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ADT7490

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Alternatively, OS- or BIOS-level software can timestamp when

the system is powered on. If an SMBALERT is generated due to

the THERM timer limit being exceeded, another timestamp can

be taken. The difference in time can be calculated for a fixed

THERM timer limit time. For example, if it takes one week for a

THERM timer limit of 2.914 sec to be exceeded, and the next

time it takes only 1 hour, this is an indication of a serious degrada-

tion in system performance.

Configuring the THERM Pin as an Output

In addition to monitoring THERM as an input, the ADT7490 can

optionally drive THERM low as an output. When PROCHOT is

bidirectional, THERM can be used to throttle the processor by

asserting PROCHOT. The user can preprogram system-critical

thermal limits. If the temperature exceeds a thermal limit by

0.25°C, THERM asserts low. If the temperature is still above the

thermal limit on the next monitoring cycle, THERM stays low.

THERM remains asserted low until the temperature is equal to

or below the thermal limit. Because the temperature for that

channel is measured only once for every monitoring cycle, after

THERM asserts, it is guaranteed to remain low for at least one

monitoring cycle.

The THERM pin can be configured to assert low if the Remote 1

THERM, local THERM, Remote 2 THERM or PECI tempera-

ture limits are exceeded by 0.25°C. The THERM temperature

limit registers are at Register 0x6A, Register 0x6B, and Register

0x6C, respectively. Setting Bits [5:7] of Configuration Register 5

(0x7C) enables the THERM output feature for the Remote 1,

local, and Remote 2 temperature channels, respectively. Figure 36

shows how the THERM pin asserts low as an output in the event

of a critical overtemperature.

MONITORING

CYCLE

TEMP

THERM LIMIT

0.25°C

THERM LIMIT

THERM

Figure 36. Asserting THERM as an Output, Based on Tripping THERM Limits

An alternative method of disabling THERM is to program the

THERM temperature limit to –63°C or less in Offset 64 mode,

or −128°C or less in twos complement mode; that is, for

THERM temperature limit values less than –63°C or –128°C,

respectively, THERM is disabled.

Enabling and Disabling THERM on Individual Channels

The THERM pin can be enabled/disabled for individual or

combinations of temperature channels using Bits [7:5] of

Configuration Register 5 (0x7C).

THERM Hysteresis

Setting Bit 0 of Configuration Register 7 (0x11) disables

THERM hysteresis.

If THERM hysteresis is enabled and THERM is disabled (Bit 2

of Configuration Register 4, 0x7D), the THERM event is not

reflected in the status register and the fans do not go to full

speed. If THERM hysteresis is disabled and THERM is disabled

(Bit 2 of Configuration Register 4, 0x7D) and assuming the

appropriate pin is configured as THERM, the THERM pin

asserts low when a THERM event occurs.

If THERM and THERM hysteresis are both enabled, the

THERM output asserts as expected.

THERM Operation in Manual Mode

In manual mode, THERM events do not cause fans to go to full

speed, unless Bit 5 of Configuration Register 1 (0x40) is set to 1.

Additionally, Bit 3 of Configuration Register 4 (0x7D) can be

used to select PWM speed on THERM event (100% or

maximum PWM).

Bit 2 in Configuration Register 4 (0x7D) can be set to disable

THERM events from affecting the fans.

FAN DRIVE USING PWM CONTROL

The ADT7490 uses pulse-width modulation (PWM) to control

fan speed. This relies on varying the duty cycle (or on/off ratio)

of a square wave applied to the fan to vary the fan speed. The

external circuitry required to drive a fan using PWM control is

extremely simple. For 4-wire fans, the PWM drive might need

only a pull-up resistor. In many cases, the 4-wire fan PWM

input has a built-in, pull-up resistor.

The ADT7490 PWM frequency can be set to a selection of

low frequencies or a single high PWM frequency. The low

frequency options are used for 3-wire fans, while the high

frequency option is usually used with 4-wire fans.

For 3-wire fans, a single N-channel MOSFET is the only drive

device required. The specifications of the MOSFET depend on

the maximum current required by the fan being driven and

the input capacitance of the FET. Because a 10 kΩ (or greater)

resistor must be used as a PWM pull-up, an FET with large

input capacitance can cause the PWM output to become

distorted and adversely affect the fan control range. This is a

requirement only when using high frequency PWM mode.

Typical notebook fans draw a nominal 170 mA, therefore, SOT

devices can be used where board space is a concern. In

desktops, fans typically draw 250 mA to 300 mA each. If several

fans are driven in parallel from a single PWM output or drive

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