Datasheet

ADT7408
Rev. 0 | Page 18 of 24
SMBUS/I
2
C COMMUNICATIONS
The data registers in the ADT7408 are selected by the pointer
register. At power-up the pointer register is set to 0x00, the
location for the capability register. The pointer register latches
the last location to which it was set. Each data register falls into
one of the following three types of user accessibility:
• Read only
• Write only
• Write/Read same address
A write to the ADT7408 always includes the address byte and
the pointer byte. A write to any register other than the pointer
register requires two data bytes.
Reading data from the ADT7408 occurs in one of the following
two ways:
• If the location latched in the pointer register is correct,
then the read simply consists of an address byte,
followed by retrieving the two data bytes.
• If the pointer register needs to be set, then an address
byte, pointer byte, repeat start, and another address
byte accomplish a read.
The data byte has the most significant bit first. At the end of a
read, the ADT7408 accepts either acknowledge (ACK) or no
acknowledge (NO ACK) from the master. No acknowledge is
typically used as a signal for the slave that the master has read
its last byte. It typically takes the ADT7408 100 ms to measure
the temperature.
Writing Data to a Register
With the exception of the pointer register, all other registers are
16 bits wide, so two bytes of data are written to these registers.
Writing two bytes of data to these registers consists of the serial
bus address, the data register address written to the pointer
register, followed by the two data bytes written to the selected
data register (see
Figure 14). If more than the required number
of data bytes is written to a register, then the register ignores
these extra data bytes. To write to a different register, another
start or repeated start is required.
D15 D13D14 D12 D10D11 D9 D8 D7 D5D6 D4 D2D3 D1 D0
9911
FRAME 3
MOST SIGNIFICANT DATA BYTE
FRAME 4
LEAST SIGNIFICAN DATA BYTE
SCL
(CONTINUED)
SDA
(CONTINUED)
SCL
SDA
STOP
BY
MASTER
A6 A4A5 A3 A1A2 A0 R/W D7 D5D6 D4 D2D3 D1 D0
9911
FRAME 1
SERIAL BUS ADDRESS BYTE
FRAME 2
POINTER BYTE
ACK
BY
TS
ACK
BY
TS
STOP
BY
MASTER
START BY
MASTER
05716-009
ACK
BY
TS
ACK
BY
TS
Figure 14. Writing to the Address Pointer Register, Followed by Two Bytes of Data