ADT7481ARMZ-R7 datasheet(15/20 Pages) ONSEMI

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ADT7481

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When reading data from a register there are two possible

scenarios:

• If the address pointer register value of the ADT7481 is

unknown or not the desired value, it is necessary to set it

to the correct value before data can be read from the

desired data register. This is done by performing a write

to the ADT7481 as before, but only the data byte

containing the register read address is sent, as data is not

to be written to the register. This is shown in 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 (shown in Figure 17).

• If the address pointer register is already at the desired

address, data can be read from the corresponding data

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. This is

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

address pointer register.

Remember that some of the ADT7481 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 register must be written to the address pointer before data

can be read from that register.

ALERT Output

Pin 8 can be 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. Several

ALERT outputs can be wire−OR’ed together, so that the

common line will go 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 it may be used 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 connected to the master. When

the SMBALERT line is pulled low by one of the devices, the

following procedure occurs as illustrated in Figure 18.

Figure 18. Use of SMBALERT

ALERT RESPONSE

ADDRESS

MASTER SENDS

ARA AND READ

COMMAND

DEVICE SENDS

ITS ADDRESS

START

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 with a low ALERT output responds to

the alert response address, and the master reads the

address from the responding device. An LSB of 1

is added because the device address is comprised

of seven bits. The address of the device is now

known and it can be interrogated in the usual way.

4. If more than one device has a low ALERT output,

the one with the lowest device address will have

priority, in accordance with normal SMBus

arbitration.

5. Once the ADT7481 has responded to the alert

response address, it will reset 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 with low ALERT outputs

respond.

Masking the ALERT Output

The ALERT output can be masked for local, Remote 1,

Remote 2 or all three channels. This is done by setting the

appropriate mask bits in either the Configuration 1 register

(read address = 0x03, write address = 0x09) or in the

consecutive ALERT register (address = 0x22)

To mask ALERTs due to local temperature, set Bit 5 of the

consecutive ALERT register to 1. Default = 0.

To mask ALERTs due to Remote 1 temperature, set Bit 1 of

the Configuration 1 register to 1. Default = 0.

To mask ALERTs due to Remote 2 temperature, set Bit 0 of

the Configuration 1 register to 1. Default = 0.

To mask ALERTs due to any channel, set Bit 7 of the

Configuration 1 register to 1. Default = 0.

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