eZ80L92 MCU
Product Specification
143
Wait
State
Start Counting
High Period
CLK1 Signal
Counter
Reset
CLK2 Signal
SCL Signal
2
Figure 34. Clock Synchronization In I C Protocol
Arbitration
A master may start a transfer only if the bus is free. Two or more masters may generate a
START condition within the minimum hold time of the START condition which results in
a defined START condition to the bus. Arbitration takes place on the SDA line, while the
SCL line is at the High level, in such a way that the master which transmits a High level,
while another master is transmitting a Low level switches off its data output stage because
the level on the bus doesn't correspond to its own level.
Arbitration can continue for many bits. Its first stage is comparison of the address bits. If
the masters are each trying to address the same device, arbitration continues with compar-
ison of the data. Because address and data information about the I2C bus is used for arbi-
tration, no information is lost during this process. A master which loses the arbitration can
generate clock pulses until the end of the byte in which it loses the arbitration.
If a master also incorporates a slave function and it loses arbitration during the addressing
stage, it's possible that the winning master is trying to address it. The losing master must
switch over immediately to its slave-receiver mode. Figure 34 illustrates the arbitration
procedure for two masters. Of course, more may be involved (depending on how many
masters are connected to the bus). The moment there is a difference between the internal
data level of the master generating DATA 1 and the actual level on the SDA line, its data
output is switched off, which means that a High output level is then connected to the bus.
As a result, the data transfer initiated by the winning master is not affected. Because con-
trol of the I2C bus is decided solely on the address and data sent by competing masters,
there is no central master, nor any order of priority on the bus.
Special attention must be paid if, during a serial transfer, the arbitration procedure is still
in progress at the moment when a repeated START condition or a STOP condition is trans-
mitted to the I2C bus. If it is possible for such a situation to occur, the masters involved
must send this repeated START condition or STOP condition at the same position in the
format frame. In other words, arbitration is not allowed between:
PS013015-0316
I2C Serial I/O Interface
ZILOG [ ZILOG, INC. ]
