LM87
Functional Description
(Continued)
8800 RPM on FAN1 and FAN2. Schmitt-Trigger input cir-
cuitry 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
and
Section 13.0.
These include:
Provide control and con-
•
Configuration Registers:
figuration.
•
Channel Mode Register:
Controls the functionality of
the dual purpose input pins, scaling for internal Vcc mea-
surement, and operation of some IRQ inputs.
Two registers to provide
•
Interrupt Status Registers:
status of each WATCHDOG limit or Interrupt event.
Reading the Status Registers clears any active bits.
Two registers to
•
Interrupt Status Mirror Registers:
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 indi-
vidual Interrupt sources, as well as separate masking for
each of the two hardware Interrupt outputs.
•
CI Clear Register:
Allows transmitting a 20 ms (mini-
mum) 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.
Enable and control the
•
Extended Mode Register:
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 ad-
justed by the user. The values stored at 13h and 14h can
be locked down by setting bits 1 and 2 of Configuration
Register 2.
The DAC digital output, moni-
•
Value and Limit RAM:
toring 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 Regis-
ter
— 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
When the LM87 is started, it cycles through each measure-
ment in sequence, and it continuously loops through the
sequence approximately once every 0.4 s. Each measured
value is compared to values stored in WATCHDOG, or Hard-
ware High Limit registers. When the measured value violates
the programmed limit the LM87 will set a corresponding
Interrupt in the Interrupt Status Registers. The hardware
Interrupt line INT# is fully programmable with separate
masking of each Interrupt source. In addition, the Configu-
ration Register has a control bit to enable or disable the
hardware Interrupt. Another hardware Interrupt line avail-
able, THERM# is used to signal temperature specific events.
Having a dedicated interrupt for these conditions allows
specific actions to be taken for thermal events. This output is
enabled by setting bit 2 of Configuration Register 1.
The Chassis Intrusion input is designed to accept an active
high signal from an external circuit that activates and latches
when the case is removed from the computer.
9
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