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).

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.

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