ADT7483A Datasheet, PDF (18/24 Pages) Analog Devices – Dual Channel Temperature Sensor and Over Temperature Alarm

ADT7483A
1
9
1
9
SCL
SDA
1
0
START BY
MASTER
0
1
1
0
1
R/W
D7
D6
D5
D4
D3
D2
D1
FRAME 1
SERIAL BUS ADDRESS BYTE
ACK. BY
ADT7483A
FRAME 2
DATA BYTE FROM ADT7483A
Figure 18. Reading from a Previously Selected Register
D0
ACK. BY STOP BY
MASTER MASTER
When reading data from a register there are two possibilities:
• If the address pointer register value of the ADT7483A 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 ADT7483A as before, but only the data byte containing
the register read address is sent, as 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 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 Figure 17 can
be omitted.
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 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 shown in Figure 19.
MASTER
RECEIVES
SMBALERT
START
ALERT RESPONSE
ADDRESS
MASTER SENDS
ARA AND READ
COMMAND
RD ACK
DEVICE
ADDRESS
NO
ACK
STOP
DEVICE SENDS
ITS ADDRESS
Figure 19. Use of SMBALERT
1. SMBALERT is pulled low.
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.
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 device address is seven bits, so an LSB of 1 is
added. The address of the device is now known and it can
be interrogated in the usual way.
• Remember that some of the ADT7483A 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 13 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 VDD. 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.
4. If more than one device’s ALERT output is low, the one
with the lowest device address will have priority, in
accordance with normal SMBus arbitration.
5. Once the ADT7483A 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.
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