Test Circuit
DS100040-6
FIGURE 3. Digital Output Load Test Circuitry
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 in-
tended to be connected to the several power supplies
present in a a typical computer. Temperature can be con-
verted to a 9-bit or 12-bit two’s complement word with reso-
lutions 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
+
volts
and a transition level of approximately V
+
/2 volts. Full scale
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 de-
tailed in
Figure 4.
These include:
Configuration Register:
Provides control and configura-
tion.
Interrupt Status Registers:
Two registers to provide sta-
tus of each WATCHDOG limit or Interrupt event.
Interrupt Mask Registers:
Allows masking of individual
Interrupt sources, as well as separate masking for each of
both hardware Interrupt outputs.
Fan Divisor/RST_OUT/OS Registers:
Bits 0-5 of this reg-
ister 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 con-
trolled 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 Regis-
ter limits, and are located at 20h-3Fh, including two unused
bytes in the upper locations.
When the LM80 is started, it cycles through each measure-
ment in sequence, and it continuously loops through the se-
quence 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 program-
mable with masking of each Interrupt source, and masking 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 In-
terrupts.
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
latches when the case is removed from the computer.
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