PIC18FXX20
Example 18-1 shows the calculation of the baud rate
error for the following conditions:
• FOSC = 16 MHz
• Desired Baud Rate = 9600
• BRGH = 0
18.1 USART Baud Rate Generator
(BRG)
The BRG supports both the Asynchronous and Syn-
chronous modes of the USARTs. 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 (TXSTAx<2>) 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 FOSC, the nearest
integer value for the SPBRGx register can be calcu-
lated using the formula in Table 18-1. From this, the
error in baud rate can be determined.
• SYNC = 0
It may be advantageous to use the high baud rate
(BRGH = 1), even for slower baud clocks. This is
because the equation in Example 18-1 can reduce the
baud rate error in some cases.
Writing a new value to the SPBRGx 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 RXx pin (either RC7/RX1/DT1 or
RG2/RX2/DT2) is sampled three times by a majority
detect circuit to determine if a high or a low level is
present at the pin.
EXAMPLE 18-1:
Desired Baud Rate
CALCULATING BAUD RATE ERROR
=
FOSC / (64 (X + 1))
Solving for X:
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 18-1: 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))
N/A
Legend: X = value in SPBRGx (0 to 255)
TABLE 18-2: REGISTERS ASSOCIATED WITH BAUD RATE GENERATOR
Value on
all other
RESETS
Value on
Name
Bit 7
Bit 6
Bit 5
Bit 4
Bit 3
Bit 2
Bit 1
Bit 0
POR, BOR
TXSTAx
RCSTAx
CSRC
SPEN
TX9
RX9
TXEN SYNC
—
BRGH TRMT TX9D 0000 -010
0000 -010
0000 000x
0000 0000
SREN CREN ADDEN FERR OERR RX9D 0000 000x
SPBRGx Baud Rate Generator Register
0000 0000
Legend: x= unknown, - = unimplemented, read as '0'. Shaded cells are not used by the BRG.
Note: Register names generically refer to both of the identically named registers for the two USART modules, where
‘x’ indicates the particular module. Bit names and RESET values are identical between modules.
DS39609A-page 200
Advance Information
2003 Microchip Technology Inc.
MICROCHIP [ MICROCHIP ]
