PCF8574_PCF8574A

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Product data sheet

Rev. 5 — 27 May 2013

12 of 33

NXP Semiconductors

PCF8574; PCF8574A

9.3 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 (see Figure 14). The acknowledge bit is an active LOW level (generated

by the receiving device) that indicates to the transmitter that the data transfer was

successful.

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 wants to issue an

acknowledge bit 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 bit 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 13. System configuration

002aaa966

MASTER

TRANSMITTER/

RECEIVER

SLAVE

RECEIVER

SLAVE

TRANSMITTER/

RECEIVER

MASTER

TRANSMITTER

MASTER

TRANSMITTER/

RECEIVER

SDA

SCL

MULTIPLEXER

SLAVE

002aaa987

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