PIC17C75X Datasheet, PDF (110/320 Pages) Microchip Technology – High-Performance 8-Bit CMOS EPROM Microcontrollers

PIC17C75X
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
SYNC
Mode
Baud Rate
0 Asynchronous
1
Synchronous
FOSC/(64(X+1))
FOSC/(4(X+1))
X = value in SPBRG (0 to 255)
Example 14-1 shows the calculation of the baud rate
error for the following conditions:
FOSC = 16 MHz
Desired Baud Rate = 9600
SYNC = 0
EXAMPLE 14-1: CALCULATING BAUD
RATE ERROR
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%
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 outputting the
new baud rate.
TABLE 14-3: REGISTERS ASSOCIATED WITH BAUD RATE GENERATOR
Address
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
(Note1)
13h, Bank 0 RCSTA1
SPEN RX9 SREN CREN —
FERR OERR RX9D 0000 -00x 0000 -00u
15h, Bank 0 TXSTA1
CSRC TX9 TXEN SYNC —
17h, Bank 0 SPBRG1 Baud rate generator register
—
TRMT TX9D 0000 --1x 0000 --1u
xxxx xxxx uuuu uuuu
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
xxxx xxxx uuuu uuuu
Legend: x = unknown, u = unchanged, - = unimplemented read as a '0', shaded cells are not used by the Baud Rate
Generator.
Note 1: Other (non power-up) resets include: external reset through MCLR and Watchdog Timer Reset.
DS30264A-page 110
Preliminary
© 1997 Microchip Technology Inc.