ADT7470 Analog Devices, Inc., ADT7470 Datasheet - Page 10

• Manufacturer Part Number: ADT7470
• Description: Temperature Sensor Hub And Fan Controller
• Manufacturer: Analog Devices, Inc.

ADT7470
The first data byte is the address of the internal data register
to be written to, which is stored in the address pointer register.
The second data byte is the data to be written to the internal
data register.
How data is read from a register depends on whether or not
the address pointer register value is known.
If the ADT7470 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 ADT7470 as before,
but only the data byte containing the register address is sent,
because data cannot be written to the register. This is shown
in Figure 8.
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 9.
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 the operation shown in Figure 8 can be omitted.
Note the following:
•
•
•
•
WRITE OPERATIONS
The SMBus specification defines several protocols for different
types of read and write operations. The protocols used in the
ADT7470 are discussed in the following sections. The following
abbreviations are used in the diagrams:
S—Start
P—Stop
R—Read
W—Write
A—Acknowledge
A —No Acknowledge
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.
In Figure 7 to Figure 9, the serial bus address is shown as
the default value 01011(A1)(A0), where A1 and A0 are set
by the address select mode function previously defined.
In addition to supporting the send byte and receive byte
protocols, the ADT7470 also supports the read byte
protocol. See System Management Bus Specifications
Rev. 2.0 for more information.
If it is required to perform several read or write operations
in succession, the master can send a repeat start condition
instead of a stop condition to begin a new operation.
Rev. B | Page 10 of 40
The ADT7470 uses the following SMBus write protocols.
Send Byte
In this protocol, the master device sends a single command byte
to a slave device, as follows:
1.
2.
3.
4.
5.
6.
For the ADT7470, the send byte protocol is used to write a
register address to RAM for a subsequent single byte read
from the same address. This is shown in Figure 10.
If it is required to read data from the register immediately after
setting up the address, the master can assert a repeat start con-
dition immediately after the final ACK and carry out a single-
byte read without asserting an intermediate stop condition.
Write Byte
In this operation, the master device sends a command byte and
one data byte to the slave device, as follows:
1.
2.
3.
4.
5.
6.
7.
8.
This is shown in Figure 11.
S
1
The master device asserts a start condition on SDA.
The master sends the 7-bit slave address followed by
the write bit (low).
The addressed slave device asserts ACK on SDA.
The master sends a command code.
The slave asserts ACK on SDA.
The master asserts a stop condition on SDA,
and the transaction ends.
The master device asserts a start condition on SDA.
The master sends the 7-bit slave address followed by the
write bit (low).
The addressed slave device asserts ACK on SDA.
The master sends a command code.
The slave asserts ACK on SDA.
The master sends a data byte.
The slave asserts ACK on SDA.
The master asserts a stop condition on SDA to end
the transaction.
ADDRESS
Figure 10. Setting a Register Address for Subsequent Read
1
S
SLAVE
2
ADDRESS
SLAVE
Figure 11. Single-Byte Write to a Register
2
W
3
A
W
A
3
REGISTER
ADDRESS
4
REGISTER
ADDRESS
4
A
5
DATA
5
A
6
P
6
7
A
8
P