LM3S8630_1 Datasheet, PDF (351/550 Pages) Texas Instruments – Stellaris® LM3S8630 Microcontroller

Stellaris® LM3S8630 Microcontroller
14.2.1.1
START and STOP Conditions
The protocol of the I²C bus defines two states to begin and end a transaction: START and STOP.
A High-to-Low transition on the SDA line while the SCL is High is defined as a START condition,
and a Low-to-High transition on the SDA line while SCL is High is defined as a STOP condition.
The bus is considered busy after a START condition and free after a STOP condition. See Figure
14-3 on page 351.
Figure 14-3. START and STOP Conditions
SDA
SCL
START
condition
STOP
condition
SDA
SCL
14.2.1.2
Data Format with 7-Bit Address
Data transfers follow the format shown in Figure 14-4 on page 351. After the START condition, a
slave address is sent. This address is 7-bits long followed by an eighth bit, which is a data direction
bit (R/S bit in the I2CMSA register). A zero indicates a transmit operation (send), and a one indicates
a request for data (receive). A data transfer is always terminated by a STOP condition generated
by the master, however, a master can initiate communications with another device on the bus by
generating a repeated START condition and addressing another slave without first generating a
STOP condition. Various combinations of receive/send formats are then possible within a single
transfer.
Figure 14-4. Complete Data Transfer with a 7-Bit Address
SDA
MSB
LSB
R/S
ACK
MSB
LSB
ACK
SCL
1
2
7
8
9
Slave address
1
2
7
8
9
Data
The first seven bits of the first byte make up the slave address (see Figure 14-5 on page 351). The
eighth bit determines the direction of the message. A zero in the R/S position of the first byte means
that the master will write (send) data to the selected slave, and a one in this position means that
the master will receive data from the slave.
Figure 14-5. R/S Bit in First Byte
MSB
LSB
R/S
Slave address
14.2.1.3
Data Validity
The data on the SDA line must be stable during the high period of the clock, and the data line can
only change when SCL is Low (see Figure 14-6 on page 352).
June 22, 2010
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Texas Instruments-Production Data