LM81CIMT-31 National Semiconductor, LM81CIMT-31 Datasheet - Page 14

• Manufacturer Part Number: LM81CIMT-31
• Description: Serial Interface ACPI-Compatible Microprocessor System
• Manufacturer: National Semiconductor

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Functional Description
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:
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.
A negative 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.
Measure-
Vccp1, Vccp2
Voltage
ments
−12V
(V
−5V
2.5 Vin
3.3 Vin
12 Vin
Input
5 Vin
S
)
40 k
40 k
R2
LSB size
17.2 mV
62.5 mV
14.1 mV
13 mV
26 mV
141 k
66.7
R1
k
Vin for
2.5V
3.3V
2.7V
+5V
+5V
192
12V
5V
V
(Continued)
+
(ADC code
Analog In-
Voltage
+1.25V
+1.25V
Vin for
3.320V
4.383V
6.641V
15.93V
3.586V
puts
89)
255
at
14
The resistors were selected by setting R2 = 40 k
calculating R2 using the following equation:
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.
table provide approximately 1.25V at the Vccp inputs.
FIGURE 6. Input Examples. Resistor values shown in
R1 = [(1.25V − V
S
) ÷ (V
+
− 1.25V)] x 40 k
DS100072-11
and then