ADM1021A

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10

Table 4. Configuration Register Bit Assignments

Bit

Name

Function

Power−On

Default

7

MASK1

0 = ALERT Enabled

1 = ALERT Masked

0

6

RUN/STOP

0 = Run

1 = Standby

0

5 to 0

Reserved

0

Conversion Rate Register

The lowest three bits of this register are used to program

the conversion rate by dividing the ADC clock by 1, 2, 4, 8,

16, 32, 64, or 128 to give conversion times from 125 ms

(Code 0x07) to 16 seconds (Code 0x00). This register can be

written to and read back over the SMBus. The higher five

bits of this register are unused and must be set to 0. Use of

slower conversion times greatly reduces the device power

consumption, as shown in Table 5.

Table 5. Conversion Rate Register Code

Data

Conversion/

Sec

Average Supply Current

0x00

0.0625

150

0x01

0.125

150

0x02

0.25

150

0x03

0.5

150

0x04

1

150

0x05

2

150

0x06

4

160

0x07

8

180

0x08 to 0xFF

Reserved

Limit Registers

The ADM1021A has four limit registers to store local and

remote and high and low temperature limits. These registers

can be written to and read back over the SMBus. The high

limit registers perform a > comparison, while the low limit

registers perform a < comparison. For example, if the high

limit register is programmed as a limit of 80

°C, measuring

81

°C results in an alarm condition. Even though the

temperature measurement range is from 0

° to 127°C, it is

possible to program the limit register with negative values.

This is for backwards compatibility with the ADM1021.

Offset Register

An offset register is provided at Address 0x11. This allows

the user to remove errors from the measured remote

temperature. These errors can be introduced by clock noise

and PCB track resistance. See Table 6 for an example of

offset values.

The offset value is stored as an 8−bit, twos complement

value. The value of the offset is negative if the MSB of

Register 0x11 is 1, and is positive if the MSB of Register

0x11 is 0. This value is added to the remote temperature. The

offset register defaults to 0 at powerup. The offset register

range is −128

°C to +127°C.

Table 6. Offset Values

Offset Register

Remote Temperature

(0x11)

Offset

Value

(With Offset)

(Without

Offset)

1111 1100

−4

°C

14

°C

18

°C

1111 1111

−1

°C

17

°C

18

°C

0000 0000

0

°C

18

°C

18

°C

0000 0001

+1

°C

19

°C

18

°C

0000 0100

+4

°C

22

°C

18

°C

One−Shot Register

The one−shot register is used to initiate a single

conversion and comparison cycle when the ADM1021A is

in standby mode, after which the device returns to standby.

This is not a data register as such, and it is the write operation

that causes the one−shot conversion. The data written to this

address is irrelevant and is not stored.

Serial Bus Interface

Control of the ADM1021A is carried out via the serial bus.

The ADM1021A is connected to this bus as a slave device,

under the control of a master device. Note that the SMBus

and SCL pins are three−stated when the ADM1021A is

powered down and will not pull down the SMBus.

Address Pins

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 ADM1021A has two address pins, ADD0 and ADD1,

to allow selection of the device address so that several

ADM1021A’s can be used on the same bus, and/or to avoid

conflict with other devices. Although only two address pins

are provided, these are three−state and can be grounded, left

addresses are possible, as shown in Table 7.

It should be noted that the state of the address pins is only

sampled at powerup, so changing them after powerup has no

effect.

ADD0

ADD1

Device Address

0

0

0011 000

0

NC

0011 001

0

1

0011 010

NC

0

0101 001

NC

NC

0101 010

NC

1

0101 011

1

0

1001 100

1

NC

1001 101

1

1

1001 110

1. ADD0 and ADD1 are sampled at powerup only.