PCA9540B_4

© NXP B.V. 2009. All rights reserved.

Product data sheet

Rev. 04 — 3 September 2009

7 of 22

NXP Semiconductors

PCA9540B

7.3 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 Figure 10).

7.4 Acknowledge

The number of data bytes transferred between the START and the STOP conditions from

transmitter to receiver is not limited. Each byte of eight bits is followed by one

acknowledge bit. The acknowledge bit is a HIGH level put on the bus by the transmitter,

whereas the master generates an extra acknowledge related clock pulse.

A slave receiver which is addressed must generate an acknowledge after the reception of

each byte. Also, a master must generate an acknowledge after the reception of each byte

that has been clocked out of the slave transmitter. The device that acknowledges has to

pull down the SDA line during the acknowledge clock pulse so that the SDA line is stable

LOW during the HIGH period of the acknowledge related clock pulse; set-up and hold

times must be taken into account.

A master receiver must signal an end of data to the transmitter by not generating an

acknowledge on the last byte that has been clocked out of the slave. In this event, the

transmitter must leave the data line HIGH to enable the master to generate a STOP

condition.

Fig 10. System configuration

002aaa966

MASTER

TRANSMITTER/

RECEIVER

SLAVE

RECEIVER

SLAVE

TRANSMITTER/

RECEIVER

MASTER

TRANSMITTER

MASTER

TRANSMITTER/

RECEIVER

SDA

SCL

MULTIPLEXER

SLAVE

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S

START

condition

9

8

2

1

clock pulse for

acknowledgement

not acknowledge

acknowledge

data output

by transmitter

data output

by receiver

SCL from master