MC74

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5

Figure 1. SMBus Protocols

DATA

S

ADDRESS

WR

ACK

7 Bits

COMMAND

DATA

P

8 Bits

ACK

ACK

S

ADDRESS

WR

ACK

7 Bits

COMMAND

RD

NACK

8 Bits

S

ADDRESS

7 Bits

ACK

ACK

8 Bits

P

DATA

RD

NACK

S

ADDRESS

7 Bits

ACK

8 Bits

P

Slave Address

Command Byte: selects

Data Byte: data goes

which register you are

writing to.

into the register set

by the command byte.

Slave Address

Command Byte: selects

which register you are

reading from.

Slave Address: repeated

due to change in data–

flow direction.

Data Byte: reads from

the register set by the

command byte.

Data Byte: reads data from

the register commanded by

the last Read Byte.

S = Start Condition

P = Stop Condition

Shaded = Slave Transmission

Write Byte Format

Read Byte Format

Receive Byte Format

8 Bits

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.