ADT7475 Analog Devices, Inc., ADT7475 Datasheet - Page 10

• Manufacturer Part Number: ADT7475
• Description: Dbcool Remote Thermal Monitor And Fan Controller
• Manufacturer: Analog Devices, Inc.

ADT7475
SERIAL BUS INTERFACE
On PCs and servers, control of the ADT7475 is carried out
using the SMBus. The ADT7475 is connected to this bus as
a slave device under the control of a master controller, which
is usually (but not necessarily) the ICH.
The ADT7475 has a fixed 7-bit serial bus address of 0101110 or
0x2E. The read/write bit must be added to get the 8-bit address
(01011100 or 0x5C). Data is sent over the serial bus in sequences
of nine clock pulses, that is, eight bits of data followed by an
acknowledge bit from the slave device. Transitions on the data
line must occur during the low period of the clock signal and
remain stable during the high period because a low-to-high
transition when the clock is high may be interpreted as a stop
signal. The number of data bytes that can be transmitted over
the serial bus in a single read or write operation is limited only
by what the master and slave devices can handle.
When all data bytes are read or written, stop conditions are
established. In write mode, the master pulls the data line high
during the tenth clock pulse to assert a stop condition. In read
mode, the master device overrides the acknowledge bit
by pulling the data line high during the low period before the
ninth clock pulse; this is known as no acknowledge. The master
takes the data line low during the low period before the tenth
clock pulse, and then high during the tenth clock pulse to assert
a stop condition.
Any number of bytes of data can be transferred over the serial
bus in one operation, but it is not possible to mix read and write
in one operation because the type of operation is determined
at the beginning and cannot subsequently be changed without
starting a new operation.
SDA
SCL
START BY
MASTER
Figure 14. Writing a Register Address to the Address Pointer Register, Then Writing Data to the Selected Register
1
0
1
SERIAL BUS ADDRESS BYTE
0
1
FRAME 1
SDA (CONTINUED)
1
SCL (CONTINUED)
1
0
Rev. B | Page 10 of 68
R/W
ADT7475
ACK. BY
D7
9
1
D7
D6
1
In the ADT7475, write operations contain either one or two
bytes, and read operations contain one byte. To write data to
one of the device data registers or read data from it, the address
pointer register must be set so that the correct data register is
addressed, and then data can be written to that register or read
from it. The first byte of a write operation always contains an
address that is stored in the address pointer register. If data is
to be written to the device, the write operation contains a
second data byte that is written to the register selected by the
address pointer register.
This write operation is shown in Figure 14. The device address
is sent over the bus, and then R/ W is set to 0. This is followed
by two data bytes. The first data byte is the address of the
internal data register to write to, which is stored in the address
pointer register. The second data byte is the data to write to the
internal data register.
When reading data from a register, there are two possibilities:
•
•
D6
D5
If the ADT7475 address pointer register value is unknown
or not the desired value, it must first be set to the correct
value before data can be read from the desired data register.
This is done by performing a write to the ADT7475 as
before, but only the data byte containing the register
address is sent, because no data is written to the register
(see Figure 15).
A read operation is then performed consisting of the serial
bus address; the R/ W bit set to 1, followed by the data byte
read from the data register (see Figure 16).
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 (see Figure 16).
ADDRESS POINTER REGISTER BYTE
D5
D4
DATA BYTE
FRAME 3
D4
D3
FRAME 2
D3
D2
D2
D1
D1
D0
ADT7475
ACK. BY
D0
9
ADT7475
ACK. BY
STOP BY
MASTER
9