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Intel® PXA26x Processor Family Developer’s Manual 9-5
Inter-Integrated Circuit Bus Interface Unit
Figure 9-2 shows the relationship between the SDA and SCL lines for START and STOP
conditions.
9.3.3.1 START Condition
The START condition (ICR[START]=1, ICR[STOP]=0) initiates a master transaction or repeated
START. Before it sets the START ICR bit, software must load the
target slave address and the R/
nW bit in the IDBR (see Section 9.9.2, “I2C Data Buffer Register- IDBR”). The START and the
IDBR contents are transmitted on the I
2
C bus after the ICR[TB] bit is set. The I
2
C bus stays in
master-transmit mode for write requests and enters master-receive mode for read requests. For a
repeated start, a change in read or write, or a change in the target slave address, the IDBR contains
the updated target slave address and the R/nW bit. A repeated start enables a master to make
multiple transfers to different slaves without surrendering the bus.
The START condition is not cleared by the I
2
C unit. If the I
2
C loses arbitration while initiating a
START, it may re-attempt the START when the bus is freed. See Section 9.4.4, “Arbitration” for
details on how the I
2
C unit functions in those circumstances.
Table 9-4. START and STOP Bit Definitions
STOP
bit
STAR
T bit
Condition Notes
0 0 No START or STOP
I
2
C unit sends a no START or STOP condition. Used when
multiple data bytes need to be transferred.
01
START Condition and
Repeated START
I
2
C unit sends a START condition and transmit the 8-bit IDBR’s
contents. The IDBR must contain the 7-bit address and the R/nW
bit before a START is initiated.
For a repeated start, the IDBR contains the target slave address
and the R/nW bit. This allows a master to make multiple transfers
to different slaves without giving up the bus.
The interface stays in master-transmit mode for writes and
transitions to master-receive mode for reads.
1 X STOP Condition
In master-transmit mode, the I
2
C unit transmits the 8-bit IDBR and
sends a STOP condition on the I
2
C bus.
In master-receive mode, the ICR[ACKNAK] must be changed to a
negative ACK (see Section 9.4.3, “Inter-Integrated Circuit
Acknowledge”). The I
2
C unit transmits the NAK bit, receives the
data byte in the IDBR, and sends a STOP condition on the I
2
C
bus.
Figure 9-2. Start and Stop Conditions
SDA
SCL
Start Condition
~
~
~
~
~
~
Stop Condition