Datasheet

© 2007 Microchip Technology Inc. DS39605F-page 135
PIC18F1220/1320
16.2 EUSART 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 (BAUDCTL<3>)
selects 16-bit mode.
The SPBRGH:SPBRG register pair controls the period
of a free running timer. In Asynchronous mode, bits
BRGH (TXSTA<2>) and BRG16 also control the baud
rate. In Synchronous mode, bit BRGH is ignored.
Table 16-1 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 SPBRGH:SPBRG registers can be
calculated using the formulas in Table 16-1. From this,
the error in baud rate can be determined. An example
calculation is shown in Example 16-1. Typical baud
rates and error values for the various asynchronous
modes are shown in Table 16-2. 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 SPBRGH:SPBRG registers
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.
16.2.1 POWER MANAGED MODE
OPERATION
The system clock is used to generate the desired baud
rate; however, when a power managed mode is
entered, the clock source may be operating at a differ-
ent frequency than in PRI_RUN mode. In Sleep mode,
no clocks are present and in PRI_IDLE mode, the
primary clock source continues to provide clocks to the
Baud Rate Generator; however, in other power
managed modes, the clock frequency will probably
change. This may require the value in SPBRG to be
adjusted.
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
and make sure that the receive operation is Idle before
changing the system clock.
16.2.2 SAMPLING
The data on the RB4/AN6/RX/DT/KBI0 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 16-1: BAUD RATE FORMULAS
EXAMPLE 16-1: CALCULATING BAUD RATE ERROR
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 SPBRGH:SPBRG register pair
For a device with FOSC of 16 MHz, desired baud rate of 9600, Asynchronous mode, 8-bit BRG:
Desired Baud Rate = F
OSC/(64 ([SPBRGH:SPBRG] + 1))
Solving for SPBRGH:SPBRG:
X = ((F
OSC/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%