Monitor. LM79 Datasheet
Microprocessor System Hardware Monitor
The LM79 is a highly integrated Data Acquisition system for
hardware monitoring of servers, Personal Computers, or
virtually any microprocessor based system. In a PC, the
LM79 can be used to monitor power supply voltages, tem-
peratures, and fan speeds. Actual values for these inputs
can be read at any time, and programmable WATCHDOG
limits in the LM79 activate a fully programmable and
maskable interrupt system with two outputs.
The LM79 has an on-chip temperature sensor, 5 positive
analog inputs, two inverting inputs (for monitoring negative
voltages), and an 8-bit ADC. An input is provided for the
overtemperature outputs of additional temperature sensors
and this is linked to the interrupt system. The LM79 provides
inputs for three fan tachometer outputs. Additional inputs are
provided for Chassis Intrusion detection circuits, 5 VID moni-
tor inputs, and chainable interrupt. The LM79 provides both
ISA and Serial Bus interfaces. A 32-byte auto-increment
RAM is provided for POST (Power On Self Test) code stor-
Compared to the LM78, the LM79 has the following differ-
• an additional VID input pin
• an additional register for device identification
• open drain Power Switch Bypass Output
n Temperature sensing
n 5 positive voltage inputs
n 2 op amps for negative voltage monitoring
n 3 fan speed monitoring inputs
n Input for additional temperature sensors
n Chassis Intrusion Detector input
n WATCHDOG comparison of all monitored values
n POST code storage RAM
n ISA and I2C™ Serial Bus interfaces
j Voltage monitoring
j Temperature Accuracy
−10˚C to +100˚C
j Supply Voltage
j Supply Current
j ADC Resolution
1 mA typ
10 µA typ
n System Hardware Monitoring for Servers and PCs
n Office Electronics
n Electronic Test Equipment and Instrumentation
−10˚C ≤ TA ≤ +100˚C
# Indicates Active Low (“Not”)
I2C® is a registered trademark of the Phillips Corporation.
© 2001 National Semiconductor Corporation DS100036
8 Digital I/O
1 Digital Input
15 1 Digital I/O
An active low standard ISA bus I/O Read Control.
An active low standard ISA bus I/O Write Control.
The reference clock for the ISA bus. Typically ranges from 4.167 MHz to
8.33 MHz. The minimum clock frequency this input can handle is 1 Hz.
Bi-directional ISA bus Data lines. D0 corresponds to the low order bit,
with D7 the high order bit.
+5V VCC power. Bypass with the parallel combination of 10 µF
(electolytic or tantalum) and 0.1 µF (ceramic) bypass capacitors.
Internally connected to all digital circuitry.
Chainable SMI (System Management Interrupt) Input. This is an active
low input that propagates the SMI signal to the SMI output of the LM79
via SMI Mask Register Bit 6 and SMI enable Bit 1 of the Configuration
An active high input from an external circuit which latches a Chassis
Intrusion event. This line can go high without any clamping action
regardless of the powered state of the LM79. The LM79 provides an
internal open drain on this line, controlled by Bit 7 of NMI Mask Register
2, to provide a minimum 20 ms reset of this line.
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