LM77CIMM-5 NSC [National Semiconductor], LM77CIMM-5 Datasheet - Page 13

• Manufacturer Part Number: LM77CIMM-5
• Description: 9-Bit Sign Digital Temperature Sensor and Thermal Window Comparator with Two-Wire Interface
• Manufacturer: NSC [National Semiconductor]

3.0 Application Hints
The temperature response graph in Figure 7 depicts a typi-
cal application designed to meet ACPI requirements. In this
type of application, the temperature scale is given an arbi-
trary value of granularity , or the window within which tem-
perature notification events should occur. The LM77 can be
programmed to the window size chosen by the designer, and
will issue interrupts to the processor whenever the window
limits have been crossed. The internal flags permit quick
determination of whether the temperature is rising or falling.
The T_CRIT limit would typically use its separate output to
activate hardware shutdown circuitry separate from the pro-
cessor. This is done because it is expected that if tempera-
ture has gotten this high that the processor may not be
responding. The separate circuitry can then shut down the
system, usually by shutting down the power supply.
Note that the INT and T_CRIT_A outputs are separate, but
can be wire-or’d together. Alternatively the T_CRIT_A can be
diode or’d to the INT line in such a way that a T_CRIT_A
event activates the INT line, but an INT event does not
activate the T_CRIT_A line. This may be useful in the event
that it is desirable to notify both the processor and separate
T_CRIT_A shutdown circuitry of a critical temperature alarm
at the same time (maybe the processor is still working and
can coordinate a graceful shutdown with the separate shut-
down circuit).
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To implement ACPI compatible sensing it is necessary to
sense whenever the temperature goes outside the window,
issue an interrupt, service the interrupt, and reprogram the
window according to the desired granularity of the tempera-
ture scale. The reprogrammed window will now have the
current temperature inside it, ready to issue an interrupt
whenever the temperature deviates from the current window.
To understand this graph, assume that at the left hand side
the system is at some nominal temperature. For the 1st
event temperature rises above the upper window limit,
T
interrupt by querying the LM77’s status bits and determines
that T
rising. The system then reprograms the temperature limits to
a value higher by an amount equal to the desired granularity.
Note that in Event Interrupt Mode, reprogramming the limits
has caused a second, known, interrupt to be issued since
temperature has been returned within the window. In Com-
parator Interrupt Mode, the LM77 simply stops issuing inter-
rupts.
The 2nd event is another identical rise in temperature. The
3rd event is typical of a drop in temperature. This is one of
the conditions that demonstrates the power of the LM77, as
the user receives notification that a lower limit is exceeded in
such a way that temperature is dropping.
The Critical Alarm Event activates the separate T_CRIT_A
output. Typically, this would feed circuitry separate from the
processor on the assumption that if the system reached this
temperature, the processor might not be responding.
HIGH
, causing INT to go active. The system responds to the
HIGH
was exceeded, indicating that temperature is
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