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ABOV Semiconductor Co., Ltd.
11.12.16 USIn I2C Start / Repeated Start / Stop
One master can issue a START (S) condition to notice other devices connected to the SCLn, SDAn lines that it will
use the bus. A STOP (P) condition is generated by the master to release the bus lines so that other devices can use it.
A high to low transition on the SDAn line while SCLn is high defines a START (S) condition.
A low to high transition on the SDAn line while SCLn is high defines a STOP (P) condition.
START and STOP conditions are always generated by the master. The bus is considered to be busy after START
condition. The bus is considered to be free again after STOP condition, ie, the bus is busy between START and STOP
condition. If a repeated START condition (Sr) is generated instead of STOP condition, the bus stays busy. So, the
START and repeated START conditions are functionally identical.
Figure 11.68 START and STOP Condition (USIn)
11.12.17 USIn I2C Data Transfer
Every byte put on the SDAn line must be 8-bits long. The number of bytes that can be transmitted per transfer is
unlimited. Each byte has to be followed by an acknowledge bit. Data is transferred with the most significant bit (MSB)
first. If a slave can’t receive or transmit another complete byte of data until it has performed some other function, it can
hold the clock line SCLn LOW to force the master into a wait state. Data transfer then continues when the slave is
ready for another byte of data and releases clock line SCLn.
Figure 11.69 Data Transfer on the I2C-Bus (USIn)
START or Repeated
START Condition
STOP or Repeated
START Condition
Acknowledgement
Signal form Slave
Acknowledgement
Signal form Slave
Byte Complete,
Interrupt within Device
Clock line held low while
interrupts are served.
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