UM10204 All information provided in this document is subject to legal disclaimers. © NXP B.V. 2012. All rights reserved.
User manual Rev. 5 — 9 October 2012 11 of 64
NXP Semiconductors
UM10204
I
2
C-bus specification and user manual
3.1.7 Clock synchronization
Two masters can begin transmitting on a free bus at the same time and there must be a
method for deciding which takes control of the bus and complete its transmission. This is
done by clock synchronization and arbitration. In single master systems, clock
synchronization and arbitration are not needed.
Clock synchronization is performed using the wired-AND connection of I
2
C interfaces to
the SCL line. This means that a HIGH to LOW transition on the SCL line causes the
masters concerned to start counting off their LOW period and, once a master clock has
gone LOW, it holds the SCL line in that state until the clock HIGH state is reached (see
Figure 7
). However, if another clock is still within its LOW period, the LOW to HIGH
transition of this clock may not change the state of the SCL line. The SCL line is therefore
held LOW by the master with the longest LOW period. Masters with shorter LOW periods
enter a HIGH wait-state during this time.
When all masters concerned have counted off their LOW period, the clock line is released
and goes HIGH. There is then no difference between the master clocks and the state of
the SCL line, and all the masters start counting their HIGH periods. The first master to
complete its HIGH period pulls the SCL line LOW again.
In this way, a synchronized SCL clock is generated with its LOW period determined by the
master with the longest clock LOW period, and its HIGH period determined by the one
with the shortest clock HIGH period.
3.1.8 Arbitration
Arbitration, like synchronization, refers to a portion of the protocol required only if more
than one master is used in the system. Slaves are not involved in the arbitration
procedure. A master may start a transfer only if the bus is free. Two masters may
generate a START condition within the minimum hold time (t
HD;STA
) of the START
condition which results in a valid START condition on the bus. Arbitration is then required
to determine which master will complete its transmission.
Arbitration proceeds bit by bit. During every bit, while SCL is HIGH, each master checks to
see if the SDA level matches what it has sent. This process may take many bits. Two
masters can actually complete an entire transaction without error, as long as the
Fig 7. Clock synchronization during the arbitration procedure
CLK
1
CLK
2
SCL
counter
reset
wait
state
start counting
HIGH period
mbc632
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