Datasheet

© 2008 Microchip Technology Inc. DS39646C-page 251
PIC18F8722 FAMILY
20.1 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 EUSART. 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
(BAUDCONx<3>) also control the baud rate. In
Synchronous mode, BRGH is ignored. Table 20-1
shows the formula for computation of the baud rate for
different EUSART modes which only apply in Master
mode (internally generated clock).
Given the desired baud rate and F
OSC, the nearest
integer value for the SPBRGHx:SPBRGx registers can
be calculated using the formulas in Table 20-1. From
this, the error in baud rate can be determined. An
example calculation is shown in Example 20-1. Typical
baud rates and error values for the various Asynchro-
nous modes are shown in Table 20-2. It may be
advantageous 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.
20.1.1 OPERATION IN POWER-MANAGED
MODES
The device clock is used to generate the desired baud
rate. When one of the power-managed modes is
entered, the new clock source may be operating at a
different frequency. This may require an adjustment to
the value in the SPBRGx register pair.
20.1.2 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 RXx pin.
TABLE 20-1: BAUD RATE FORMULAS
Configuration Bits
BRG/EUSART Mode Baud Rate Formula
SYNC BRG16 BRGH
000 8-bit/Asynchronous F
OSC/[64 (n + 1)]
001 8-bit/Asynchronous
F
OSC/[16 (n + 1)]
010 16-bit/Asynchronous
011 16-bit/Asynchronous
F
OSC/[4 (n + 1)]10x 8-bit/Synchronous
11x 16-bit/Synchronous
Legend: x = Don’t care, n = value of SPBRGHx:SPBRGx register pair