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8574TS

Remote 8-bit I/O expander for I2C-BUS

Philips Electronics 

Philips Semiconductors

Remote 8-bit I/O expander for I2C-bus

Product specification

PCF8574

6 CHARACTERISTICS OF THE I2C-BUS

The I2C-bus is for 2-way, 2-line communication between

different ICs or modules. The two lines are a serial data

line (SDA) and a serial clock line (SCL). Both lines must be

connected to a positive supply via a pull-up resistor when

connected to the output stages of a device. Data transfer

may be initiated only when the bus is not busy.

6.1 Bit transfer

One data bit is transferred during each clock pulse. The

data on the SDA line must remain stable during the HIGH

period of the clock pulse as changes in the data line at this

time will be interpreted as control signals (see Fig.5).

6.2 Start and stop conditions

Both data and clock lines remain HIGH when the bus is not

busy. A HIGH-to-LOW transition of the data line, while the

clock is HIGH is defined as the start condition (S).

A LOW-to-HIGH transition of the data line while the clock

is HIGH is defined as the stop condition (P) (see Fig.6).

6.3 System configuration

A device generating a message is a ‘transmitter’, a device

receiving is the ‘receiver’. The device that controls the

message is the ‘master’ and the devices which are

controlled by the master are the ‘slaves’ (see Fig.7).

handbook, full pagewidth

SDA

SCL

data line

stable;

data valid

change

of data

allowed

Fig.5 Bit transfer.

MBC621

handbook, full pagewidth

SDA

SDA

SCL

S

SCL

P

START condition

STOP condition

MBC622

Fig.6 Definition of start and stop conditions.

SDA

SCL

MASTER

TRANSMITTER /

RECEIVER

2002 Nov 22

SLAVE

RECEIVER

SLAVE

TRANSMITTER /

RECEIVER

MASTER

TRANSMITTER

Fig.7 System configuration.

MASTER

TRANSMITTER /

RECEIVER

MBA605

7

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