PIC18F2420/2520/4420/4520
advantageous to use the high baud rate (BRGH = 1) or
the 16-bit BRG to reduce the baud rate error, or
18.1
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 (BAUDCON<3>)
selects 16-bit mode.
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.
The SPBRGH:SPBRG register pair controls the period
of a free-running timer. In Asynchronous mode, bits,
BRGH (TXSTA<2>) and BRG16 (BAUDCON<3>), also
control the baud rate. In Synchronous mode, BRGH is
ignored. Table 18-1 shows the formula for computation
of the baud rate for different EUSART modes which
only apply in Master mode (internally generated clock).
18.1.1
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 SPBRG register pair.
Given the desired baud rate and FOSC, the nearest
integer value for the SPBRGH:SPBRG registers can be
calculated using the formulas in Table 18-1. From this,
the error in baud rate can be determined. An example
calculation is shown in Example 18-1. Typical baud
rates and error values for the various Asynchronous
modes are shown in Table 18-2. It may be
18.1.2
SAMPLING
The data on the RX 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 18-1: BAUD RATE FORMULAS
Configuration Bits
BRG/EUSART Mode
Baud Rate Formula
SYNC
BRG16
BRGH
0
0
0
0
1
1
0
0
1
1
0
1
0
1
0
1
x
x
8-Bit/Asynchronous
8-Bit/Asynchronous
16-Bit/Asynchronous
16-Bit/Asynchronous
8-Bit/Synchronous
16-Bit/Synchronous
FOSC/[64 (n + 1)]
FOSC/[16 (n + 1)]
FOSC/[4 (n + 1)]
Legend: x= Don’t care, n = value of SPBRGH:SPBRG register pair
© 2008 Microchip Technology Inc.
DS39631E-page 205
MICROCHIP [ MICROCHIP ]
