The following steps explain a complete
transaction in F/S mode.
- The controller initiates data transfer by generating a start
condition. Figure 7-4 shows that he start condition is when a high-to-low
transition occurs on the SDA line while SCL is high. All
I2C-compatible devices recognize a start condition.
- The controller then generates the SCL pulses, and transmits the
7-bit address and the read/write direction bit (R/W) on the
SDA line. During all transmissions, the controller makes sure that data are
valid. Figure 7-5 shows that a valid data condition requires the SDA line to
be stable during the entire high period of the clock pulse. All devices
recognize the address sent by the controller and compare the address to the
respective internal fixed address. Only the target device with a matching
address generates an acknowledge by pulling the SDA line low during the entire
high period of the 9th SCL cycle (see Figure 7-3). When the controller detects this acknowledge, the
communication link with a target has been established.
- The controller generates further SCL cycles to transmit
(R/W bit 0) or receive (R/W bit 1)
data to the target. In either case, the receiver must acknowledge the data sent
by the transmitter. The acknowledge signal can be generated by the controller or
by the target, depending on which is the receiver. The 9-bit valid data
sequences consists of 8-data bits and 1 acknowledge-bit, and can continue as
long as necessary.
- Figure 7-4 shows that to signal the end of the data transfer, the
controller generates a stop condition by pulling the SDA line from low-to-high
while the SCL line is high. This action releases the bus and stops the
communication link with the addressed target. All I2C-compatible
devices recognize the stop condition. Upon receipt of a stop condition, the bus
is released, and all target devices then wait for a start condition followed by
a matching address.
Figure 7-4 Start and Stop
Conditions Figure 7-5 Bit Transfer on the
I2C Bus