Functional Description
MCF548x Reference Manual, Rev. 3
Freescale Semiconductor 28-11

28.4.6 Repeated Start

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 a different mode without releasing the bus.
Various combinations of read/write formats are then possible:
• The first example in Figure 28-11 is the case of a master-transmitter transmitting to a
slave-receiver. The transfer direction is not changed.
• The second example in Figure 28-11 is the master reading slave data immediately after the first
byte. At the moment of the first acknowledge, the master-transmitter becomes a master-receiver
and the slave-receiver becomes a slave-transmitter.
• In the third example in Figure 28-11, the START condition and slave address are both repeated
using the repeated START signal. This is to communicate with the same slave in a different mode
without releasing the bus. The master transmits data to the slave first, and then the master reads
data from the slave by reversing the R/W bit.
Figure 28-11. Data Transfer, Combined Format

28.4.7 Clock Synchronization and Arbitration

I2C is a true multi-master bus that allows more than one master to be connected to it. If two or more masters
try to control the bus at the same time, a clock synchronization procedure determines the bus clock.
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. Once 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.
ST A
7-bit Slave Address 0A
DATA DATA A/A
R/W
SP
From Master to Slave
From Slave to Master
ST = Start
SP = Stop
A = Acknowledge (SDA low)
A = Not Acknowledge (SDA high)
ST A
7-bit Slave Address 1A
DATA DATA A
R/W
SP
ST A
1A
DATA
R/W
Rept A
7-bit Slave 0A
DATA DATA A/A
R/
W
SP
Rept ST = Repeated Start
ST Address
7-bit Slave
Address
Example 1:
Example 2:
Example 3:
Master Reads from Slave Master Writes to Slave
Note: No acknowledge on the last byte.