參數(shù)資料
型號(hào)： EVAL-ADUC834QSZ
廠(chǎng)商： Analog Devices Inc
文件頁(yè)數(shù)： 57/80頁(yè)
文件大小： 0K
描述： KIT DEV QUICK START ADUC834
標(biāo)準(zhǔn)包裝： 1
系列： QuickStart™ 套件
類(lèi)型： MCU
適用于相關(guān)產(chǎn)品： ADuC834
所含物品： 評(píng)估板，線(xiàn)纜，電源，軟件和文檔
REV. A
–60–
ADuC834
BAUD RATE GENERATION USING TIMER 3
The high integer dividers in a UART block means that high
speed baud rates are not always possible using some particular
crystals. e.g., using a 12 MHz crystal, a baud rate of 115200 is
not possible. To address this problem the ADuC834 has added
a dedicated baud rate timer (Timer 3) specifically for generating
highly accurate baud rates.
Timer 3 can be used instead of Timer 1 or Timer 2 for generating
very accurate high speed UART baud rates including 115200
and 230400. Timer 3 also allows a much wider range of baud
rates to be obtained. In fact, every desired bit rate from 12 bits
to 393216 bits can be generated to within an error of
± 0.8%.
Timer 3 also frees up the other three timers allowing them to be
used for different applications. A block diagram of Timer 3 is
shown in Figure 57.
(1 + T3FD/64)
2
T3 RX/TX
CLOCK
CORE
CLK*
T3EN
RX
CLOCK
TX CLOCK
TIMER 1/TIMER 2
RX CLOCK (FIG 56)
FRACTIONAL
DIVIDER
0
1
TIMER 1/TIMER 2
TX CLOCK (FIG 56)
16
2DIV
*THE CORE CLOCK IS THE OUTPUT OF THE PLL (SEE "ON-CHIP PLL")
Figure 57. Timer 3, UART Baud Rates
Two SFRs (T3CON and T3FD) are used to control Timer 3.
T3CON is the baud rate control SFR, allowing Timer 3 to be
used to set up the UART baud rate, and setting up the binary
divider (DIV).
Table XXXIII. T3CON SFR Bit Designations
Bit
Name
Description
7
T3EN
Set to enable Timer 3 to generate the baud
rate. When set PCON.7, T2CON.4 and
T2CON.5 are ignored. Cleared to let the baud
rate be generated as per a standard 8052.
6
–––
Reserved for future use
5
–––
Reserved for future use
4
–––
Reserved for future use
3
–––
Reserved for future use
2
DIV2
Binary Divider Factor
1
DIV1
DIV2
DIV1
DIV0
Bin Divider
0
DIV0
0
1
00
1
2
01
0
4
01
1
8
10
0
16
10
1
32
11
0
64
11
1
128
The appropriate value to write to the DIV2-1-0 bits can be
calculated using the following formula where fCORE is the output
of the PLL as described in the “On-Chip PLL” description.
Note: The DIV value must be rounded down.
DIV
f
Baud Rate
CORE
=
×
()
log
32
2
T3FD is the fractional divider ratio required to achieve the
required baud rate. We can calculate the appropriate value for
T3FD using the following formula.
Note: T3FD should be rounded to the nearest integer.
TFD
f
Baud Rate
CORE
DIV
3
2
64
=
×
Once the values for DIV and T3FD are calculated, the actual
baud rate can be calculated using the following formula:
Actual Baud Rate
f
TFD
CORE
DIV
=
×
×+
()
2
23
64
For a baud rate of 115200 while operating from the maximum
core frequency (CD = 0) we have:
DIV
TFD
Dh
=×
()
==
=×
()
×
() ==
log
/
/ log
.
..
12582912 32
115200
2
1 77
1
32
12 582912
2
115200
64
45 22
2
1
Therefore, the actual baud rate is 115439 bits.
Table XXXIV. Commonly Used Baud Rates Using Timer 3
Ideal
%
Baud
CD
DIV
T3CON
T3FD
Error
230400
0
80H
2DH
0.2
115200
0
1
81H
2DH
0.2
115200
1
0
80H
2DH
0.2
57600
0
2
82H
2DH
0.2
57600
1
81H
2DH
0.2
57600
2
0
80H
2DH
0.2
38400
0
3
83H
12H
0.1
38400
1
2
82H
12H
0.1
38400
2
1
81H
12H
0.1
38400
3
0
80H
12H
0.1
19200
0
4
84H
12H
0.1
19200
1
3
83H
12H
0.1
19200
2
82H
12H
0.1
19200
3
1
81H
12H
0.1
19200
4
0
80H
12H
0.1
9600
0
5
85H
12H
0.1
9600
1
4
84H
12H
0.1
9600
2
3
83H
12H
0.1
9600
3
2
82H
12H
0.1
9600
4
1
81H
12H
0.1
9600
5
0
80H
12H
0.1
38400
0
3
83H
12H
0.1
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