Datasheet

LM87

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SNAS034J –APRIL 2000–REVISED MARCH 2013

WATCHDOG LIMIT COMPARISONS AND INTERRUPT STRUCTURE

Figure 13. Interrupt Structure

Figure 13 depicts the Interrupt Structure of the LM87. The LM87 can generate Interrupts as a result of each of its

internal WATCHDOG registers on the analog, temperature, and fan inputs.

External Interrupts can come from the following sources. While the label suggests a specific type or source of

Interrupt, this label is not a restriction of its usage, and it could come from any desired source:

• Chassis Intrusion: This is an active high interrupt from any type of device that detects and captures

chassis intrusion violations. This could be accomplished mechanically, optically, or electrically, and circuitry

external to the LM87 is expected to latch the event. The design of the LM87 allows this input to go high even

with no power applied to the LM87, and no clamping or other interference with the line will occur. This line

can also be pulled low for at least 20 ms by the LM87 to reset a typical Chassis Intrusion circuit. This reset is

activated by setting Bit 7 of CI Clear Register (46h) high. The bit in the Register is self-clearing.

• THERM# Input: This is an active low interrupt that would typically be generated by an external temperature

monitoring system. If the THERM# output is currently inactive and this input is pulled low by an external

circuit, the THERM# Interrupt Status bit will be set. In addition, the DAC output will be forced to full scale

operation while THERM# is pulled low by the external source. This allows a separate thermal sensor to

override the current fan speed setting in an overtemperature situation not sensed by the LM87. The DAC

setting will return to normal when the THERM# input is deactivated and the DAC setting register is unaffected

by the THERM# input condition.

• IRQ0-2: These are active low inputs from any type of external interrupt source. If enabled via the Channel

Mode Register (16h) the INT# output will be activated whenever these inputs are pulled low. Since there are

no dedicated ISR bits that correspond to the IRQ inputs, the VID status bits can be read to determine which

Product Folder Links: LM87