Datasheet

 1999-2013 Microchip Technology Inc. Preliminary DS41124D-page 79
PIC16C745/765
11.1 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.
Table 11-1 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 F
INT, the nearest inte-
ger value for the SPBRG register can be calculated
using the formula in Table 11-1. From this, the error in
baud rate can be determined.
It may be advantageous to use the high baud rate
(BRGH = 1) even for slower baud clocks. This is
because the F
INT/(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.
11.1.1 SAMPLING
The data on the RC7/RX/DT pin is sampled three times
near the center of each bit time by a majority detect cir-
cuit to determine if a high or a low level is present at the
RX pin.
TABLE 11-1: BAUD RATE FORMULA
TABLE 11-2: BAUD RATES FOR
SYNCHRONOUS MODE
TABLE 11-3: BAUD RATES FOR
ASYNCHRONOUS MODE
(BRGH = 0)
SYNC BRGH = 0 (Low Speed) BRGH = 1 (High Speed)
0
1
(Asynchronous) Baud Rate = FINT/(64(SPBRG+1))
(Synchronous) Baud Rate = F
INT/(4(SPBRG+1))
Baud Rate = F
INT/(16(SPBRG+1))
NA
Desired
Baud
24 MHz
Actual
Baud
% of Error SPBRG
300  
1200  
2400  
4800  
9600  
19200  
38400 38461.54 0.16 155
57600 57692.31 0.16 103
115200 115384.62 0.16 51
230400 230769.23 0.16 25
460800 461538.46 0.16 12
921600 1000000.00 8.51 5
Desired
Baud
24 MHz
Actual
Baud
% of Error SPBRG
300 
1200 
2400 2403.85 0.16 155
4800 4807.69 0.16 77
9600 9615.38 0.16 38
19200 19736.84 2.80 18
38400 41666.67 8.51 8
57600 62500.00 8.51 5
115200 125000.00 8.51 2
230400 
460800 
921600 