LM77

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SNIS103F – JUNE 1999 – REVISED MARCH 2013

Figure 13. Typical Pointer Set Followed by Immediate Read from Configuration Register

Figure 14. Configuration Register Write

Comparison Register Write

Figure 15. Timing Diagrams

Application Hints

The temperature response graph in Figure 16 depicts a typical application designed to meet ACPI requirements.

In this type of application, the temperature scale is given an arbitrary value of "granularity", or the window within

which temperature 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 processor. This is done because it is expected that if temperature 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 shutdown 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 temperature 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

temperature is 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 Comparator Interrupt

Mode, the LM77 simply stops issuing interrupts.

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.

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