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Inter-Integrated Circuit Bus Controller Module (I
2
C)
PXN20 Microcontroller Reference Manual, Rev. 1
32-12
Freescale Semiconductor
Figure 32-10. I
2
C Bus Transmission Signals
32.4.1.1
START Signal
When the bus is free, i.e.,no master device is engaging the bus (both SCL and SDA lines are at logical
high), a master may initiate communication by sending a START signal. As shown in
START signal is a high-to-low transition of SDA while SCL is high. This signal denotes the beginning of
a new data transfer (each data transfer may contain several bytes of data) and brings all slaves out of their
idle states.
Figure 32-11. Start and Stop conditions
SCL
SDA
Start
Signal
Ack
Bit
1
2
3
4
5
6
7
8
MSB
LSB
1
2
3
4
5
6
7
8
MSB
LSB
Stop
Signal
No
SCL
SDA
1
2
3
4
5
6
7
8
MSB
LSB
1
2
5
6
7
8
MSB
LSB
Repeated
3
4
9 9
AD0 AD1 AD2 AD3 AD4 AD5 AD6 R/W
XXX
D0
D1 D2
D3
D4
D5
D6
D7
Calling Address
Read/
Data Byte
AD0 AD1 AD2 AD3 AD4 AD5 AD6 R/W
AD0 AD1 AD2 AD3 AD4 AD5 AD6 R/W
New Calling Address
9
9
XX
Ack
Bit
Write
Start
Signal
Start
Signal
Ack
Bit
Calling Address
Read/
Write
Stop
Signal
No
Ack
Bit
Read/
Write
SDA
SCL
START condition
STOP condition
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