Datasheet
LM87
SNAS034J –APRIL 2000–REVISED MARCH 2013
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FUNCTIONAL DESCRIPTION
GENERAL DESCRIPTION
The LM87 provides 7 analog inputs, an internal junction type temperature sensor, two remote junction
temperature sensing channels, a Delta-Sigma ADC (Analog-to-Digital Converter), a DAC output, 2 fan speed
counters, WATCHDOG registers, and a variety of inputs and outputs on a single chip. A two wire SMBus™
Serial Bus interface is included. The LM87 performs power supply, temperature, fan control and fan monitoring
for personal computers.
The analog inputs are useful for monitoring several power supplies present in a typical computer. The LM87
includes internal resistor dividers that scale external Vccp1, Vccp2, +2.5V, +5.0 V, +12 V and internal +3.3V
power supply voltages to a 3/4 scale nominal ADC output. Two additional inputs, +AIN1 and +AIN2 (2.5V full
scale) are input directly with no resistive dividers. The LM87 ADC continuously converts the scaled inputs to 8-bit
digital words. Measurement of negative voltages (such as -5 V and -12 V power supplies) can be accommodated
with an external resistor divider applied to the +AIN1 or +AIN2 inputs. Internal and external temperature is
converted to 8-bit two's-complement digital words with a 1 °C LSB.
Fan inputs measure the period of tachometer pulses from the fans, providing a higher count for lower fan
speeds. The fan inputs are Schmitt-Trigger digital inputs with an acceptable range of 0 V to V
+
and a transition
level of approximately V
+
/2. Full scale fan counts are 255 (8-bit counter) and this represents a stopped or very
slow fan. Nominal speeds, based on a count of 153, are programmable from 1100 to 8800 RPM on FAN1 and
FAN2. Schmitt-Trigger input circuitry is included to accommodate slow rise and fall times. An 8 bit DAC with 0 V
to 2.5 V output voltage range can be used for control of fan speed.
The LM87 has several internal registers, as shown in Figure 5, Table 1 and REGISTERS AND RAM. These
include:
• Configuration Registers: Provide control and configuration.
• Channel Mode Register: Controls the functionality of the dual purpose input pins, scaling for internal Vcc
measurement, and operation of some IRQ inputs.
• Interrupt Status Registers: Two registers to provide status of each WATCHDOG limit or Interrupt event.
Reading the Status Registers clears any active bits.
• Interrupt Status Mirror Registers: Two registers to provide status of each WATCHDOG limit or Interrupt
event. Reading the Mirror Registers does not affect the status bits.
• Interrupt Mask Registers: Allows masking of individual Interrupt sources, as well as separate masking for
each of the two hardware Interrupt outputs.
• CI Clear Register: Allows transmitting a 20 ms (minimum) low pulse on the chassis intrusion pin (CI).
• VID/Fan Divisor Register: This register contains the state of the VID0-VID3 input lines and the divisor bits
for FAN1 and FAN2 inputs.
• VID4 Register: Contains the state of the VID4 input.
• Extended Mode Register: Enable and control the Alert Response operation.
• Hardware High Limit Registers: Registers at 13h, 14h, 17h and 18h where Internal and External
'Hardware' WATCHDOG temperature high limits are stored. These limits have Power On Default settings but
can be adjusted by the user. The values stored at 13h and 14h can be locked down by setting bits 1 and 2 of
Configuration Register 2.
• Value and Limit RAM: The DAC digital output, monitoring results (temperature, voltages, fan counts),
WATCHDOG limits, and Company/Stepping IDs are all contained in the Value RAM. The Value RAM consists
of a total of 33 bytes, addresses 19h - 3Fh, containing:
– byte 1 at address 19h contains the DAC Data Register
– locations 1Ah and 1Bh contain the WATCHDOG low limits for AIN1 and AIN2
– locations 1Ch - 1Fh are unassigned and do not have associated registers
– the next 10 bytes at addresses 20h -29h contain all of the results
– location 2Ah is unassigned and does not have an associated register
– the next 18 bytes at addresses 2Bh-3Ch are the remaining WATCHDOG limits
– the last 2 bytes at addresses 3Eh and 3Fh contain the Company ID and Stepping ID numbers,
respectively
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