PIC16F1938 Datasheet, PDF (301/452 Pages) Microchip Technology – 28/40/44-Pin Flash-Based, 8-Bit CMOS Microcontrollers

25.3 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 BAUDCON register selects 16-bit
mode.
The SPBRGH, SPBRGL 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 TXSTA
register and the BRG16 bit of the BAUDCON register. In
Synchronous mode, the BRGH bit is ignored.
Table 25-3 contains the formulas for determining the
baud rate. Example 25-1 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 25-3. 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 SPBRGH, SPBRGL 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.
PIC16(L)F1938/9
EXAMPLE 25-1: 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--F--H--^O--:--^S_S---^C_{P----B----R---G-----]----+-----1----}
Solving for SPBRGH:SPBRGL:
----------------F----^O----^S---^C------------------
X = -D----e---s--i--r--e---d----B----a---u---d----R----a---t--e- – 1
64
1---6---0----0--0---0---0---0--
= ------9----6---0--0-------- – 1
64
= 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---5----^–-----9---6---0---0---^- = 0.16%
9600
 2011 Microchip Technology Inc.
Preliminary
DS41574A-page 301