ADT7461A Datasheet, PDF (17/24 Pages) Analog Devices – ±1°C Temperature Monitor with Series Resistance Cancellation

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 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 (see Figure 18).
• 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.
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 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 and requires a pull-up
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.
ADT7461A
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 19):
MASTER
RECEIVES
SMBALERT
START
ALERT RESPONSE
ADDRESS
RD ACK
DEVICE
ADDRESS
NO
ACK
STOP
MASTER SENDS
ARA AND READ
COMMAND
DEVICE SENDS
ITS ADDRESS
Figure 19. Use of 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 μA if there is no SMBus activity, or 30 μA
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.
Rev. A | Page 17 of 24