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ASC7511

LOW-VOLTAGE 2-WIRE DIGITAL TEMPERATURE SENSOR WITH THERMAL ALARM

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aSC7511

Temperature Measurement

The aSC7511 measures temperature by calculating the

change in sensing a diode-connected transistor’s base-

emitter voltage (ΔVBE) when two different currents are

applied sequentially. This difference has a linear positive

slope with temperature, unlike the non-linear VBE under a

constant current.

The ΔVBE is amplified and scaled depending on the type of

transistor being measured. The typical case for the remote

measurement is to connect to the substrate thermal diode

of a CPU or ASIC. This will provide the die temperature of

that device to a high level of accuracy for the purpose of

controlling system cooling or reporting routine or over-

temperature conditions.

For each measurement cycle, an internal 2:1 multiplexer

alternates between the local and remote sensor diodes and

provides the input to a filter and amplifier. An analog-to-

digital converter takes this signal, converts it to a digital

value and stores the alternating values in the appropriate

temperature register of the aSC7511. These stored values

are continuously compared to several user-selected limit

values for sending alarm conditions or turning on an

external fan driver.

Conversions for both sensors will proceed automatically at

the default rate of 16 conversions per second until a

different rate is selected or one-shot or stand-by mode are

selected. The conversion rate is user-selected by writing to

the conversion rate register. Table 3 provides a list of

conversion rates ranging from 0.0625 to 64 conversions

per second. Sensor power consumption is directly

proportional to the conversion rate and should be taken into

account in power-limited applications as well as the impact

that power dissipation has in self-heating of the local

sensor. More details are provided in the Operation section.

Thermal Alarm and Alert Functions

The aSC7511 thermal alarm function, THERM , provides

user programmable thermostat capability and allows the

aSC7511 to function as a standalone thermostat without

constant attention over the serial interface. This signal is an

open drain output. When either the remote or local

temperature reading exceeds the selected limits it goes low

and will remain low until the measured temperature falls

below the alarm limit by the amount set into the THERM

hysteresis register (default value is 10°C).

The aSC7511 thermal alert function, ALERT , provides

another open-drain pin that is driven low when either the

internal or remote temperature is greater than the high limit

or less than or equal to the low limit selected by the user. In

addition, an SMBus Alert is generated, requiring the bus

master to inquire on the SMBus which client has

interrupted in order to re-set the alert. The ALERT pin may

be masked by the user with bit 7 of the configuration

register.

Note that limit comparisons are triggered only by a

measurement action, not by a change in limits. An actual

alarm condition may exist but will not be reported until a

measurement takes place. This should be taken into

account when using slow conversion rates.

The ALERT pin may also be configured as a second

THERM pin, THERM2 . This will behave in the same way

as the THERM pin, but will be controlled by the high limit

used for the ALERT pin, although it will not need to be

reset by the master and does not use the hysteresis value.

A more detailed discussion and examples is provided in the

Operation section.

Fault Tolerance

The number of out of limit readings required before the

ALERT is asserted may be set by writing to the

Consecutive ALERT register. The default value is 1 and

the user may select requiring 2, 3, or 4 consecutive out of

limit readings before is ALERT asserted.

Registers

The ASC7511 contains 22 8-bit registers. All of these

registers may be accessed by the user via the digital serial

interface at any time. A detailed description of these

registers and their functions is provided in the following

paragraphs. Reading and writing to registers is covered in

the SMBus Operation section. Use of the register set to

control and interrogate the aSC7511 is covered in the

Operation section. A table of the registers is provided in

Table 4.

Address Pointer Register

The Address Pointer Register is a write-only register that is

automatically written by the first byte that follows the R/ W

bit of a write transaction. If R/ W is low, It will then contain

the address to which the following byte is written, when

R/ W bit is high it will be the read address. Details of the

reading and writing sequence are covered in the Serial

Interface Operation section.

Measured Temperature Value Registers

The aSC7511 has two sensors whose measured values

are stored in registers. Remote temperature is two bytes, at

register addresses 01h and 10h. Local temperature is at

00h. The power-on default values are zero and the

aSC7511 will automatically begin filling these registers with

values after the power on reset sequence is complete.

These are read-only registers.

© Andigilog, Inc. 2006

-6-

www.andigilog.com

August 2006 - 70A04010

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