The following steps explain a complete
transaction in F/S mode.
- The controller initiates data transfer by generating a start
condition. The start condition is when a high-to-low transition occurs on the
SDA line while SCL is high, as shown in Figure 7-19. 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. A valid data condition requires the SDA line to be stable during the
entire high period of the clock pulse, as shown in Figure 7-20. 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, as shown in
Figure 7-18. 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 eight data bits and one acknowledge-bit, and can continue
as long as necessary.
- 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, as shown in Figure 7-19. 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 7-19 Start and Stop
Conditions Figure 7-20 Bit Transfer on the
I2C Bus