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ADT7461
When reading data from a register there are two possibilities:
0
ALERT RESPONSE
ADDRESS
MASTER SENDS
ARA AND READ
COMMAND
DEVICE SENDS
ITS ADDRESS
RD
START
ACK
DEVICE
ADDRESS
NO
MASTER
RECEIVES
SMBALERT
If the ADT7461’s address pointer register value 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 performing a write to the
ADT7461 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 F
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
.
.
igure 16
igure 16
Figure 17
If the address pointer register is known to be already 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 F
omitted.
can be
Notes
1.
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.
2.
Do not forget that some of the ADT7461 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 to
V
DD
. Several ALERT outputs can be wire-ORed 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 F
.
igure 18
Figure 18. 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 will have priority, in accor-
dance with normal SMBus arbitration.
5.
Once the ADT7461 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 will send the
ARA again, and so on until all devices whose ALERT
outputs were low have responded.
LOW POWER STANDBY MODE
The ADT7461 can be put into low power standby mode by
setting 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 μA if there is no SMBus activity
or 100 μA 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 0×0F), after which the device will return
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 now
outside the new limits, an ALERT is generated, even though the
ADT7461 is still in standby.
SENSOR FAULT DETECTION
The ADT7461 has sensor fault detection circuitry internally at
its D+ input. This circuit can detect situations where an external
remote diode is not connected, or is incorrectly connected, to
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