LM80CIMT-3 National Semiconductor, LM80CIMT-3 Datasheet - Page 8

• Manufacturer Part Number: LM80CIMT-3
• Manufacturer: National Semiconductor

Specifications of LM80CIMT-3

• Watchdog Timer: Yes
• Chip Enable Signals: No
• Package Type: TSSOP
• Operating Supply Voltage (min): 2.8V
• Operating Supply Voltage (max): 5.75V
• Operating Temp Range: -25C to 125C
• Operating Temperature Classification: Commercial
• Power Fail Detection: No
• Mounting: Surface Mount
• Pin Count: 24
• Battery Backup Switching: No
• Manual Reset: No
• Lead Free Status / RoHS Status: Not Compliant

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Functional Description
1.0 GENERAL DESCRIPTION
The LM80 provides 7 analog inputs, a temperature sensor, a
Delta-Sigma ADC (Analog-to-Digital Converter), 2 fan speed
counters, WATCHDOG registers, and a variety of inputs and
outputs on a single chip. A two wire Serial Bus interface is
provided. The LM80 performs power supply, temperature, fan
control and fan monitoring for personal computers.
The LM80 continuously converts analog inputs to 8-bit digital
words with a 10 mV LSB (Least Significant Bit) weighting,
yielding input ranges of 0 to 2.56V. The Analog inputs are
intended to be connected to the several power supplies
present in a a typical computer. Temperature can be convert-
ed to a 9-bit or 12-bit two's complement word with resolutions
of 0.5°C LSB or 0.0625°C LSB, respectively.
Fan inputs can be programmed to accept either fan failure
indicator or tachometer signals. Fan failure signals can be
programmed to be either active high or active low. Fan inputs
measure the period of tachometer pulses from the the fans,
providing a higher count for lower fan speeds. The fan inputs
are digital inputs with and acceptable range of 0 to V
and a transition level of approximately V
fan counts are 255 (8-bit counter), which represent a stopped
or very slow fan. Nominal speed based on a count of 153, are
programmable from 1100 to 8800 RPM. Signal conditioning
circuitry is included to accommodate slow rise and fall times.
The LM80 provides a number of internal registers, as detailed
in
Figure
Configuration Register:
configuration.
Interrupt Status Registers:
status of each WATCHDOG limit or Interrupt event.
Interrupt Mask Registers:
Interrupt sources, as well as separate masking for each of
both hardware Interrupt outputs.
4. These include:
Provides control and
Allows masking of individual
Two registers to provide
+
/2 volts. Full scale
+
volts
8
When the LM80 is started, it cycles through each measure-
ment in sequence, and it continuously loops through the
sequence approximately once every second. Each measured
value is compared to values stored in WATCHDOG, or Limit
registers. When the measured value violates the pro-
grammed limit the LM80 will set a corresponding Interrupt in
the Interrupt Status Registers. Two hardware Interrupt lines,
INT and RST_OUT/OS are available. INT is fully pro-
grammable with masking of each Interrupt source, and mask-
ing of each output. RST_OUT/OS is dedicated to the
temperature reading WATCHDOG registers. In addition, the
Fan Divisor register has control bits to enable or disable the
hardware Interrupts.
Additional digital inputs are provided for chaining of INT, out-
puts of multiple external LM75 temperature sensors via the
BTI (Board Temperature Interrupt) input, and a CI (Chassis
Intrusion) input. The Chassis Intrusion input is designed to
accept an active high signal from an external circuit that latch-
es when the case is removed from the computer.
Fan Divisor/RST_OUT/OS Registers: Bits 0-5 of this
register contain the divisor bits for FAN1 and FAN2 inputs.
Bits 6-7 control the function of the RST_OUT/OS output.
OS Configuration/Temperature Resolution Register:
The configuration of the OS (Overtemperature Shutdown)
is controlled by the lower 3 bits of this register. Bit 3
enables 12-bit temperature conversions. Bits 4-7 reflect
the lower four bits of the temperature reading for a 12-bit
resolution.
Value RAM: The monitoring results: temperature,
voltages, fan counts, and Fan Divisor/RST_OUT/OS
Register limits are all contained in the Value RAM. The
Value RAM consists of a total of 32 bytes. The first 10
bytes are all of the results, the next 20 bytes are the Fan
Divisor/RST_OUT/OS Register limits, and are located at
20h-3Fh, including two unused bytes in the upper
locations.