Datasheet
PIC17C7XX
DS30289C-page 120 1998-2013 Microchip Technology Inc.
14.1 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. Table 14-2 shows
the formula for computation of the baud rate for differ-
ent USART modes. These only apply when the USART
is in Synchronous Master mode (internal clock) and
Asynchronous mode.
Given the desired baud rate and Fosc, the nearest inte-
ger value between 0 and 255 can be calculated using
the formula below. The error in baud rate can then be
determined.
TABLE 14-2: BAUD RATE FORMULA
Example 14-1 shows the calculation of the baud rate
error for the following conditions:
F
OSC = 16 MHz
Desired Baud Rate = 9600
SYNC = 0
EXAMPLE 14-1: CALCULATING BAUD
RATE ERROR
Writing a new value to the SPBRG, causes the BRG
timer to be reset (or cleared). This ensures that the
BRG does not wait for a timer overflow before output-
ting the new baud rate.
Effects of Reset
After any device RESET, the SPBRG register is
cleared. The SPBRG register will need to be loaded
with the desired value after each RESET.
TABLE 14-3: REGISTERS ASSOCIATED WITH BAUD RATE GENERATOR
SYNC Mode Baud Rate
0
1
Asynchronous
Synchronous
F
OSC/(64(X+1))
F
OSC/(4(X+1))
X = value in SPBRG (0 to 255)
Desired Baud Rate = FOSC / (64 (X + 1))
9600 = 16000000 /(64 (X + 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%
Address Name Bit 7 Bit 6 Bit 5 Bit 4 Bit 3 Bit 2 Bit 1 Bit 0
Value on
POR,
BOR
MCLR, WDT
USART1
13h, Bank 0 RCSTA1 SPEN RX9 SREN CREN — FERR OERR RX9D 0000 -00x 0000 -00u
15h, Bank 0 TXSTA1 CSRC
TX9 TXEN SYNC — — TRMT TX9D
0000 --1x 0000 --1u
17h, Bank 0 SPBRG1 Baud Rate Generator Register 0000 0000 0000 0000
USART2
13h, Bank 4 RCSTA2 SPEN RX9 SREN CREN — FERR OERR RX9D
0000 -00x 0000 -00u
15h, Bank 4 TXSTA2 CSRC
TX9 TXEN SYNC — — TRMT TX9D
0000 --1x 0000 --1u
17h, Bank 4 SPBRG2 Baud Rate Generator Register 0000 0000 0000 0000
Legend: x = unknown, u = unchanged, - = unimplemented, read as '0'. Shaded cells are not used by the Baud Rate Generator.