Datasheet

© 2009 Microchip Technology Inc. DS40044G-page 75
PIC16F627A/628A/648A
12.1 USART Baud Rate Generator
(BRG)
The BRG supports both the Asynchronous and
Synchronous 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 F
OSC, the nearest
integer 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
EQUATION 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 FOSC/(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) and ensures the
BRG does not wait for a timer overflow before
outputting the new baud rate.
The data on the RB1/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 12-1: BAUD RATE FORMULA
TABLE 12-2: REGISTERS ASSOCIATED WITH BAUD RATE GENERATOR
Desired Baud Rate
Fosc
64 x 1+()
-----------------------=
9600
16000000
64 x 1+()
------------------------=
x25.042=
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 (Asynchronous) Baud Rate = F
OSC/(64(X+1)) Baud Rate = FOSC/(16(X+1))
1 (Synchronous) Baud Rate = F
OSC/(4(X+1)) NA
Legend: 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
Value on all
other Resets
98h TXSTA
CSRC TX9 TXEN SYNC —BRGHTRMT TX9D 0000 -010 0000 -010
18h RCSTA SPEN
RX9 SREN CREN ADEN 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 for the BRG.