參數(shù)資料
型號(hào)： NCT7491RQR2G
廠(chǎng)商： ON Semiconductor
文件頁(yè)數(shù)： 12/80頁(yè)
文件大小： 844K
描述： IC REMOTE THERMAL MONITOR 24QSOP
標(biāo)準(zhǔn)包裝： 2,500
系列： *
NCT7491
http://onsemi.com
12
R/W
0
SCL
SDA
1
0
1
1
1
0
D7
D6
D5
D4
D3
D2
D1
D0
ACK. B Y
ADT7490
STOP BY
MASTER
START B Y
MASTER
FRAME 1
SERIAL BUS ADDRESS BYTE
FRAME 2
ADDRESS POINTER REGISTER BYTE
1
1
9
ACK. B Y
ADT7490
9
Figure 5. Writing to the Address Pointer Register Only
R/W
0
SCL
SDA
1
0
1
1
1
0
D7
D6
D5
D4
D3
D2
D1
D0
NO ACK. B Y
MASTER
STOP BY
MASTER
START B Y
MASTER
FRAME 1
SERIAL BUS ADDRESS BYTE
FRAME 2
DATA BYTE FROM ADT7490
1
1
9
ACK. B Y
ADT7490
9
Figure 6. Reading Data from a Previously Selected Register
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. However, 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.
In addition to supporting the send byte and receive byte
protocols, the NCT7491 also supports the read byte protocol
(see System Management Bus Specifications Rev. 2 for more
information; this document is available from the SMBus
organization).
If several read or write operations must be performed in
succession, the master can send a repeat start condition
instead of a stop condition to begin a new operation.
Write Operations
The SMBus specification defines several protocols for
different types of read and write operations. The ones used
in the NCT7491 are discussed here. The following
abbreviations are used in the diagrams:
" S  Start
" P  Stop
" R  Read
" /W
  Write
" A  Acknowledge
" /A
  No acknowledge
The NCT7491 uses the following SMBus write protocols.
Send Byte
In this operation, the master device sends a single
command byte to a slave device, as follows:
1. The master device asserts a start condition on SDA.
2. The master sends the 7bit slave address followed
by the write bit (low).
3. The addressed slave device asserts ACK on SDA.
4. The master sends a command code.
5. The slave asserts ACK on SDA.
6. The master asserts a stop condition on SDA and
the transaction ends.
For the NCT7491, the send byte protocol is used to write
a register address to RAM for a subsequent singlebyte read
from the same address. This operation is illustrated in Figure 7.
SLAVE
ADDRESS
A
S
A    P
REGISTER
ADDRESS
2
3
1
5   6
4
Figure 7. Setting a Register Address for
Subsequent Read
W
If the master is required to read data from the register
immediately after setting up the address, it can assert a repeat
start condition immediately after the final ACK and carry
out a singlebyte 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. The master device asserts a start condition on SDA.
2. The master sends the 7bit slave address followed
by the write bit (low).
3. The addressed slave device asserts ACK on SDA.
4. The master sends a command code.
5. The slave asserts ACK on SDA.
6. The master sends a data byte.
7. The slave asserts ACK on SDA.
8. The master asserts a stop condition on SDA, and
The byte write operation is illustrated in Figure 8.
SLAVE
ADDRESS
A
DATA
S
A
A   P
REGISTER
ADDRESS
2
3
1
5
6
7   8
4
Figure 8. Single Byte Write to a Register
W
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