ADT7461AARMZ-2REEL Datasheet ADT7461AARMZ, ADT7461AARMZ-002, ADT7461AARMZ-2REEL | P13-P15

ADT7461A

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transmitted over the serial bus in a single read or

write operation is limited only by what the master

and slave devices can handle.

3. When all data bytes have been read or written,

stop conditions are established. In write mode, the

master pulls the data line high during the tenth

clock pulse to assert a stop condition. In read

mode, the master device overrides the

acknowledge bit by pulling the data line high

during the low period before the ninth clock pulse.

This is known as no acknowledge. The master

takes the data line low during the low period

before the tenth clock pulse, then high during the

tenth clock pulse to assert a stop condition.

Any number of bytes of data are transferable over

the serial bus in one operation, but it is not

possible to mix read and write in one operation

because the type of operation is determined at the

beginning and cannot subsequently be changed

without starting a new operation. For the

ADT7461A, write operations contain either one or

two bytes, while read operations contain one byte.

To write data to one of the device data registers, or to read

data from it, the address pointer register must be set so that

the correct data register is addressed. The first byte of a write

operation always contains a valid address that is stored in the

address pointer register. If data is to be written to the device,

the write operation contains a second data byte that is written

to the register selected by the address pointer register.

This procedure is illustrated in Figure 15. The device

address is sent over the bus followed by R/W

set to 0. This

is followed by two data bytes. The first data byte is the

address of the internal data register to be written to, which

is stored in the address pointer register. The second data byte

is the data to be written to the internal data register.

Figure 15. Writing a Register Address to the Address Pointer Register, then Writing Data to the Selected Register

Figure 16. Writing to the Address Pointer Register Only

Figure 17. Reading from a Previously Selected Register

D7 D6 D5 D4 D3 D2 D1 D0

FRAME 3

DATA BYTE

ACK. BY

ADT7461A

STOP BY

MASTER

SDATA (CONTINUED)

SCLK (CONTINUED)

FRAME 1

SERIAL BUS ADDRESS BYTE

ACK. BY

ADT7461A

SDATA

SCLK

START BY

MASTER

FRAME 2

ADDRESS POINTER REGISTER BYTE

D7 D6 D5 D4 D3 D2 D1 D0A6 A5 A4 A3 A2 A1 A0 R/W

ACK. BY

ADT7461A

1 19 9

FRAME 1

SERIAL BUS ADDRESS BYTE

ACK. BY

ADT7461A

SDATA

SCLK

START BY

MASTER

FRAME 2

ADDRESS POINTER REGISTER BYTE

D7 D6 D5 D4 D3 D2 D1 D0A6 A5 A4 A3 A2 A1 A0 R/W

ACK. BY

ADT7461A

1 19 9

STOP BY

MASTER

FRAME 1

SERIAL BUS ADDRESS BYTE

ACK. BY

ADT7461A

SDATA

SCLK

START BY

MASTER

FRAME 2

ADDRESS POINTER REGISTER BYTE

D7 D6 D5 D4 D3 D2 D1 D0A6 A5 A4 A3 A2 A1 A0 R/W

ACK. BY

ADT7461A

1 19 9

STOP BY

MASTER

ADT7461A

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When reading data from a register there are two

possibilities.

• If the address pointer register value of the ADT7461A

is unknown or not the desired value, it is first necessary

to set it to the correct value before data can be read

from the desired data register. This is done by writing

to the ADT7461A as before, but only the data byte

containing the register read address is sent, because

data is not to be written to the register see Figure 16.

A read operation is then performed consisting of the

serial bus address, R/W

bit set to 1, followed by the

data byte read from the data register see Figure 17.

• If the address pointer register is known to be at the

desired address, data can be read from the

corresponding data register without first writing to the

address pointer register and the bus transaction shown

in Figure 16 can be omitted.

Notes:

• It is possible to read a data byte from a data register

without first writing to the address pointer register.

However, if the address pointer register is already at the

correct value, it is not possible to write data to a register

without writing to the address pointer register because

the first data byte of a write is always written to the

address pointer register.

• Some of the registers have different addresses for read

and write operations. The write address of a register

must be written to the address pointer if data is to be

written to that register, but it may not be possible to

read data from that address. The read address of a

data can be read from that register.

ALERT Output

This is applicable when Pin 6 is configured as an ALERT

output. The ALERT output goes low whenever an

out−of−limit measurement is detected, or if the remote

temperature sensor is open circuit. It is an open−drain output

and requires a pullup resistor. Several ALERT

outputs can

be wire−OR’ed together, so that the common line goes low

if one or more of the ALERT

outputs goes low.

The ALERT

output can be used as an interrupt signal to a

processor, or as an SMBALERT

. Slave devices on the SMBus

cannot normally signal to the bus master that they want to

talk, but the SMBALERT

function allows them to do so.

One or more ALERT

outputs can be connected to a

common SMBALERT

line that is connected to the master.

When the SMBALERT

line is pulled low by one of the

devices, the following procedure occurs (see Figure 18):

Figure 18. Use of SMBALERT

ALERT RESPONSE

ADDRESS

MASTER SENDS

ARA AND READ

COMMAND

DEVICE SENDS

ITS ADDRESS

RDSTART ACK

DEVICE

ADDRESS

ACK

STOP

MASTER

RECEIVES

SMBALERT

1. SMBALERT is pulled low.

2. Master initiates a read operation and sends the

alert response address (ARA = 0001 100). This is

a general call address that must not be used as a

specific device address.

3. The device whose ALERT

output is low responds

to the alert response address and the master reads

its device address. As the device address is seven

bits, an LSB of 1 is added. The address of the

device is now known and it can be interrogated in

the usual way.

4. If more than one device’s ALERT

output is low,

the one with the lowest device address takes

priority, in accordance with normal SMBus

arbitration.

Once the ADT7461A has responded to the alert response

address, it resets its ALERT

output, provided that the error

condition that caused the ALERT

no longer exists. If the

SMBALERT

line remains low, the master sends the ARA

again, and so on until all devices whose ALERT

outputs

were low have responded.

Low Power Standby Mode

The ADT7461A can be put into low power standby mode

by setting Bit 6 of the configuration register. When Bit 6 is

low, the ADT7461A operates normally. When Bit 6 is high,

the ADC is inhibited, and any conversion in progress is

terminated without writing the result to the corresponding

value register. However, the SMBus is still enabled. Power

consumption in the standby mode is reduced to 5 mA if there

is no SMBus activity, or 30 mA if there are clock and data

signals on the bus.

When the device is in standby mode, it is possible to

initiate a one−shot conversion of both channels by writing to

the one−shot register (Address 0x0F), after which the device

returns to standby. It does not matter what is written to the

one−shot register, all data written to it is ignored. It is also

possible to write new values to the limit register while in

standby mode. If the values stored in the temperature value

registers are outside the new limits, an ALERT

is generated,

even though the ADT7461A is still in standby.

ADT7461A

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Sensor Fault Detection

At its D+ input, the ADT7461A contains internal sensor

fault detection circuitry. This circuit can detect situations

where an external remote diode is either not connected or

incorrectly connected to the ADT7461A. A simple voltage

comparator trips if the voltage at D+ exceeds V

− 1 V

(typical), signifying an open circuit between D+ and D−.

The output of this comparator is checked when a conversion

is initiated. Bit 2 of the status register (open flag) is set if a

fault is detected. If the ALERT

pin is enabled, setting this

flag causes ALERT

to assert low.

If the user does not wish to use an external sensor with the

ADT7461A, tie the D+ and D− inputs together to prevent

continuous setting of the open flag.

The ADT7461A Interrupt System

The ADT7461A has two interrupt outputs, ALERT and

THERM

. Both have different functions and behavior.

ALERT

is maskable and responds to violations of software

programmed temperature limits or an open−circuit fault on

the external diode. THERM

is intended as a fail−safe

interrupt output that cannot be masked.

If the external or local temperature exceeds the

programmed high temperature limits, or equals or exceeds

the low temperature limits, the ALERT

output is asserted

low. An open−circuit fault on the external diode also causes

ALERT

to assert. ALERT is reset when serviced by a master

reading its device address, provided the error condition has

gone away and the status register has been reset.

The THERM

output asserts low if the external or local

temperature exceeds the programmed THERM

limits.

THERM

temperature limits should normally be equal to or

greater than the high temperature limits. THERM

is reset

automatically when the temperature falls back within the

THERM

limit. The external and local limits are set by

default to 85°C. A hysteresis value can be programmed; in

which case, THERM

resets when the temperature falls to the

limit value minus the hysteresis value. This applies to both

local and remote measurement channels. The power−on

hysteresis default value is 10°C, but this can be

reprogrammed to any value after powerup.

The hysteresis loop on the THERM

outputs is useful when

THERM

is used, for example, as an on/off controller for a

fan. The user’s system can be set up so that when THERM

asserts, a fan is switched on to cool the system. When

THERM

goes high again, the fan can be switched off.

Programming a hysteresis value protects from fan jitter,

where the temperature hovers around the THERM

limit, and

the fan is constantly switched.

Table 10. THERM Hysteresis

THERM Hysteresis Binary Representation

0°C 0 000 0000

1°C 0 000 0001

10°C 0 000 1010

Figure 19 shows how the THERM and ALERT outputs

operate. The ALERT

output can be used as a SMBALERT

to signal to the host via the SMBus that the temperature has

risen. The user can use the THERM

output to turn on a fan

to cool the system, if the temperature continues to increase.

This method ensures that there is a fail−safe mechanism to

cool the system, without the need for host intervention.

Figure 19. Operation of the ALERT and THERM

Interru

HIGH TEMP LIMIT

RESET BY MASTER

ALERT

THERM

1005C

TEMPERATURE

905C

805C

705C

605C

505C

405C

THERM LIMIT

THERM LIMIT−HYSTERESI

• If the measured temperature exceeds the high

temperature limit, the ALERT

output asserts low.

• If the temperature continues to increase and exceeds the

THERM

limit, the THERM output asserts low. This can

be used to throttle the CPU clock or switch on a fan.

• The THERM output deasserts (goes high) when the

temperature falls to THERM

limit minus hysteresis. In ,

the default hysteresis value of 10°C is shown.

• The ALERT output deasserts only when the

temperature has fallen below the high temperature

limit, and the master has read the device address and

cleared the status register.

• Pin 6 on the ADT7461A can be configured as either an

ALERT

output or as an additional THERM output.

• THERM2 asserts low when the temperature exceeds the

programmed local and/or remote high temperature

limits. It is reset in the same manner as THERM and is

not maskable.

• The programmed hysteresis value also applies to

THERM2

Figure 20 shows how THERM

and THERM2 operate

together to implement two methods of cooling the system.

In this example, the THERM2

limits are set lower than the

THERM

limits. The THERM2 output is used to turn on a

fan. If the temperature continues to rise and exceeds the

THERM

limits, the THERM output provides additional

cooling by throttling the CPU.

P1-P3 P4-P6 P7-P9 P10-P12 P13-P15 P16-P18 P19-P19

ADT7461AARMZ-2REEL

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Manufacturer:

ON Semiconductor

Description:

SENSOR DIGITAL -40C-120C MICRO8

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