PIC18F2220/2320/4220/4320
It may be advantageous to use the high baud rate
18.2 USART Baud Rate Generator (BRG)
(BRGH = 1), even for slower baud clocks, because the
FOSC/(16 (X + 1)) equation can reduce the baud rate
error in some cases.
The BRG supports both the Asynchronous and
Synchronous modes of the USART. It is a dedicated
8-bit Baud Rate Generator. The SPBRG register
controls the period of a free-running 8-bit timer. In
Asynchronous mode, bit BRGH (TXSTA<2>) also con-
trols the baud rate. In Synchronous mode, bit BRGH is
ignored. Table 18-1 shows the formula for computation
of the baud rate for different USART modes which only
apply in Master mode (internal clock).
Writing a new value to the SPBRG register 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.
18.2.1
POWER MANAGED MODE
OPERATION
Given the desired baud rate and FOSC, the nearest
integer value for the SPBRG register can be calculated
using the formula in Table 18-1. From this, the error in
baud rate can be determined.
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, 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.
Example 18-1 shows the calculation of the baud rate
error for the following conditions:
• FOSC = 16 MHz
• Desired Baud Rate = 9600
• BRGH = 0
• SYNC = 0
18.2.2
SAMPLING
The data on the RC7/RX/DT 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.
EXAMPLE 18-1:
CALCULATING BAUD RATE ERROR
Desired Baud Rate
= FOSC/(64 (X + 1))
Solving for X:
X
X
X
= ((FOSC/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%
TABLE 18-1: BAUD RATE FORMULA
SYNC
BRGH = 0 (Low Speed)
BRGH = 1 (High Speed)
Baud Rate = FOSC/(16 (X + 1))
N/A
0(Asynchronous)
1(Synchronous)
Baud Rate = FOSC/(64 (X + 1))
Baud Rate = FOSC/(4 (X + 1))
Legend: X = value in SPBRG (0 to 255)
TABLE 18-2: REGISTERS ASSOCIATED WITH BAUD RATE GENERATOR
Value on
all other
Resets
Value on
POR, BOR
Name
Bit 7
Bit 6
Bit 5
Bit 4
Bit 3
Bit 2
Bit 1
Bit 0
TXSTA
RCSTA
SPBRG
CSRC
SPEN
TX9
RX9
TXEN SYNC
—
BRGH TRMT TX9D 0000 -010
0000 -010
0000 -00x
0000 0000
SREN CREN ADDEN FERR OERR RX9D 0000 -00x
0000 0000
Baud Rate Generator Register
Legend: x= unknown, - = unimplemented, read as ‘0’. Shaded cells are not used by the BRG.
DS39599C-page 198
2003 Microchip Technology Inc.
MICROCHIP [ MICROCHIP ]
