ADT7481

Rev. 0 | Page 13 of 24

CONVERSION RATE/CHANNEL

SELECTOR REGISTER

The conversion rate/channel selector register for reads is at

Address 0x04, and at Address 0x0A for writes. The four LSBs

of this register are used to program the conversion times from

15.5 ms (Code 0x0A) to 16 seconds (Code 0x00). To program

the ADT7481 to perform continuous measurements, set the

conversion rate register to 0x0B. For example, a conversion rate

of eight conversions/second means that beginning at 125 ms

intervals, the device performs a conversion on the local and the

remote temperature channels.

This register can be written to, and read back from, the SMBus.

The default value of this register is 0x08, giving a rate of 16

conversions per second. Using slower conversion times greatly

reduces the device power consumption.

Bit 7 in this register can be used to disable averaging of the

temperature measurements. All temperature channels are

measured by default. It is possible to configure the ADT7481 to

measure the temperature of one channel only. This can be

configured using Bit 4 and Bit 5 (see Table 10).

Table 10. Conversion Rate/Channel Selector Register (Read Address 0x04, Write Address 0x0A)

Bit

Mnemonic

Function

7

Averaging

Setting this bit to 1 disables averaging of the temperature measurements at the slower conversion rates

(averaging cannot take place at the three faster rates, so setting this bit has no effect). When default = 0,

averaging is enabled.

6

Reserved

Reserved for future use. Do not write to this bit.

<5:4>

Channel

Selector

These bits are used to select the temperature measurement channels:

00 = round robin = default = all channels measured

01 = local temperature only measured

10 = Remote 1 temperature only measured

11 = Remote 2 temperature only measured

<3:0>

Conversion

Rates

These bits set how often the ADT7481 measures each temperature channel. Conversion rates are as follows:

Conversions/sec

Time (seconds)

0000 = 0.0625

16

0001 = 0.125

8

0010 = 0.25

4

0011 = 0.5

2

0100 = 1

1

0101 = 2

500 m

0110 = 4

250 m

0111 = 8

125 m

1000 = 16 = default

62.5 m

1001 = 32

31.25 m

1010 = 64

15.5 m

1011 = continuous measurements

73 m (averaging enabled)

LIMIT REGISTERS

The ADT7481 has three limits for each temperature channel:

high, low, and THERM temperature limits for local, Remote 1,

and Remote 2 temperature measurements. The remote tempera-

ture high and low limits span two registers each to contain an

upper and lower byte for each limit. There is also a THERM

hysteresis register. All limit registers can be written to, and read

back from, the SMBus. See Table 15 for details of the limit register

addresses and power-on default values.

When Pin 8 is configured as an ALERT output, the high limit

registers perform a > comparison while the low limit registers

perform a ≤ comparison. For example, if the high limit register

is programmed with 80°C, then measuring 81°C will result in

an out-of-limit condition, setting a flag in the status register.

If the low limit register is programmed with 0°C, measuring

0°C or lower will result in an out-of-limit condition.

Exceeding either the local or remote THERM limit asserts

THERM low. When Pin 8 is configured as THERM2, exceeding

either the local or remote high limit asserts THERM2 low. A

default hysteresis value of 10°C is provided that applies to both

THERM channels. This hysteresis value may be reprogrammed.

It is important to remember that the temperature limits data

format is the same as the temperature measurement data for-

mat. So if the temperature measurement uses the default binary

scale, then the temperature limits also use the binary scale. If

the temperature measurement scale is switched, however, the

temperature limits do not automatically switch. The user must

reprogram the limit registers to the desired value in the correct

data format. For example, if the remote low limit is set at 10°C

and the default binary scale is being used, the limit register

value should be 0000 1010b. If the scale is switched to offset

binary, the value in the low temperature limit register should be

reprogrammed to be 0100 1010b.