NXP Semiconductors PXN2020, Reference Manual, Page: 1058 / 1376

Inter-Integrated Circuit Bus Controller Module (I
2
C)
PXN20 Microcontroller Reference Manual, Rev. 1
32-14 Freescale Semiconductor
32.4.1.5 Repeated START Signal
As shown in Figure 32-10 , a repeated START signal is a START signal generated without first generating
a STOP signal to terminate the communication. This is used by the master to communicate with another
slave or with the same slave in different mode (transmit/receive mode) without releasing the bus.
32.4.1.6 Arbitration Procedure
The Inter-IC bus is a true multi-master bus that allows more than one master to be connected on it. If two
or more masters try to control the bus simultaneously, a clock synchronization procedure determines the
bus clock, for which the low period is equal to the longest clock low period and the high is equal to the
shortest among the masters. The relative priority of the contending masters is determined by a data
arbitration procedure. A bus master loses arbitration if it transmits logic “1” while another master transmits
logic 0. The losing masters immediately switch to slave receive mode and stop driving the SDA output. In
this case, the transition from master to slave mode does not generate a STOP condition. Meanwhile, a
status bit is set by hardware to indicate loss of arbitration.
32.4.1.7 Clock Synchronization
Because wire-AND logic is performed on the SCL line, a high-to-low transition on the SCL line affects all
the devices connected on the bus. The devices start counting their low period and after a device's clock has
gone low, it holds the SCL line low until the clock high state is reached. However, the change of low to
high in this device clock may not change the state of the SCL line if another device clock remains within
its low period. Therefore, synchronized clock SCL is held low by the device with the longest low period.
Devices with shorter low periods enter a high wait state during this time (see
Figure 32-12 ). When all
engaged devices have counted off their low period, the synchronized clock SCL line is released and pulled
high. There is then no difference between the device clocks and the state of the SCL line and all the devices
start counting their high periods. The first device to complete its high period pulls the SCL line low again.
Figure 32-12. I
2
C Bus Clock Synchronization
SCL1
SCL2
SCL
Internal Counter Reset
WAIT
Start Counting High Period