PIC18CXX2
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.
15.1
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 15-1 shows the formula for computation of the
baud rate for different USART modes, which only apply
in master mode (internal clock).
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.
15.1.1 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.
Given the desired baud rate and Fosc, the nearest inte-
ger value for the SPBRG register can be calculated
using the formula in Table 15-1. From this, the error in
baud rate can be determined.
Example 15-1 shows the calculation of the baud rate
error for the following conditions:
FOSC = 16 MHz
Desired Baud Rate = 9600
BRGH = 0
SYNC = 0
EXAMPLE 15-1: CALCULATING BAUD RATE ERROR
Desired Baud rate
Solving for X:
= Fosc / (64 (X + 1))
X
X
X
= ( (Fosc / Desired Baud rate) / 64 ) - 1
= ((16000000 / 9600) / 64) - 1
= [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%
TABLE 15-1: BAUD RATE FORMULA
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))
NA
X = value in SPBRG (0 to 255)
TABLE 15-2: REGISTERS ASSOCIATED WITH BAUD RATE GENERATOR
Value on
POR,
BOR
Value on all
other resets
Name
Bit 7
Bit 6
Bit 5
Bit 4
Bit 3
Bit 2
Bit 1
Bit 0
TXSTA
RCSTA
SPBRG
CSRC
TX9
TXEN SYNC
—
—
BRGH TRMT TX9D 0000 -010 0000 -010
FERR OERR RX9D 0000 -00x 0000 -00x
0000 0000 0000 0000
SPEN
RX9
SREN CREN
Baud Rate Generator Register
Legend: x = unknown, - = unimplemented read as '0'.
Shaded cells are not used by the BRG.
7/99 Microchip Technology Inc.
Preliminary
DS39026B-page 153
MICROCHIP [ MICROCHIP ]
