PCF8574_PCF8574A

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Product data sheet

Rev. 5 — 27 May 2013

13 of 33

NXP Semiconductors

PCF8574; PCF8574A

10. Application design-in information

10.1 Bidirectional I/O expander applications

In the 8-bit I/O expander application shown in Figure 15, P0 and P1 are inputs, and

P2 to P7 are outputs. When used in this configuration, during a write, the input (P0 and

P1) must be written as HIGH so the external devices fully control the input ports.

The desired HIGH or LOW logic levels may be written to the ports used as outputs (P2 to

P7). If 10

A internal output HIGH is not enough current source, the port needs external

pull-up resistor. During a read, the logic levels of the external devices driving the input

ports (P0 and P1) and the previous written logic level to the output ports (P2 to P7) will be

read.

The GPIO also has an interrupt line (INT) that can be connected to the interrupt logic of

the microcontroller. By sending an interrupt signal on this line, the remote I/O informs the

microprocessor that there has been a change of data on its ports without having to

10.2 How to read and write to I/O expander (example)

In the application example of PCF8574 shown in Figure 15, the microcontroller wants to

control the P3 switch ON and the P7 LED ON when the temperature sensor P0 changes.

1. When the system power on:

Core Processor needs to issue an initial command to set P0 and P1 as inputs and

P[7:2] as outputs with value 1010 00 (LED off, MP3 off, camera on, audio off,

switch off and latch off).

2. Operation:

When the temperature changes above the threshold, the temperature sensor signal

will toggle from HIGH to LOW. The INT will be activated and notifies the ‘core

processor’ that there have been changes on the input pins. Read the input register.

If P0 = 0 (temperature sensor has changed), then turn on LED and turn on switch.

3. Software code:

//System Power on

// write to PCF8574 with data 1010 0011b to set P[7:2] outputs and P[1:0] inputs

<S> <0100 0000> <ACK> <1010 0011> <ACK> <P>//Initial setting for PCF9574

Fig 15. Bidirectional I/O expander application

002aah384

temperature sensor

battery status

control for latch

control for switch

control for audio

control for camera

control for MP3

P0

P1

P2

P3

P4

P5

P6

P7

SDA

SCL

INT

A0

A1

A2

CORE

PROCESSOR