Serial Host Interface
Characteristics Of The I2C Bus

MOTOROLA DSP56012 User’s Manual 5-21

•Data valid—The state of the data line represents valid data when, after a Start
event, the data line is stable for the duration of the high period of the clock
signal. The data on the line may be changed during the low period of the clock
signal. There is one clock pulse per bit of data.
Each 8-bit word is followed by one acknowledge bit. This acknowledge bit is a high
level put on the bus by the transmitter when the master device generates an extra
acknowledge-related clock pulse. A slave receiver that is addressed is obliged to
generate an acknowledge after the reception of each byte. Also, a master receiver
must generate an acknowledge after the reception of each byte that has been clocked
out of the slave transmitter. The device that acknowledges has to pull down the SDA
line during the acknowledge clock pulse in such a way that the SDA line is stable low
during the high period of the acknowledge-related clock pulse (see Figure 5-9).
By definition, a device that generates a signal is called a “transmitter,” and the device
that receives the signal is called a “receiver.” The device that controls the signal is
called the “master” and the devices that are controlled by the master are called
“slaves”. A master receiver must signal an end-of-data to the slave transmitter by not
generating an acknowledge on the last byte that has been clocked out of the slave
device. In this case the transmitter must leave the data line high to enable the master
Figure 5-8 I2C Start and Stop Events
Figure 5-9 Acknowledgment on the I2C Bus
S P
Start Event Stop Event
SDA
SCL
AA0423
Start
Event Clock Pulse For
Acknowledgment
S
12 89
SCL From
Master Device
Data Output
by Transmitter
Data Output
by Receiver
AA0424