PIC16F87X Datasheet, PDF (99/218 Pages) Microchip Technology – 28/40-pin 8-Bit CMOS FLASH Microcontrollers

PIC16F87X
10.1 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 10-1 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 10-1. 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 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.
10.1.1 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.
TABLE 10-1: 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 10-2: REGISTERS ASSOCIATED WITH BAUD RATE GENERATOR
Address Name Bit 7 Bit 6 Bit 5 Bit 4
Bit 3
Bit 2 Bit 1 Bit 0
98h
TXSTA CSRC TX9 TXEN SYNC
—
BRGH TRMT TX9D
18h
RCSTA SPEN RX9 SREN CREN ADDEN FERR OERR RX9D
99h
SPBRG Baud Rate Generator Register
Legend: x = unknown, - = unimplemented, read as '0'. Shaded cells are not used by the BRG.
Value on:
POR,
BOR
0000 -010
0000 000x
0000 0000
Value on
all other
RESETS
0000 -010
0000 000x
0000 0000
 2001 Microchip Technology Inc.
DS30292C-page 97