ADT7421

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11

Table 8. List of Registers

Read Address

(Hex)

Write Address

(Hex)

Name

Power−On Default

Binary

Hex

Decimal

Not Applicable

Not Applicable

Address Pointer

Undefined

00

Not Applicable

Local Temperature Value

0000 0000

0x00

01

Not Applicable

External Temperature Value High Byte

0000 0000

0x00

02

Not Applicable

Status

Undefined

03

09

Configuration

0000 1000

0x08

04

0A

Conversion Rate

0000 1000

0x06

05

0B

Local Temperature High Limit

0101 0101

0x55

85°C

06

0C

Local Temperature Low Limit

0000 0000

0x00

0°C

07

0D

External Temperature High Limit High Byte

0101 0101

0x55

85°C

08

0E

External Temperature Low Limit High Byte

0000 0000

0x00

0°C

Not Applicable

0F

One−Shot

10

Not Applicable

External Temperature Value Low Byte

0000 0000

0x00

11

11

External Temperature Offset High Byte

0000 0000

0x00

12

12

External Temperature Offset Low Byte

0000 0000

0x00

13

13

External Temperature High Limit Low Byte

0000 0000

0x00

14

14

External Temperature Low Limit Low Byte

0000 0000

0x00

19

19

External THERM Limit

0101 0101

0x55

85°C

20

20

Local THERM Limit

0101 0101

0x55

85°C

21

21

THERM Hysteresis

0000 1010

0x0A

10°C

22

22

Consecutive ALERT

0000 0001

0x01

3D

Not Applicable

Device ID

0010 0001

0x21

FE

Not Applicable

Manufacturer ID

0100 0001

0x41

FF

Not Applicable

Die Revision Code

Serial Bus Interface

Control of the ADT7421 is carried out via the serial bus.

The ADT7421 is connected to this bus as a slave device,

under the control of a master device.

The ADT7421 has an SMBus timeout feature. When this

is enabled, the SMBus times out after typically 25 ms of no

activity. However, this feature is not enabled by default.

Bits 6 and 7 of the consecutive alert register

(Address = 0x22) should be set to enable it.

Addressing the Device

In general, every SMBus device has a 7−bit device

address, except for some devices that have extended 10−bit

addresses. When the master device sends a device address

over the bus, the slave device with that address responds.

The ADT7421is available with one device address, 0x4C

(1001 100b). The ADT7421−2 is also available.

The ADT7421−2 has an SMBus address of 0x4D (1001

101b). This is to allow two ADT7421 devices on the same

bus, or if the default address conflicts with an existing device

on the SMBus. The serial bus protocol operates as follows:

The master initiates a data transfer by establishing a start

condition, defined as a high−to−low transition on SDATA,

the serial data line, while SCLK, the serial clock line,

remains high. This indicates that an address/data stream

follows. All slave peripherals connected to the serial bus

respond to the start condition and shift in the next eight bits,

consisting of a 7−bit address (MSB first) plus an R/W bit,

which determines the direction of the data transfer, that is,

whether data is written to, or read from, the slave device. The

peripheral whose address corresponds to the transmitted

address responds by pulling the data line low during the low

period before the ninth clock pulse, known as the

acknowledge bit. All other devices on the bus remain idle

while the selected device waits for data to be read from or

written to it. If the R/W bit is a 0, the master writes to the

slave device. If the R/W bit is a 1, the master reads from the

slave device.

1. Data is sent over the serial bus in a sequence of

nine clock pulses, eight bits of data followed by an

acknowledge bit from the slave device. Transitions

on the data line must occur during the low period

of the clock signal and remain stable during the

high period, since a low−to−high transition when

the clock is high can be interpreted as a stop

signal. The number of data bytes that can be

transmitted over the serial bus in a single read or