ADT7490_16 datasheet(10/75 Pages) ONSEMI

Part #	ADT7490

Description	Remote Thermal Monitor and Fan Controller

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ADT7490

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10

Theory of Operation

The ADT7490 is a complete thermal monitor and multiple

fan controller for any system requiring thermal monitoring

and cooling. The device communicates with the system via

a serial system management bus. The serial bus controller

has a serial data line for reading and writing addresses and

data (Pin 1), and an input line for the serial clock (Pin 2). All

control and programming functions for the ADT7490 are

performed over the serial bus. In addition, Pin 14 can be

reconfigured as an SMBALERT output to signal

out-of-limit conditions.

Feature Comparisons Between the ADT7490 and

ADT7476A

The ADT7490 is pin and register map compatible with the

ADT7476A. The new or additional features are detailed in

the following sections.

PECI Input

CPU thermal information is provided through the PECI

input. The ADT7490 has PECI master capabilities and can

read the CPU thermal information through the PECI

interface. Each CPU address can have up to two PECI

domains. The ADT7490 has the ability to record four PECI

temperature readings corresponding to the four PECI

addresses of 0x30 to 0x33. The hotter of the two domains at

any given address is stored in the PECI value registers. A

PECI reading is a negative value, in degrees Celsius, which

represents the offset from the thermal control circuit (TCC)

activation temperature. PECI information is not converted

to absolute temperature reading. PECI information is in a

16-bit twos complement value; however, the ADT7490

records the sign bit as well as the bits from 12:6 in the 16-bit

PECI payload. See the Platform Environment Control

Interface (PECI) Specification from Intel® for more details

on the PECI data format. The PECI format is represented in

Table 6.

Table 6. PECI DATA FORMAT

MSB Upper Nibble

MSB Lower Nibble

S

x

Sign Bit

Integer Value (0°C to 127°C)

There are associated high and low limits for each PECI

reading that can be programmed. The limit values take the

same format as the PECI reading. Therefore, the

programmed limits are not absolute temperatures but a

relative offset in degrees Celsius from the TCC activation

temperature. An out-of-limit event is recorded as follows:

High Limit > Comparison Performed

Low Limit

≤ Comparison Performed

An out-of-limit event is recorded in the associated status

register and can be used to assert the SMBALERT pin.

Temperature Data REPLACE Mode

The REPLACE mode is configured by setting Bit 4 of

Register 0x36. In this mode, the data in the existing

Remote 1 registers are replaced by PECI0 data and vice

versa. This is a legacy mode that allows the thermal data

from CPU1 to be stored in the same registers as in the

ADT7476A. This reduces the software changes in systems

transitioning from CPUs with thermal diodes to CPUs with

a PECI interface. See the PECI Temperature Measurement

section for more details.

Fan Control Using PECI Information

The CPU thermal information from PECI can be used in the

existing automatic fan control algorithms. This temperature

reading remains relative to TCC activation temperature and

the associated AFC control parameters are programmed in

relative temperatures as opposed to absolute temperatures,

and are in the same format as detailed in Table 6. PECIMIN,

TRANGE, and TCONTROL are user defined.

Figure 17. Overview of Automatic Fan Speed Control

Using PECI Thermal Information

TRANGE

PWMMAX

PWMMIN

PWM = 0%

PWM = 100%

(TMIN)

PECIMIN

(TMAX)

TCONTROL

TCC

PECI = 0

Dynamic TMIN Fan Control Mode

The automatic fan speed control incorporates a feature

called dynamic TMIN control. This intelligent fan control

feature reduces the design effort required to program the

automatic fan speed control loop and improves the system

acoustics.

VTT Input

The VTT voltage is monitored on Pin 8. This voltage is

also used as the reference voltage for the PECI interface. The

VTT voltage must be connected to the ADT7490 in order for

the PECI interface to be operational.

IMON Monitoring

The IMON input on Pin 19 can be used to monitor the IMON

output of ON Semiconductor’s VR10/VR11.1 controllers.

IMON is a voltage representation of the CPU current. Using

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