Datasheet

Table Of Contents
© 2006 Microchip Technology Inc. DS41159E-page 185
PIC18FXX8
18.1 USART Baud Rate Generator
(BRG)
The BRG supports both the Asynchronous and
Synchronous 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 register) also
controls the baud rate. In Synchronous mode, bit
BRGH is ignored. Table 18-1 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 F
OSC, the nearest
integer value for the SPBRG register can be calculated
using the formula in Table 18-1. From this, the error in
baud rate can be determined.
Example 18-1 shows the calculation of the baud rate
error for the following conditions:
F
OSC = 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.
18.1.1 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 18-1: CALCULATING BAUD RATE ERROR
TABLE 18-1: BAUD RATE FORMULA
TABLE 18-2: REGISTERS ASSOCIATED WITH BAUD RATE GENERATOR
Desired Baud Rate = FOSC/(64 (X + 1))
Solving for X:
X = ((F
OSC/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 = F
OSC/(64 (X + 1))
(Synchronous) Baud Rate = F
OSC/(4 (X + 1))
Baud Rate = F
OSC/(16 (X + 1))
NA
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 —BRGHTRMT TX9D 0000 -010 0000 -010
RCSTA SPEN
RX9 SREN CREN ADDEN FERR OERR RX9D 0000 000x 0000 000u
SPBRG Baud Rate Generator Register 0000 0000 0000 0000
Legend: x = unknown, - = unimplemented, read as ‘0’. Shaded cells are not used by the BRG.