參數資料
型號： 16LF628A
廠商： Microchip Technology Inc.
元件分類： 8位微控制器
英文描述： FLASH-Based 8-Bit CMOS Microcontrollers
中文描述： 基于閃存的8位CMOS微控制器
文件頁數： 73/168頁
文件大?。?/td> 3760K
代理商： 16LF628A
2002 Microchip Technology Inc.
Preliminary
DS40044A-page 71
PIC16F627A/628A/648A
12.1
USART Baud Rate Generator
(BRG)
The BRG supports both the Asynchronous and Syn-
chronous 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 controls the baud
rate. In Synchronous mode bit BRGH is ignored.
Table 12-1 shows the formula for computation of the
baud rate for different USART modes, which only apply
in Master mode (internal clock).
Given the desired baud rate and F
OSC
, the nearest
integer value for the SPBRG register can be calculated
using the formula in Table 12-1. From this, the error in
baud rate can be determined.
Example 12-1 shows the calculation of the baud rate
error for the following conditions:
F
OSC
= 16 MHz
Desired Baud Rate = 9600
BRGH = 0
SYNC = 0
EXAMPLE 12-1:
CALCULATING BAUD
RATE ERROR
It may be advantageous to use the high baud rate
(BRGH = 1) even for slower baud clocks. This is
because the F
OSC
/(16(X + 1)) equation can reduce the
baud rate error in some cases.
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 output-
ting the new baud rate.
TABLE 12-1:
BAUD RATE FORMULA
TABLE 12-2:
REGISTERS ASSOCIATED WITH BAUD RATE GENERATOR
Desired Baud Rate
64
x
1
+
)
----------------------
=
9600
64
x
1
+
)
16000000
=
x
25.042
=
Calculated Baud Rate
64 25
1
+
)
16000000
9615
=
=
Error
Desired Baud Rate
(Calculated Baud Rate - Desired Baud Rate)
=
= -----------------------------
9600
–
9600
0.16%
=
SYNC
BRGH = 0 (Low Speed)
BRGH = 1 (High Speed)
0
1
(Asynchronous) Baud Rate = F
OSC
/(64(X+1))
(Synchronous) Baud Rate = F
OSC
/(4(X+1))
Baud Rate= F
OSC
/(16(X+1))
NA
Legend: X = value in SPBRG (0 to 255)
Address
Name
Bit 7
Bit 6
Bit 5
Bit 4
Bit 3
Bit 2
Bit 1
Bit 0
Value on
POR
Value on all
other
RESETS
98h
TXSTA
CSRC
TX9
TXEN
SYNC
—
BRGH
TRMT
TX9D
0000 -010
0000 -010
18h
RCSTA
SPEN
RX9
SREN
CREN
ADEN
FERR
OERR
RX9D
0000 000x
0000 000x
99h
SPBRG
Baud Rate Generator Register
0000 0000
0000 0000
Legend:
x
= unknown,
-
= unimplemented read as '0'.
Shaded cells are not used by the BRG.
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