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| 型号： | ADUC845BCPZ8-3 |
| 厂商： | ANALOG DEVICES INC |
| 元件分类： | 微控制器/微处理器 |
| 英文描述： | 8-BIT, FLASH, 12.58 MHz, MICROCONTROLLER, QCC56 |
| 封装： | 8 X 8 MM, LEAD FREE, MO-220-VLLD-2, LFCSP-56 |
| 文件页数： | 91/108页 |
| 文件大小： | 1940K |
| 代理商： | ADUC845BCPZ8-3 |
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ADuC845/ADuC847/ADuC848
Rev. B | Page 83 of 108
Timer 3 Generated Baud Rates
The high integer dividers in a UART block mean that high
speed baud rates are not always possible. Also, generating baud
rates requires the exclusive use of a timer, rendering it unusable
for other applications when the UART is required. To address
this problem, the ADuC845/ADuC847/ADuC848 have 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 bps to 393216 bps 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 61.
÷ (1 + T3FD/64)
T3 Rx/Tx
CLOCK
CORE
CLK
T3EN
Rx CLOCK
Tx CLOCK
TIMER 1/TIMER 2
Rx CLOCK
FRACTIONAL
DIVIDER
0
1
TIMER 1/TIMER 2
Tx CLOCK
÷ 16
÷ 2DIV
04741-058
Figure 61. Timer 3, UART Baud Rate
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 to set up the binary
divider (DIV).
The appropriate value to write to the DIV2-1-0 bits can be
calculated using the following formula where fCORE is defined in
PLLCON SFR. Note that the DIV value must be rounded down.
DIV =
)
2
(
log
16
log
×
Rate
Baud
Frequency
Clock
Core
T3FD is the fractional divider ratio required to achieve the
required baud rate. The appropriate value for T3FD can be
calculated with the following formula:
T3FD =
Rate
Baud
Frequency
Clock
Core
DIV
×
1
2
64
Note that T3FD should be rounded to the nearest integer. Once
the values for DIV and T3FD are calculated, the actual baud
rate can be calculated with the following formula:
Actual Baud Rate =
)
64
(
2
1
+
×
T3FD
Frequency
Clock
Core
DIV
For example, to get a baud rate of 9600 while operating at a core
clock frequency of 1.5725 MHz, that is, CD = 3,
DIV = log(1572500/(16 × 9600))/log2 = 3.35 = 3
Note that the DIV result is rounded down.
T3FD = (2 × 1572500)/(231 × 9600) 64 = 18 = 12H
Therefore, the actual baud rate is 9588 bps, which gives an error
of 0.12%.
The T3CON and T3FD registers are used to control Timer 3.
T3CON – Timer 3 Control Register
SFR Address:
9EH
Power-On Default:
00H
Bit Addressable:
No
Table 55. T3CON SFR Bit Designations
Bit No.
Name
Description
7
T3BAUDEN
T3UARTBAUD Enable.
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
Not Implemented. Write Don’t Care.
5
Not Implemented. Write Don’t Care.
4
Not Implemented. Write Don’t Care.
3
Not Implemented. Write Don’t Care.
2, 1, 0
DIV2, DIV1, DIV0
Binary Divider
DIV2
DIV1
DIV0
0
Binary Divider 0. See Table 57.
0
1
Binary Divider 1. See Table 57.
0
1
0
Binary Divider 2. See Table 57.
0
1
Binary Divider 3. See Table 57.
1
0
Binary Divider 4. See Table 57.
1
0
1
Binary Divider 5. See Table 57.
1
0
Binary Divider 6. See Table 57.
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