I2C System Configuration

24.3 I2C System Configuration

The I2C module uses a serial data line (SDA) and a serial clock line (SCL) for data transfer. For I2C compliance, all devices connected to these two signals must have open drain or open collector outputs. The logic AND function is exercised on both lines with external pull-up resistors.

Out of reset, the I2C default state is as a slave receiver. Thus, when not programmed to be a master or responding to a slave transmit address, the I2C module should return to the default slave receiver state. See Section 24.6.1, “Initialization Sequence,” for exceptions.

NOTE

The I2C module is designed to be compatible with the Philips I2C bus protocol. For information on system configuration, protocol, and restrictions, see The I2C Bus Specification, Version 2.1.

24.4 I2C Protocol

Normally, a standard communication is composed of the following parts:

1.START signal—When no other device is bus master (both SCL and SDA lines are at logic high), a device can initiate communication by sending a START signal (see A in Figure 24-2). A START signal is defined as a high-to-low transition of SDA while SCL is high. This signal denotes the beginning of a data transfer (each data transfer can be several bytes long) and awakens all slaves.

Interrupt bit set

SCL held low while

(Byte complete)

Interrupt is serviced

 

 

msb

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

lsb

 

 

 

SCL

1

 

2

3

4

5

6

7

8

9

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

msb

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

lsb

 

 

 

1

 

2

3

4

5

6

7

8

9

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

SDA

 

 

AD7 AD6 AD5 AD4 AD3 AD2 AD1 R/W

 

 

 

XXX D7 D6 D5 D4 D3 D2 D1

D0

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

A

 

 

START

 

Calling Address

 

 

 

 

 

 

 

 

ACK

 

 

 

 

 

 

 

 

 

Data Byte

 

 

 

 

 

 

 

 

 

 

 

 

 

R/W

 

 

 

 

 

 

 

 

 

 

 

 

No

STOP

 

Signal

 

 

 

 

B

 

 

 

 

 

C

Bit

D

 

 

 

E

 

ACK

Signal

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Bit

 

 

 

F

Figure 24-2. I2C Standard Communication Protocol

2.Slave address transmission—The master sends the slave address in the first byte after the START signal (B). After the seven-bit calling address, it sends the R/W bit (C), which tells the slave data transfer direction.

Each slave must have a unique address. An I2C master must not transmit its own slave address; it cannot be master and slave at the same time.

MOTOROLA

Chapter 24. I2C Interface

24-3

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Motorola MCF5281, MCF5282 user manual 24.3 I2C System Configuration, 24.4 I2C Protocol