ADT7461AARMZ datasheet(13/24 Pages) AD

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ADT7461A

Rev. A | Page 13 of 24

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 con-

version times greatly reduces the device power consumption.

Table 10. Conversion Rate Register Codes

Code

Conversion/Second

Time (Seconds)

0x00

0.0625

0x01

0.125

0x02

0.25

0x03

0.5

0x04

0x05

500 m

0x06

250 m

0x07

125 m

0x08

62.5 m

0x09

31.25 m

0x0A

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 power-up (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 tem-

perature 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.

The ALERT interrupt latch is not reset by reading the status

the master reading the device address, provided the error condi-

tion has gone away and the status register flag bits are reset.

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