ADT7421ARZ-REEL View Datasheet(PDF) - ON Semiconductor

Part Name

Description

MFG CO.

ADT7421ARZ-REEL

Precision Temperature Sensor with Beta Compensation (for <45 nm Geometries)

ON Semiconductor 

ADT7421

The user can switch between measurement ranges at any

time. Switching the range likewise switches the data format.

The next temperature result following the switching is

reported back to the register in the new format. However, the

contents of the limit registers do not change. It is up to the

user to ensure that when the data format changes, the limit

registers are reprogrammed as necessary. More information

on this is found in the Limit Registers section.

ADT7421 Registers

The ADT7421 contains 22, 8−bit registers in total. These

registers store the results of remote and local temperature

measurements, high and low temperature limits, and

configure and control the device. See the section through the

Consecutive ALERT Register section of this data sheet for

more information on the ADT7421 registers. Additional

details are shown in Table 3 through Table 7. The entire

register map is available in Table 8.

Address Pointer Register

The address pointer register itself does not have, nor does

it require, an address because the first byte of every write

operation is automatically written to this register. The data

in this first byte always contains the address of another

register on the ADT7421 that is stored in the address pointer

register. It is to this register address that the second byte of

a write operation is written, or to which a subsequent read

operation is performed.

The power−on default value of the address pointer register

is 0x00. Therefore, if a read operation is performed

immediately after power−on, without first writing to the

address pointer, the value of the local temperature is returned

because its register address is 0x00.

Temperature Value Registers

The ADT7421 has three registers to store the results of

local and remote temperature measurements. These

registers can only be written to by the ADC and can be read

by the user over the SMBus. The local temperature value

register is at Address 0x00.

The external temperature value high byte register is at

Address 0x01, with the low byte register at Address 0x10.

The power−on default for all three registers is 0x00.

Configuration Register

The configuration register is Address 0x03 at read and

Address 0x09 at write. Its power−on default is 0x08. Only

five bits of the configuration register are used. Bit 0, Bit 1,

and Bit 3 are reserved; the user does not write to them.

Bit 7 of the configuration register masks the ALERT

output. If Bit 7 is 0, the ALERT output is enabled. This is the

power−on default. If Bit 7 is set to 1, the ALERT output is

disabled. This applies only if Pin 6 is configured as ALERT.

If Pin 6 is configured as THERM2, then the value of Bit 7

has no effect.

If Bit 6 is set to 0, which is power−on default, the device

is in operating mode with ADC converting. If Bit 6 is set to

1, the device is in standby mode and the ADC does not

convert. The SMBus does, however, remain active in standby

mode; therefore, values can be read from or written to the

ADT7421 via the SMBus. The ALERT and THERM outputs

are also active in standby mode. Changes made to the

registers in standby mode that affect the THERM or ALERT

outputs cause these signals to be updated.

Bit 4 switches beta cancellation on and off. With Bit 4 set

to zero beta cancellation is set on. If using a discrete

transistor as the sensing element, Beta Cancellation should

be switched off by setting Bit 4 to 1.

Bit 5 determines the configuration of Pin 6 on the

ADT7421. If Bit 5 is 0 (default), then Pin 6 is configured as

an ALERT output. If Bit 5 is 1, then Pin 6 is configured as

a THERM2 output. Bit 7, the ALERT mask bit, is only active

when Pin 6 is configured as an ALERT output. If Pin 6 is set

up as a THERM2 output, then Bit 7 has no effect.

Bit 2 sets the temperature measurement range. If Bit 2 is

0 (default value), the temperature measurement range is set

between 0°C to +125°C. Setting Bit 2 to 1 sets the

measurement range to the extended temperature range

(−40°C to +125°C).

Table 3. Configuration Register Bit Assignments

Bit

Name

Function

Power−On

Default

7 MASK1

0 = ALERT Enabled

0

1 = ALERT Masked

6 RUN/STOP

0 = Run

0

1 = Standby

5 ALERT/

0 = ALERT

0

THERM2

1 = THERM2

4 Beta Enable 0 = Beta Compensation

0

On

1 = Beta Compensation

Off

3 Reserved

Reserved

1

2 Temperature 0 = 0°C to 125°C

0

Range Select 1 = Extended Range

1 Reserved

Reserved

0

0 Reserved

Reserved

0

Conversion Rate Register

The conversion rate register is Address 0x04 at read and

Address 0x0A at write. The lowest four bits of this register

are used to program the conversion rate. For example, a

conversion rate of five conversions per second means that

beginning at 200 ms intervals, the device performs a

conversion on the internal and the external temperature

channels.

The conversion rate register can be written to and read

back over the SMBus. The higher four bits of this register are

unused and must be set to 0. The default value of this register

is 0x06, giving a rate of 4 conversions per second. Use of

slower conversion times greatly reduces the device power

consumption.

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