ADT7461

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14

When reading data from a register there are two

possibilities.

1. If the ADT7461’s address pointer register value 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

writing to the ADT7461 as before, but only the

data byte containing the register read address is

sent, since data is not to be written to the register.

This is shown in Figure 17.

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. This is

shown in Figure 18.

2. 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 17 can be omitted.

Although it is possible to read a data byte from a data

register without first writing to the address pointer register,

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.

Also, 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 register must be written to the

address pointer before 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

be wire-ORed together, so 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 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 that is connected to the master.

When the SMBALERT line is pulled low by one of the

devices, the procedure shown in Figure 19 occurs.

Figure 19. Use of SMBALERT

ALERT RESPONSE

ADDRESS

MASTER SENDS

ARA AND READ

COMMAND

DEVICE SENDS

ITS ADDRESS

RD

START

ACK

DEVICE

ADDRESS

NO

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 can be interrogated in

the usual way.

4. If the ALERT output is low on more than one

device, the one with the lowest device address has

priority, in accordance with normal SMBus

arbitration.

5. Once the ADT7461 has responded to the alert

response address, it resets its ALERT output,

provided the error condition that caused the

ALERT no longer exists. If the SMBALERT line

remains low, the master sends the ARA again; this

sequence continues until all devices whose

ALERT out-puts were low have responded.

Low Power Standby Mode

The ADT7461 can be put into low power standby mode

by set-ting Bit 6 of the configuration register. When Bit 6 is

low, the ADT7461 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.

The SMBus is still enabled. Power consumption in the

standby mode is reduced to less than 10

mA if there is no

SMBus activity or 100

mA if there are clock and data signals

on the bus.

When the device is in standby mode, it is still 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, as 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 now outside the new limits, an ALERT is

generated even though the ADT7461 is still in standby.