PIC18F6X2X Datasheet, PDF (221/386 Pages) Microchip Technology – 64/80-Pin High Performance, 64-Kbyte Enhanced FLASH Microcontrollers with A/D

PIC18F6X2X/8X2X
19.1 USART Baud Rate Generator (BRG)
The BRG is a dedicated 8-bit or 16-bit generator that
supports both the Asynchronous and Synchronous
modes of the USART. By default, the BRG operates in
8-bit mode; setting the BRG16 bit (BAUDCONx<3>)
selects 16-bit mode.
The SPBRGHx:SPBRGx register pair controls the
period of a free running timer. In Asynchronous mode,
bits BRGH (TXSTAx<2>) and BRG16 also control the
baud rate. In Synchronous mode, bit BRGH is ignored.
Table 19-1 shows the formula for computation of the
baud rate for different USART modes which only apply
in Master mode (internally generated clock).
Given the desired baud rate and FOSC, the nearest
integer value for the SPBRGHx:SPBRGx registers can
be calculated using the formulas in Table 19-1. From
this, the error in baud rate can be determined. An
example calculation is shown in Example 19-1. Typical
baud rates and error values for the various Asynchro-
nous modes are shown in Table 19-2. It may be advan-
tageous to use the high baud rate (BRGH = 1) or the
16-bit BRG to reduce the baud rate error, or achieve a
slow baud rate for a fast oscillator frequency.
Writing a new value to the SPBRGHx:SPBRGx regis-
ters causes the BRG timer to be reset (or cleared). This
ensures the BRG does not wait for a timer overflow
before outputting the new baud rate.
19.1.1 SAMPLING
The data on the RXx pin (either RC7/RX1/DT1 or RG2/
RX2/DT2) 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 19-1: BAUD RATE FORMULAS
Configuration Bits
SYNC
BRG16
BRGH
BRG/USART Mode
0
0
0
0
1
1
Legend:
0
0
8-bit/Asynchronous
0
1
8-bit/Asynchronous
1
0
16-bit/Asynchronous
1
1
16-bit/Asynchronous
0
x
8-bit/Synchronous
1
x
16-bit/Synchronous
x = Don’t care, n = value of SPBRGHx:SPBRGx register pair
Baud Rate Formula
FOSC / [64 (n+1)]
FOSC / [16 (n+1)]
FOSC / [4 (n+1)]
EXAMPLE 19-1: CALCULATING BAUD RATE ERROR
For a device with FOSC of 16 MHz, desired baud rate of 9600, Asynchronous mode, 8-bit BRG:
Desired Baud Rate = FOSC / (64 ([SPBRGHx:SPBRGx] + 1))
Solving for SPBRGHx:SPBRGx:
X = ((FOSC / Desired Baud Rate)/64) – 1
= ((16000000 / 9600) / 64) – 1
= [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 19-2: REGISTERS ASSOCIATED WITH BAUD RATE GENERATOR
Name
Bit 7 Bit 6 Bit 5 Bit 4
Bit 3
Bit 2
Bit 1
Bit 0
Value on
Value on all
POR, BOR other RESETS
TXSTAx
CSRC TX9 TXEN SYNC SENDB BRGH TRMT TX9D
RCSTAx
SPEN RX9 SREN CREN ADDEN FERR OERR RX9D
BAUDCONx —
RCIDL
—
SCKP BRG16
—
WUE ABDEN
SPBRGHx Baud Rate Generator Register, High Byte
SPBRGx Baud Rate Generator Register, Low Byte
Legend: x = unknown, - = unimplemented, read as ‘0’. Shaded cells are not used by the BRG.
0000 -010
0000 -00x
-1-1 0-00
0000 0000
0000 0000
0000 -010
0000 -00x
-1-1 0-00
0000 0000
0000 0000
 2003 Microchip Technology Inc.
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