PIC18FXX39
DS30485A-page 168
Preliminary
2002 Microchip Technology Inc.
17.1
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 17-1 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 17-1. From this, the error in
baud rate can be determined.
error for the following conditions:
FOSC = 16 MHz
Desired Baud Rate = 9600
BRGH = 0
SYNC = 0
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
outputting the new baud rate.
17.1.1
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.
EXAMPLE 17-1:
CALCULATING BAUD RATE ERROR
TABLE 17-1:
BAUD RATE FORMULA
TABLE 17-2:
REGISTERS ASSOCIATED WITH BAUD RATE GENERATOR
Desired Baud Rate
= FOSC / (64 (X + 1))
Solving for X:
X
= ( (FOSC / Desired Baud Rate) / 64 ) – 1
X
= ((16000000 / 9600) / 64) – 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%
SYNC
BRGH = 0 (Low Speed)
BRGH = 1 (High Speed)
0
1
(Asynchronous) Baud Rate = FOSC/(64(X+1))
(Synchronous) Baud Rate = FOSC/(4(X+1))
Baud Rate = FOSC/(16(X+1))
N/A
Legend: X = value in SPBRG (0 to 255)
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
TXSTA
CSRC
TX9
TXEN
SYNC
—BRGH
TRMT
TX9D
0000 -010
RCSTA
SPEN
RX9
SREN
CREN
ADDEN
FERR
OERR
RX9D
0000 -00x
SPBRG
Baud Rate Generator Register
0000 0000
Legend: x = unknown, - = unimplemented, read as '0'. Shaded cells are not used by the BRG.