MC74 Datasheet, PDF (5/12 Pages) ON Semiconductor – Serial Digital Temperature Sensor

MC74
Write Byte Format
S
ADDRESS
7 Bits
Slave Address
Read Byte Format
WR
ACK
COMMAND ACK DATA ACK
P
8 Bits
8 Bits
Command Byte: selects
which register you are
writing to.
Data Byte: data goes
into the register set
by the command byte.
S ADDRESS WR
7 Bits
Slave Address
ACK COMMAND ACK
8 Bits
S ADDRESS RD ACK
7 Bits
Command Byte: selects
which register you are
reading from.
Slave Address: repeated
due to change in data–
flow direction.
DATA NACK P
8 Bits
Data Byte: reads from
the register set by the
command byte.
Receive Byte Format
S ADDRESS RD ACK DATA NACK P
7 Bits
8 Bits
Data Byte: reads data from
the register commanded by
the last Read Byte.
S = Start Condition
P = Stop Condition
Shaded = Slave Transmission
Figure 1. SMBus Protocols
Start Condition (START)
The MC74 continuously monitors the SDA and SCL lines
for a start condition (a HIGH to LOW transition of SDA
while SCL is HIGH) and will not respond until this condition
is met.
Address Byte
Immediately following the Start Condition, the host must
transmit the address byte to the MC74. The states of A1 and
A0 determine the 7–bit SMBus address for the MC74. The
7–bit address transmitted in the serial bit stream must match
for the MC74 to respond with an Acknowledge (indicating
the MC74 is on the bus and ready to accept data). The eighth
bit in the Address Byte is a Read–Write Bit. This bit is a 1
for a read operation or 0 for a write operation. During the
first phase of any transfer this bit will be set = 0 to indicate
that the command byte is being written.
Acknowledge (ACK)
Acknowledge (ACK) provides a positive handshake
between the host and the MC74. The host releases SDA after
transmitting eight bits, then generates a ninth clock cycle to
allow the MC74 to pull the SDA line LOW to acknowledge
that it successfully received the previous eight bits of data or
address.
Data Byte
After a successful ACK of the address byte, the host must
transmit the data byte to be written or clock out the data to
be read. (See the appropriate timing diagrams. ) ACK will
be generated after a successful write of a data byte into the
MC74.
Stop Condition (STOP)
Communications must be terminated by a stop condition
(a LOW to HIGH transition of SDA while SCL is HIGH).
The Stop Condition must be communicated by the
transmitter to the MC74. NOTE: Refer to Timing Diagrams
for serial bus timing.
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