Datasheet

PIC16C77X
 1999-2013 Microchip Technology Inc. Advance Information DS30275B-page 99
9.1 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.
Ta ble 9 - 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 inte-
ger value for the SPBRG register can be calculated
using the formula in Tabl e 9- 1. From this, the error in
baud rate can be determined.
Example 9-1 shows the calculation of the baud rate
error for the following conditions:
F
OSC = 16 MHz
Desired Baud Rate = 9600
BRGH = 0
SYNC = 0
EXAMPLE 9-1: CALCULATING BAUD RATE
ERROR
It may be advantageous to use the high baud rate
(BRGH = 1) even for slower baud clocks. This is
because the F
OSC/(16(X + 1)) equation can reduce the
baud rate error in some cases.
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.
9.1.1 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.
TABLE 9-1 BAUD RATE FORMULA
TABLE 9-2 REGISTERS ASSOCIATED WITH BAUD RATE GENERATOR
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%
SYNC BRGH = 0 (Low Speed) BRGH = 1 (High Speed)
0
1
(Asynchronous) Baud Rate = FOSC/(64(X+1))
(Synchronous) Baud Rate = F
OSC/(4(X+1))
Baud Rate= F
OSC/(16(X+1))
NA
X = value in SPBRG (0 to 255)
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
98h TXSTA
CSRC TX9 TXEN SYNC —BRGHTRMT TX9D
0000 -010 0000 -010
18h RCSTA SPEN RX9 SREN CREN ADDEN FERR OERR RX9D
0000 000x 0000 000x
99h SPBRG Baud Rate Generator Register
0000 0000 0000 0000
Legend: x = unknown, - = unimplemented read as '0'. Shaded cells are not used by the BRG.
774.book Page 99 Tuesday, January 29, 2013 12:02 PM