?2002 Microchip Technology Inc.
DS21462C-page 7
TC74
FIGURE 3-1:
SMBus/I
2
C Protocols.
3.3
START Condition (S)
The TC74 continuously monitors the SDA and SCLK
lines for a START condition (a high-to-low transition of
SDA while SCLK is high) and will not respond until this
condition is met.
3.4
Address Byte
Immediately following the START condition, the host
must transmit the address byte to the TC74. The states
of A2, A1 and A0 determine the SMBus/I
2
C address for
the TC74. The 7-bit address transmitted in the serial bit
stream must match for the TC74 to respond with an
Acknowledge (indicating the TC74 is on the bus and
ready to accept data). The 8-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, indicating that the
command byte is being written.
3.5
Acknowledge (ACK)
Acknowledge (ACK) provides a positive handshake
between the host and the TC74. The host releases
SDA after transmitting 8 bits. The host then generates
a ninth clock cycle to allow the TC74 to pull the SDA
line low. This action acknowledges that the TC74
successfully received the previous 8 bits of data or
address.
3.6
Data Byte
After a successful ACK of the address byte, the host
must transmit the data byte to be written, or clock-in the
data to be read (see the appropriate timing diagrams).
ACK will be generated upon a successful write of a
data byte into the TC74.
3.7
STOP Condition (P)
Communications must be terminated by a STOP
condition (a low-to-high transition of SDA while SCLK
is high). The STOP condition must be communicated
by the transmitter to the TC74. Refer to Figure 1-1,
Timing Diagrams , for serial bus timing.
S
Address
WR
ACK
Command
ACK
Data
ACK
P
8 Bits
7 Bits
8 Bits
Slave Address
Command Byte: selects
which register you are
writing to.
Data Byte: data goes
into the register set
by the command byte.
Write Byte Format
Read Byte Format
S
Address
WR
ACK
Command
ACK
S
Address
RD ACK Data NACK
P
7 Bits
8 Bits
7 Bits
8 Bits
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.
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 or Write
Byte transmission.
S = START Condition
P = STOP Condition
Shaded = Slave Transmission
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