PIC16F62X
12.1
USART Baud Rate Generator (BRG)
EXAMPLE 12-1: CALCULATING BAUD RATE
ERROR
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 12-1 shows the formula for computation of the
baud rate for different USART modes which only apply
in master mode (internal clock).
Desired Baud rate = Fosc / (64 (X + 1))
9600 =
16000000 /(64 (X + 1))
X
=
Î25.042° = 25
Calculated Baud Rate=16000000 / (64 (25 + 1))
=
=
9615
Error
(Calculated Baud Rate - Desired Baud Rate)
Desired Baud Rate
Given the desired baud rate and Fosc, the nearest inte-
ger value for the SPBRG register can be calculated
using the formula in Table 12-1. From this, the error in
baud rate can be determined.
=
=
(9615 - 9600) / 9600
0.16%
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.
Example 12-1 shows the calculation of the baud rate
error for the following conditions:
FOSC = 16 MHz
Desired Baud Rate = 9600
BRGH = 0
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.
SYNC = 0
TABLE 12-1: 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 12-2: REGISTERS ASSOCIATED WITH BAUD RATE GENERATOR
Value on Value on all
POR other resets
Address Name
Bit 7
Bit 6
Bit 5
Bit 4
Bit 3
Bit 2
Bit 1
Bit 0
0000 -010 0000 -010
98h
TXSTA
CSRC TX9 TXEN SYNC
—
BRGH TRMT TX9D
0000 -00x 0000 -00x
0000 0000 0000 0000
18h
99h
RCSTA SPEN RX9 SREN CREN ADEN FERR OERR RX9D
SPBRG Baud Rate Generator Register
Legend: x = unknown, -= unimplemented read as '0'. Shaded cells are not used by the BRG.
1999 Microchip Technology Inc.
Preliminary
DS40300B-page 73
MICROCHIP [ MICROCHIP ]
