PIC18F23K22 Datasheet, PDF (275/492 Pages) Microchip Technology – 28/40/44-Pin, Low-Power, High-Performance Microcontrollers with nanoWatt XLP Technology

PIC18(L)F2X/4XK22
16.3 EUSART Baud Rate Generator
(BRG)
The Baud Rate Generator (BRG) is an 8-bit or 16-bit
timer that is dedicated to the support of both the
asynchronous and synchronous EUSART operation.
By default, the BRG operates in 8-bit mode. Setting the
BRG16 bit of the BAUDCONx register selects 16-bit
mode.
The SPBRGHx:SPBRGx register pair determines the
period of the free running baud rate timer. In
Asynchronous mode the multiplier of the baud rate
period is determined by both the BRGH bit of the
TXSTAx register and the BRG16 bit of the BAUDCONx
register. In Synchronous mode, the BRGH bit is ignored.
Table contains the formulas for determining the baud
rate. Example 16-1 provides a sample calculation for
determining the baud rate and baud rate error.
Typical baud rates and error values for various
Asynchronous modes have been computed for your
convenience and are shown in Table 16-5. It may be
advantageous to use the high baud rate (BRGH = 1),
or the 16-bit BRG (BRG16 = 1) to reduce the baud rate
error. The 16-bit BRG mode is used to achieve slow
baud rates for fast oscillator frequencies.
Writing a new value to the SPBRGHx, SPBRGx
register pair causes the BRG timer to be reset (or
cleared). This ensures that the BRG does not wait for a
timer overflow before outputting the new baud rate.
If the system clock is changed during an active receive
operation, a receive error or data loss may result. To
avoid this problem, check the status of the RCIDL bit to
make sure that the receive operation is Idle before
changing the system clock.
EXAMPLE 16-1: 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 = -6--4----_--[---S---P---B----R----G----H-F---x-^O--:--^S_S--^C-P----B---R----G----x---]----+------1---_
Solving for SPBRGHx:SPBRGx:
X = -D---------e-----s-----i---r-----e----F-d-------^O_6--B-----4-^S--a----^C--u------d--------R--------a------t----e-- – 1
= -1-----6------09--------066-----004-----00------0-------0--- – 1
= 25.042 = 25
Calculated Baud Rate = _{6--1-4--6--}-0-2--0-5--0---+0---0--1-0---_
= 9615
Error = ^{C-----a--l--c---.---B---a---u--D--d---e-R-s---ia-r--t-ee--d---–}--B--D-a---eu---sd--i--r-R-e---ad---t-e-B---a---u---d-----R---a---t--e---
= ^---9---6---1---95---6-^–--0---09---6---0---0---^- = 0.16%
TABLE 16-3: BAUD RATE FORMULAS
Configuration Bits
SYNC
BRG16
BRGH
BRG/EUSART 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)]
 2010 Microchip Technology Inc.
Preliminary
DS41412B-page 275