Motorola ColdFire MCF5281 - Manual (Page 465)

I 2 C Interface
Freescale Semiconductor 24-11
24.3.7 Clock Synchronization and Arbitration
I
2
C is a true multi-master bus that allows more than one master connected to it. If two or more master
devices simultaneously request control of the bus, a clock synchronization procedure determines the bus
clock. Because wire-AND logic is performed on the I2C_SCL line, a high-to-low transition on the
I2C_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 I2C_SCL line low until the clock high state is reached.
However, change of low to high in this device’s clock may not change the state of the I2C_SCL line if
another device clock remains within its low period. Therefore, synchronized clock I2C_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 24-12 ). When all
devices concerned have counted off their low period, the synchronized clock (I2C_SCL) line is released
and pulled high. At this point, the device clocks and the I2C_SCL line are synchronized, and the devices
start counting their high periods. The first device to complete its high period pulls the I2C_SCL line low
again.
Figure 24-12. Clock Synchronization
A data arbitration procedure determines the relative priority of the contending masters. A bus master loses
arbitration if it transmits logic 1 while another master transmits logic 0. The losing masters immediately
switch over to slave receive mode and stop driving I2C_SDA output (see
Figure 24-13 ). In this case,
transition from master to slave mode does not generate a STOP condition. Meanwhile, hardware sets
I2SR[IAL] to indicate loss of arbitration.
Figure 24-13. Arbitration Procedure
Internal Counter Reset
Wait
Start counting high period
I2C_SCL1
I2C_SCL2
I2C_SCL
Master 2 Loses Arbitration,
and becomes slave-receiver
I2C_SCL
I2C_SDA by
Master1
I2C_SDA by
Master2
I2C_SDA
MCF5282 and MCF5216 ColdFire Microcontroller User’s Manual, Rev. 3