PCA8574_PCA8574A_2

© NXP B.V. 2007. All rights reserved.

Product data sheet

Rev. 02 — 14 May 2007

8 of 27

NXP Semiconductors

PCA8574/74A

8.3 Reading from a port (Input mode)

All ports programmed as input should be set to logic 1. To read, the master

(microcontroller) first addresses the slave device after it receives the interrupt. By setting

the last bit of the byte containing the slave address to logic 1 the Read mode is entered.

The data bytes that follow on the SDA are the values on the ports.

If the data on the input port changes faster than the master can read, this data may be

lost.

Fig 8.

Write mode (output)

A5 A4 A3 A2 A1 A0

0

A

SA6

slave address

START condition

R/W

acknowledge

from slave

002aac120

P6

1

P7

data 1

A

acknowledge

from slave

12345678

SCL

9

SDA

A

acknowledge

from slave

write to port

data output from port

P5

data 2

DATA 2 VALID

P4 P3 P2 P1 P0

P7

P4 P3 P2 P1 P0

0

P5

P5

DATA 1 VALID

P5 output voltage

P5 pull-up output current

INT

A LOW-to-HIGH transition of SDA while SCL is HIGH is defined as the STOP condition (P). Transfer of data can be stopped

at any moment by a STOP condition. When this occurs, data present at the last acknowledge phase is valid (Output mode).

Input data is lost.

Fig 9.

Read input port register

A5 A4 A3 A2 A1 A0

1

A

SA6

slave address

START condition

R/W

acknowledge

from slave

002aac121

data from port

A

acknowledge

from master

SDA

1

no acknowledge

from master

read from

port

data into

port

data from port

DATA 1

DATA 4

INT

DATA 4

DATA 2

DATA 3

P

STOP

condition