ADT7461A

http://onsemi.com

10

Table 9. 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

ALERT/

THERM2

0 = ALERT

1 = THERM2

0

4, 3

Reserved

0

2

Temperature

Range Select

0 = 0°C to 127°C

1 = Extended Range

0

1, 0

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 by dividing the

internal oscillator clock by 1, 2, 4, 8, 16, 32, 64, 128, 256,

512, or 1024 to give conversion times from 15.5 ms

(Code 0x0A) to 16 seconds (Code 0x00). For example, a

conversion rate of eight conversions per second means that

beginning at 125 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 0x08, giving a rate of 16 conversions per second. Use of

slower conversion times greatly reduces the device power

consumption.

Table 10. CONVERSION RATE REGISTER CODES

Code

Conversion/

Second

Time (Seconds)

0x00

0.0625

16

0x01

0.125

8

0x02

0.25

4

0x03

0.5

2

0x04

1

1

0x05

2

500 m

0x06

4

250 m

0x07

8

125 m

0x08

16

62.5 m

0x09

32

31.25 m

0x0A

64

15.5 m

0x0B to 0xFF

Reserved

Limit Registers

The ADT7461A has eight limit registers: high, low, and

THERM temperature limits for both local and remote

temperature measurements. The remote temperature 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 over, the SMBus. See Table 14 for details of the

limit register addresses and their power-on default values.

When Pin 6 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 results 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 results in an out-of-limit

condition.

Exceeding either the local or remote THERM limit asserts

THERM low. When Pin 6 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 can be reprogrammed to any value after

powerup (Register Address 0x21).

It is important to remember that the temperature limits

data format is the same as the temperature measurement data

format. Therefore, if the temperature measurement uses

default binary, 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 with the

default binary scale, the limit register value is 0000 1010b.

If the scale is switched to offset binary, the value in the low

temperature limit register needs to be reprogrammed to

0100 1010b.

Status Register

The status register is a read-only register at Address 0x02.

It contains status information for the ADT7461A.

When Bit 7 of the status register is high, it indicates that the

ADC is busy converting. The other bits in this register flag the

out-of-limit temperature measurements (Bit 6 to Bit 3, and

Bit 1 to Bit 0) and the remote sensor open circuit (Bit 2).

If Pin 6 is configured as an ALERT output, the following

applies: If the local temperature measurement exceeds its

limits, Bit 6 (high limit) or Bit 5 (low limit) of the status

register asserts to flag this condition. If the remote

temperature measurement exceeds its limits, then Bit 4

(high limit) or Bit 3 (low limit) asserts. Bit 2 asserts to flag

an open circuit condition on the remote sensor. These five

flags are NOR’ed together, so if any of them is high, the

ALERT interrupt latch is set and the ALERT output goes

low.

Reading the status register clears the five flags, Bit 6 to

Bit 2, provided the error conditions causing the flags to be

set have gone away. A flag bit can be reset only if the

corresponding value register contains an in-limit

measurement or if the sensor is good.