ADT7481 datasheet(8/20 Pages) ONSEMI

Part #	ADT7481

Description	Dual Channel Temperature Sensor and Overtemperature Alarm

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ADT7481

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Table 1. Register Address for the Temperature Values

Temperature

Channel

Register Address,

MSBs

Register Address,

LSBs

Local

0x00

N/A

Remote 1

0x01

0x10 (2 MSBs)

Remote 2

0x30

0x33 (2 MSBs)

If Bit 3 of the Configuration 1 register is set to 1, then the

Remote 2 temperature values can be read from the following

register addresses:

Remote 2, MSBs = 0x01

Remote 2, LSBs = 0x10

The above is true only when Bit 3 of the Configuration 1

register is set. To read the Remote 1 temperatures, this bit

needs to be switched back to 0.

Only the two MSBs in the remote temperature low byte

are used. This gives the remote temperature measurement a

resolution of 0.25

°C. Table 2 shows the data format for the

remote temperature low byte.

Table 2. Extended Temperature Resolution

(Remote Temperature Low Byte)

Extended Resolution

Remote Temperature Low Byte

0.00°C

0 000 0000

0.25°C

0 100 0000

0.50°C

1 000 0000

0.75°C

1 100 0000

When reading the full remote temperature value,

including both the high and low byte, the two registers

should be read LSB first and then the MSB. This is because

reading the LSB will cause the MSB to be locked until it is

read. This is to guarantee that the two values read are derived

from the same temperature measurement. The MSB register

updates only after it has been read. The MSB will not lock

if a SMBus repeat start is used between reading the two

registers. There needs to be a stop between reading the LSB

and MSB.

If the LSB register is read but not the MSB register, then

fail−safe protection is provided by the THERM and ALERT

signals which update with the latest temperature measurements

rather than the register values.

Temperature Measurement Range

The temperature measurement range for both local and

remote measurements is, by default, 0

°C to +127°C.

However, the ADT7481 can be operated using an extended

temperature range. The temperature range in the extended

mode is −64

°C to +191°C. The user can switch between these

two temperature ranges by setting or clearing Bit 2 in the

Configuration 1 register. A valid result is available in the next

measurement cycle after changing the temperature range.

Bit 2 Configuration Register 2 = 0 = 0

°C to +127°C = default

Bit 2 Configuration Register 2 = 1 = −64

°C to +191°C

In extended temperature mode, the upper and lower

temperatures that can be measured by the ADT7481 are

limited by the remote diode selection. While the temperature

registers can have values from −64

°C to +191°C, most

temperature sensing diodes have a maximum temperature

range of −55

°C to +150°C.

Note that while both local and remote temperature

measurements can be made while the part is in extended

temperature mode, the ADT7481 should not be exposed to

temperatures greater than those specified in the Absolute

section. Furthermore, the device is only guaranteed to operate

as specified at ambient temperatures from −40

°C to +120°C.

Temperature Data Format

The ADT7481 has two temperature data formats. When

the temperature measurement range is from 0

°C to +127°C

(default), the temperature data format is binary for both local

and remote temperature results. See the Temperature

Measurement Range section for information on how to

switch between the two data formats.

When the measurement range is in extended mode, an

offset binary data format is used for both local and remote

results. Temperature values in the offset binary data format

are offset by +64. Examples of temperatures in both data

formats are shown in Table 3.

Table 3. Temperature Data Format

(Local and Remote Temperature High Byte)

Temperature

Binary

Offset Binary (Note 1)

−55°C

0 000 0000

(Note 2)

0 000 1001

110°C

0 000 0000

0 100 0000

+1°C

0 000 0001

0 100 0001

+10°C

0 000 1010

0 100 1010

+25°C

0 001 1001

0 101 1001

+50°C

0 011 0010

0 111 0010

+75°C

0 100 1011

1 000 1011

+100°C

0 110 0100

1 010 0100

+125°C

0 111 1101

1 011 1101

+127°C

0 111 1111

1 011 1111

+150°C

0 111 1111

(Note 3)

1 101 0110

1. Offset binary scale temperature values are offset by +64.

2. Binary scale temperature measurement returns 0 for all

temperatures <0°C.

3. Binary scale temperature measurement returns 127 for all

temperatures >127°C.

The user may switch between measurement ranges at any

time. Switching the range will also switch the data format.

The next temperature result following the switching will be

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

contents of the limit registers will 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 can be found in the Limit Registers section.

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