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PCA9540

2-channel I2C multiplexer

Philips Electronics 

Philips Semiconductors

2-channel I2C multiplexer

Product specification

PCA9540

CHANNEL SELECTION

A SC0x/SD0x downstream pair, or channel, is selected by the

contents of the control register. This register is written after the

PCA9540 has been addressed. The 2 LSBs of the control byte are

used to determine which channel is to be selected. When a channel

is selected, the channel will become active after a stop condition has

been placed on the I2C bus. This ensures that all SCx/SDx lines will

be in a HIGH state when the channel is made active, so that no

false conditions are generated at the time of connection.

CONTROL BYTE

7 6 5 4 3 210

X X X X X 0XX

X X X X X 100

X X X X X 101

SELECTED

CHANNEL

none

0 (SC0/SD0)

1 (SC1/SD1)

CONTROL REGISTER

7 6 5 43 2 1 0

X X X X X B2 B1 B0

Channel select bits

(read/write)

SW00497

POWER-ON RESET

During power-up the control register defaults to all zeroes causing

all the channels to be deselected.

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.

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 FIgure 1).

SDA

SCL

data line

stable;

data valid

change

of data

allowed

SW00363

Figure 1. Bit transfer

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 Figure 2).

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 Figure 3).

SDA

SDA

SCL

S

SCL

P

START condition

STOP condition

Figure 2. Definition of start and stop conditions

SW00365

SDA

SCL

MASTER

TRANSMITTER/

RECEIVER

1999 Dec 15

SLAVE

RECEIVER

SLAVE

TRANSMITTER/

RECEIVER

MASTER

TRANSMITTER

Figure 3. System configuration

4

MASTER

TRANSMITTER/

RECEIVER

SLAVE

I2C

MULTIPLEXER

SW00366

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