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參數(shù)資料
| 型號(hào)： | LM81CIMT-31 |
| 廠商： | National Semiconductor Corporation |
| 英文描述： | Serial Interface ACPI-Compatible Microprocessor System |
| 中文描述： | 串行接口ACPI的兼容微處理器系統(tǒng) |
| 文件頁數(shù)： | 14/32頁 |
| 文件大?。?/td> | 412K |
| 代理商： | LM81CIMT-31 |
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Functional Description
(Continued)
3.5 Reading Conversion Results
The conversion results are available in the Value RAM. Con-
versions can be read at any time and will provide the result
of the last conversion. Because the ADC stops, and starts a
new conversion whenever it is read, reads of any single
value should not be done more often than once every 56 ms.
When reading all values, allow at least 0.82 seconds be-
tween reading groups of values. Reading more frequently
than once every 0.82 seconds can also prevent complete
updates of Interrupt Status Registers and Interrupt Output’s.
A typical sequence of events upon power on of the LM81
would consist of:
1.
Set WATCHDOG Limits
2.
Set Interrupt Masks
3.
Start the LM81 monitoring process
4.0 ANALOG INPUTS
All analog input voltages are digitized to 8-bits of resolution.
All analog inputs, except for Vccp1 and Vccp2, include inter-
nal resistor attenuators. The theoretical LSB size, theoretical
voltage input required for an ADC reading of 192 (3/4 scale)
and 255 (full scale) for each analog input is detailed in the
table below:
Input
LSB size
Vin for
192
2.5V
3.3V
5V
12V
2.7V
Vin for
255
3.320V
4.383V
6.641V
15.93V
3.586V
2.5 Vin
3.3 Vin
5 Vin
12 Vin
Vccp1, Vccp2
Thus monitoring power supplies within a system can be eas-
ily accomplished by tying the Vccp, +2.5 Vin, +3.3 Vin,
+5 Vin and +12 Vin analog inputs to the corresponding sys-
tem supply. A digital reading can be converted to a voltage
by simply multiplying the decimal value of the reading by the
LSB size.
For inputs with attenuators the input impedance is greater
than 90 k
. Vccp inputs do not have resistor attenuators and
are are directly tied to the ADC, therefore having a much
larger input impedance.
Anegative power supply voltage can be applied to a Vccp in-
put through a resistor divider referenced to a known positive
DC voltage as show in Figure 6 The resistor values shown
in the table below for the circuit of Figure 6 will provide ap-
proximately 1.25V at the Vccp analog inputs of the LM81 for
a nominal reading of 89.
13 mV
17.2 mV
26 mV
62.5 mV
14.1 mV
Voltage
Measure-
ments
(V
S
)
R2
R1
V
+
Voltage
at
Analog In-
puts
(ADC code
89)
+1.25V
+1.25V
12V
5V
40 k
40 k
141 k
66.7
k
+5V
+5V
The resistors were selected by setting R2 = 40 k
and then
calculating R2 using the following equation:
R1 = [(1.25V V
S
)
÷
(V
+
1.25V)] x 40 k
The maximum R1 can be is restricted by the DC input cur-
rent of a Vccp input.
Inputs with internal resistor dividers (+2.5 Vin, +3.3 Vin or
+5 Vin, +12 Vin) can have voltage applied that exceeds the
power supply up to: 3.6V for +2.5 Vin, 4.6V for +3.3 Vin, 6.8V
for +5 Vin, and 15V for +12 Vin. The Vccp inputs have a
parasitic diode to the positive supply, so care should be
taken not to forward bias this diode. All analog inputs have
internal diodes that clamp the input voltage when going be-
low ground thus limiting the negative analog input voltage
range to 50 mV. Violating the analog input voltage range of
any analog input has no detrimental effect on the other ana-
log inputs. External resistors should be included to limit input
currents to the values given in the ABSOLUTE MAXIMUM
RATINGS for Input Current At Any Pin whenever exceeding
the analog input voltage range, even on an un-powered
LM81. Inputs with external attenuator networks will usually
meet these requirements. If it is possible for inputs without
attenuators (such as Vccp1 and Vccp2) to be turned on while
LM81 is powered off, additional resistors of about 10 k
should be added in series with the inputs to limit the input
current.
4.1 Analog Input Interrupts
A WATCHDOG window comparison on the analog inputs
can activate the INT interrupt output. A converted input volt-
age that is above its respective HIGH limit or less than or
equal to its LOW limit will cause a flag to be set in its Inter-
rupt Status Register. This flag will activate the INT output
when its mask bit is set low. Mask bits are found in the Inter-
rupt Mask Registers.
5.0 LAYOUT AND GROUNDING
A separate, low-impedance ground plane for analog ground,
which provides a ground point for both GND pins, voltage di-
viders and other analog components, will provide best per-
formance but is not mandatory. Analog components such as
voltage dividers should be located physically as close as
possible to the LM81.
DS100072-11
FIGURE 6. Input Examples. Resistor values shown in
table provide approximately 1.25V at the Vccp inputs.
www.national.com
14
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