Microchip Technology Inc.

DS30234D-page 107

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