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ADM1021AARQZ-R Datasheet(PDF) 12 Page - ON Semiconductor
ONSEMI [ON Semiconductor]
ADM1021AARQZ-R Datasheet(HTML) 12 Page - ON Semiconductor
/ 15 page
Figure 15. Writing to the Address Pointer Register Only
SERIAL BUS ADDRESS BYTE
ADDRESS POINTER REGISTER BYTE
Figure 16. Reading Data from a Previously Selected Register
SERIAL BUS ADDRESS BYTE
FRAME 2 DATA BYTE FROM ADM1021A
When reading data from a register there are two
1. If the ADM1021A’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 ADM1021A as
before, but only the data byte containing the
register read address is sent, because data is not to
be written to the register. This is shown in
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 16.
2. 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, so Figure 15
can be omitted.
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; this is because the first
data byte of a write is always written to the
address pointer register.
2. Remember that the ADM1021A 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 is not 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.
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 and requires a 10 k
pullup to V
. Several ALERT outputs can be wire−ANDed
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 can be used as an SMBALERT. Slave devices
on the SMBus cannot normally signal to the master that they
want to talk, but the SMBALERT function allows them to do
One or more ALERT outputs are 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 shown in Figure 17.
Figure 17. Use of SMBALERT
ARA AND READ
START ALERT RESPONSE ADDRESS RD ACK
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. The address of the device is
now known and it can be interrogated in the usual
4. If more than one device’s ALERT output is low,
the one with the lowest device address has priority,
in accordance with normal SMBus arbitration.
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