PIC16C77X Datasheet, PDF (99/200 Pages) Microchip Technology – 28/40-Pin, 8-Bit CMOS Microcontrollers w/ 12-Bit A/D

PIC16C77X
9.1 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 9-1 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 9-1. From this, the error in
baud rate can be determined.
Example 9-1 shows the calculation of the baud rate
error for the following conditions:
FOSC = 16 MHz
Desired Baud Rate = 9600
BRGH = 0
SYNC = 0
TABLE 9-1 BAUD RATE FORMULA
EXAMPLE 9-1: 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.
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.
9.1.1 SAMPLING
The data on the RC7/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.
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))
NA
TABLE 9-2 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 0000 -010 0000 -010
18h
RCSTA SPEN RX9 SREN CREN ADDEN 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.
0000 000x
0000 0000
© 1999 Microchip Technology Inc.
Advance Information
DS30275A-page 99