C8051F310 datasheet(147/228 Pages) SILABS

147 / 228 page

Rev. 1.7

147

C8051F310/1/2/3/4/5/6/7

The direction bit (R/W) occupies the least-significant bit position of the address byte. The direction bit is set

to logic 1 to indicate a "READ" operation and cleared to logic 0 to indicate a "WRITE" operation.

All transactions are initiated by a master, with one or more addressed slave devices as the target.

The

master generates the START condition and then transmits the slave address and direction bit. If the trans-

action is a WRITE operation from the master to the slave, the master transmits the data a byte at a time

waiting for an ACK from the slave at the end of each byte. For READ operations, the slave transmits the

data waiting for an ACK from the master at the end of each byte. At the end of the data transfer, the master

generates a STOP condition to terminate the transaction and free the bus. Figure 14.3 illustrates a typical

SMBus transaction.

Figure 14.3. SMBus Transaction

14.3.1. Arbitration

A master may start a transfer only if the bus is free. The bus is free after a STOP condition or after the SCL

and SDA lines remain high for a specified time (see Section “14.3.4. SCL High (SMBus Free) Timeout”

on page 148). In the event that two or more devices attempt to begin a transfer at the same time, an arbi-

tration scheme is employed to force one master to give up the bus. The master devices continue transmit-

ting until one attempts a HIGH while the other transmits a LOW. Since the bus is open-drain, the bus will

be pulled LOW. The master attempting the HIGH will detect a LOW SDA and lose the arbitration. The win-

ning master continues its transmission without interruption; the losing master becomes a slave and

receives the rest of the transfer if addressed. This arbitration scheme is non-destructive: one device

always wins, and no data is lost.

SLA6

SDA

SLA5-0

R/W

D6-0

SCL

Slave Address + R/W

Data Byte

START

ACK

NACK

STOP

C8051F310 Datasheet(PDF) 147 Page - Silicon Laboratories