PIC16F627A Datasheet, PDF (73/168 Pages) Microchip Technology – FLASH-Based 8-Bit CMOS Microcontrollers

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 FOSC, 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:
FOSC = 16 MHz
Desired Baud Rate = 9600
BRGH = 0
SYNC = 0
EXAMPLE 12-1: CALCULATING BAUD
RATE ERROR
Desired Baud Rate = _6---4---^F-₍--_x-^o---^s_+---^c--1----₎
9600
=
1----6---0---0---0---0---0---0-
64(x + 1)
x = 25.042
Calculated Baud Rate
=
--1---6---0---0---0---0---0---0---
64(25 + 1)
=
9615
Error
=
(---C----a--l--c---u---l-a---t--e--d-----B----a--u---d-----R----a---t-e---------D----e---s--i--r--e---d----B----a---u---d-----R----a--t--e---)
Desired Baud Rate
= ^{9----6---1---5----–-----9---6---0---0-} = 0.16%
9600
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.
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))
Legend: X = value in SPBRG (0 to 255)
BRGH = 1 (High Speed)
Baud Rate= FOSC/(16(X+1))
NA
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
Value on all
other
RESETS
98h
TXSTA CSRC TX9 TXEN SYNC
—
BRGH
18h
RCSTA SPEN RX9 SREN CREN ADEN FERR
99h SPBRG
Baud Rate Generator Register
Legend: x = unknown, - = unimplemented read as '0'.
Shaded cells are not used by the BRG.
TRMT
OERR
TX9D
RX9D
0000 -010
0000 000x
0000 0000
0000 -010
0000 000x
0000 0000
 2002 Microchip Technology Inc.
Preliminary
DS40044A-page 71