HCS08 datasheet(212/290 Pages) MOTOROLA

212 / 290 page

Inter-Integrated Circuit (IIC) Module

MC9S08GB/GT

212

MOTOROLA

13.2.1.5 Repeated START Signal

As shown in Figure 13-3, 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.

13.2.1.6 Arbitration Procedure

The IIC 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 at the same time, 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

one 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 over to slave receive mode and stop driving 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.

13.2.1.7 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 is still 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 13-4). When all

devices concerned 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 13-4 IIC Clock Synchronization

SCL1

SCL2

SCL

INTERNAL COUNTER RESET

DELAY

START COUNTING HIGH PERIOD

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