Datasheet
1997-2013 Microchip Technology Inc. DS30234E-page 107
PIC16C6X
12.1 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 12-1 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 Table 12-1. From this, the error in
baud rate can be determined.
Example 12-1 shows the calculation of the baud rate
error for the following conditions:
F
OSC = 16 MHz
Desired Baud Rate = 9600
BRGH = 0
SYNC = 0
Applicable Devices
61 62 62A R6263R6364 64A R64 65 65A R65 66 67
EXAMPLE 12-1: 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 that the
BRG does not wait for a timer overflow before output-
ting the new baud rate.
Note: For the PIC16C63/R63/65/65A/R65 the
asynchronous high speed mode
(BRGH = 1) may experience a high rate of
receive errors. It is recommended that
BRGH = 0. If you desire a higher baud rate
than BRGH = 0 can support, refer to the
device errata for additional information or
use the PIC16C66/67.
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%
TABLE 12-1: BAUD RATE FORMULA
TABLE 12-2: REGISTERS ASSOCIATED WITH BAUD RATE GENERATOR
SYNC BRGH = 0 (Low Speed) BRGH = 1 (High Speed)
0
1
(Asynchronous) Baud Rate = F
OSC/(64(X+1))
(Synchronous) Baud Rate = F
OSC/(4(X+1))
Baud Rate = F
OSC/(16(X+1))
N/A
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 — FERR OERR RX9D 0000 -00x 0000 -00x
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.