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ADM1031
asserted low. In this case, both throttling and active
cooling take place. If the high temperature limit is
programmed to a lower value than the THERM
limit, exceeding the high temperature limit asserts
INT low. Software could change the speed of the
fan depending on temperature readings. If the
temperature continues to increase and exceeds the
THERM limit, THERM asserts low to throttle the
CPU and the fan runs full-speed. This allows the
system to run in performance mode, where active
cooling takes place and the CPU is only throttled
at high temperature.
Using the high temperature limit and the THERM limit in
this way allows the user to gain maximum performance from
the Remote Temperature High and Low status bits. Bit 4 of
Status Register 1 indicates that the Remote Temperature
THERM limit has been exceeded. This bit gets cleared on a
read of Status Register 1 (see Figure 20). Bit 5 indicates a
remote diode error. This bit is a 1 if a short or open is detected
on the remote temperature channel on powerup. If this bit is
set to 1 on powerup, it cannot be cleared. Bit 6 of Status
Register 2 (0 ? 03) indicates that the Local THERM limit has
been exceeded. This bit is cleared on a read of Status
Register 2. Bit 7 indicates that THERM has been pulled low
as an input. This bit can also be cleared on a read of Status
Register 2.
THERM
LIMIT
the system by only slowing it down, should it be at a critical
temperature.
Although the ADM1031 does not have a dedicated
interrupt mask register, clearing the appropriate enable bits
in Configuration Register 2 clears the appropriate interrupts
and masks out future interrupts on that channel. Disabling
interrupt bits prevents out-of-limit conditions from
THERM
INT
TEMP
5 ?
INT REARMED
generating an interrupt or setting a bit in the status registers.
Using THERM as an Input
The THERM pin is an open-drain input/output pin. When
used as an output, it signals overtemperature conditions.
When asserted low as an output, the fan is driven full-speed
if the THERM-to-Fan Enable bit is set to 1 (Bit 7 of Register
0 ? 3F). When THERM is pulled low as an input, the THERM
bit (Bit 7) of Status Register 2 is set to 1, and the fans are
driven full-speed. Note that the THERM-to-Fan Enable bit
has no effect whenever THERM is used as an input. If
THERM is pulled low as an input, and the THERM-to-Fan
Enable bit = 0, then the fans are still driven full-speed. The
THERM-to-Fan Enable bit only affects the behavior of
THERM when used as an output.
Status Registers
All out-of-limit conditions are flagged by status bits in
Status Register 1 (0 ? 02) and Status Register 2 (0 ? 03). Bit 0
(Alarm Speed) and Bit 1 (Fan Fault) of Status Register 1,
once set, can be cleared by reading Status Register 1. Once
the alarm speed bit is cleared, this bit is not reasserted on the
next monitoring cycle even if the condition still persists.
This bit can be reasserted only if the fan is no longer at alarm
speed. Bit 1 (Fan Fault) is set whenever a fan tach failure is
detected. Once cleared, it reasserts on subsequent fan tach
failures.
Bit 2 and Bit 3 of Status Register 1 and Status Register 2
are the Remote 1 and Remote 2 Temperature High and Low
status bits. Exceeding the high or low temperature limits for
the external channel sets these status bits. Reading the status
register clears these bits. However, these bits are reasserted
if the out-of-limit condition still exists on the next
STATUS REG. READ
Figure 20. Operation of THERM and INT Signals
Figure 20 shows the interaction between INT and
THERM. Once a critical temperature THERM limit is
exceeded, both INT and THERM assert low. Reading the
status registers clears the interrupt and the INT pin goes
high. However, the THERM pin remains asserted until the
measure d temperature falls 5 ? C below the exceeded
THERM limit. This feature can be used to CPU throttle or
drive a fan full speed for maximum cooling. Note that the
INT pin for that interrupt source is not rearmed until the
temperature has fallen below the THERM limit –5 ? C. This
prevents unnecessary interrupts from tying up valuable CPU
resources.
Fan Control Modes of Operation
The ADM1031 has four different modes of operation.
These modes determine the behavior of the system.
1. Automatic Fan Speed Control Mode.
2. Filtered Automatic Fan Speed Control Mode.
3. PWM Duty Cycle Select Mode (Directly Sets Fan
Speed Under Software Control).
4. RPM Feedback Mode.
Automatic Fan Speed Control
The ADM1031 has a local temperature channel and two
remote temperature channels, which can be connected to an
on-chip diode-connected transistor on a CPU. These three
temperature channels can be used as the basis for an
automatic fan speed control loop to drive fans using pulse
width modulation (PWM).
monitoring cycle. Bit 6 and Bit 7 are the Local Temperature
High and Low status bits. These behave exactly the same as
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