PIC16C765SLASHP datasheet(77/158 Pages) MICROCHIP

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Advanced Information

DS41124A-page 77

PIC16C745/765

11.1

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 11-1 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 FINT, the nearest inte-

ger value for the SPBRG register can be calculated

using the formula in Table 11-1. From this, the error in

baud rate can be determined.

It may be advantageous to use the high baud rate

(BRGH = 1) even for slower baud clocks. This is

because the FINT/(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.

11.1.1

SAMPLING

The data on the RC7/RX/DT pin is sampled three times

near the center of each bit time by a majority detect cir-

cuit to determine if a high or a low level is present at the

RX pin.

TABLE 11-1:

BAUD RATE FORMULA

SYNC

BRGH = 0 (Low Speed)

BRGH = 1 (High Speed)

(Asynchronous) Baud Rate = FINT/(64(SPBRG+1))

(Synchronous) Baud Rate = FINT/(4(SPBRG+1))

Baud Rate= FINT/(16(SPBRG+1))

TABLE 11-2:

BAUD RATES FOR SYNCHRONOUS MODE

Desired

Baud

4 MHz

6 MHz

24 MHz

Actual

Baud

% of

Error

SPBRG

Actual

Baud

% of

Error

SPBRG

Actual

Baud

% of

Error

SPBRG

300





1200





2400





4800

4807.69

0.16

207



9600

9615.38

0.16

103

9615.38

0.16

155



19200

19230.77

0.16

19230.77

0.16



38400

38461.54

0.16

38461.54

0.16

38461.54

0.16

155

57600

58823.53

2.12

57692.31

0.16

57692.31

0.16

103

115200

125000.00

8.51

115384.62

0.16

115384.62

0.16

230400

250000.00

8.51

250000.00

8.51

230769.23

0.16

460800

500000.00

8.51

500000.00

8.51

461538.46

0.16

921600

 1000000.00

8.51

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