LM80CIMT-3 National Semiconductor, LM80CIMT-3 Datasheet - Page 12

• Manufacturer Part Number: LM80CIMT-3
• Manufacturer: National Semiconductor

Specifications of LM80CIMT-3

• Watchdog Timer: Yes
• Chip Enable Signals: No
• Package Type: TSSOP
• Operating Supply Voltage (min): 2.8V
• Operating Supply Voltage (max): 5.75V
• Operating Temp Range: -25C to 125C
• Operating Temperature Classification: Commercial
• Power Fail Detection: No
• Mounting: Surface Mount
• Pin Count: 24
• Battery Backup Switching: No
• Manual Reset: No
• Lead Free Status / RoHS Status: Not Compliant

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3.0 USING THE LM80
3.1 Power On
When power is first applied, the LM80 performs a “power on
reset” on several of its registers. The power on condition of
registers is shown in
ues are not shown have power on conditions that are inde-
terminate (this includes the value RAM and WATCHDOG
limits). The ADC is inactive. In most applications, usually the
first action after power on would be to write WATCHDOG lim-
its into the Value RAM.
3.2 Resets
Configuration Register INITIALIZATION accomplishes the
same function as power on reset. The Value RAM conversion
results, and Value RAM WATCHDOG limits are not Reset and
will be indeterminate immediately after power on. If the Value
RAM contains valid conversion results and/or Value RAM
WATCHDOG limits have been previously set, they will not be
affected by a Configuration Register INITIALIZATION. Power
on reset, or Configuration Register INITIALIZATION, clear or
initialize the following registers (the initialized values are
shown in
Configuration Register INITIALIZATION is accomplished by
setting Bit 7 of the Configuration Register high. This Bit auto-
matically clears after being set.
The LM80 can be reset to it's “power on state” by taking
NTEST_IN/Reset_IN pin low for at least 50 ns.
3.3 Using the Configuration Register
The Configuration Register provides all control over the
LM80. At power on, the ADC is stopped and INT_Clear is as-
serted, clearing the INT and RST_OUT/OS hardwire outputs.
The Configuration Register starts and stops the LM80, en-
ables and disables INT outputs, clears and sets CI and
GPO I/O pins, initiates reset pulse on RST_OUT/OS pin, and
provides the Reset function described in Section 3.2.
Bit 0 of the Configuration Register controls the monitoring
loop of the LM80. Setting Bit 0 low stops the LM80 monitoring
loop and puts the LM80 in shutdown mode, reducing power
consumption. Serial Bus communication is possible with any
register in the LM80 although activity on these lines will in-
crease shutdown current, up to as much as maximum rated
supply current, while the activity takes place. Taking Bit 0 high
starts the monitoring loop, described in more detail subse-
quently.
Bit 1 of the Configuration Register enables the INT Interrupt
hardwire output when this bit is taken high.
Bit 2 of the Configuration Register defines whether the INT
pin is open source or open drain.
Bit 3 clears the INT output when taken high. The LM80 mon-
itoring function will stop until bit 3 is taken low. The content of
the Interrupt (INT) Status Registers will not be affected.
Bit4, when taken high, will initiate a 10 ms RESET signal on
the RST_OUT/OS output (when this pin is in the RST mode).
Configuration Register
Interrupt Status Register 1
Interrupt Status Register 2
Interrupt Mask Register 1
Interrupt Mask Register 2
Fan Divisor/RST_OUT/OS Register
OS Configuration/Temperature Resolution Register
Value Ram (Registers at Address 20h - 3Fh, which
include: Temperature reading, IN0-IN6 readings, FAN1
and FAN2 readings, and WATCHDOG limits)
Table
1):
Table
1. Registers whose power on val-
12
When bit 5 is taken high the CI (Chassis Intrusion) pin is reset.
Bit 6 of the configuration register sets or clears the GPO out-
put. This pin can be used in software power control by acti-
vating an external power control MOSFET.
3.4 Starting Conversions
3.4 STARTING CONVERSION The monitoring function (Ana-
log inputs, temperature, and fan speeds) in the LM80 is
started by writing to the Configuration Register and setting
INT_Clear (Bit 3), low, and Start (Bit 0), high. The LM80 then
performs a round-robin monitoring of all analog inputs, tem-
perature, and fan speed inputs approximately once a second.
If the temperature resolution is set to 12 bits one complete
monitoring function will take approximately 2 seconds. The
sequence of items being monitored corresponds to locations
in the Value RAM (except for the Temperature reading) and
is:
1.
2.
3.
4.
5.
6.
7.
8.
9.
10. Fan 2
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 the conversion is read, reads of any
single value should not be done more often than once every
120 ms. When reading all values with the temperature reso-
lution set to 9-bits, allow at least 1.5 seconds between reading
groups of values. Reading more frequently than once every
1.5 seconds can also prevent complete updates of Interrupt
Status Registers and Interrupt Outputs. If the temperature
resolution is set to 12-bit, allow at least 2.0 seconds between
reading groups of values.
A typical sequence of events upon power on of the LM80
would consist of:
1.
2.
3.
4.0 ANALOG INPUTS
The 8-bit ADC has a 10 mV LSB, yielding a 0V to 2.55V (2.56
- 1LSB) input range. This is true for all analog inputs. In PC
monitoring applications these inputs would most often be
connected to power supplies. The 2.5, 3.3, ± 5 and ± 12 volt
inputs should be attenuated with external resistors to any de-
sired value within the input range. Care should be taken not
to exceed the power supply voltage (V
A typical application, such as is shown in
lect the input voltage divider to provide 1.9V at the analog
inputs of the LM80. This is sufficiently high for good resolution
of the voltage, yet leaves headroom for upward excursions
from the supply of about 25%. To simplify the process of re-
sistor selection, set the value of R2 first. Select a value for R2
or R4 between 10 kΩ and 100 kΩ This is low enough to avoid
errors due to input leakage currents yet high enough to both
protect the inputs under overdrive conditions as well as min-
Temperature
IN0
IN1
IN2
IN3
IN4
IN5
IN6
Fan 1
Set WATCHDOG Limits
Set Interrupt Masks
Start the LM80 monitoring process
+
) at any time.
Figure
6, might se-