PCF8574P datasheet(11/24 Pages) PHILIPS

11 / 24 page

1997 Apr 02

Philips Semiconductors

Product speciﬁcation

Remote 8-bit I/O expander for I2C-bus

PCF8574

7.2

Interrupt (see Figs 12 and 13)

The PCF8574 provides an open drain output (INT) which

can be fed to a corresponding input of the microcontroller.

This gives these chips a type of master function which can

initiate an action elsewhere in the system.

An interrupt is generated by any rising or falling edge of the

port inputs in the input mode. After time tiv the signal INT is

valid.

Resetting and reactivating the interrupt circuit is achieved

when data on the port is changed to the original setting or

data is read from or written to the port which has generated

the interrupt.

Resetting occurs as follows:

• In the READ mode at the acknowledge bit after the rising

edge of the SCL signal

• In the WRITE mode at the acknowledge bit after the

HIGH-to-LOW transition of the SCL signal

• Interrupts which occur during the acknowledge clock

pulse may be lost (or very short) due to the resetting of

the interrupt during this pulse.

Each change of the I/Os after resetting will be detected

and, after the next rising clock edge, will be transmitted as

INT. Reading from or writing to another device does not

affect the interrupt circuit.

7.3

Quasi-bidirectional I/Os (see Fig.14)

A quasi-bidirectional I/O can be used as an input or output

without the use of a control signal for data direction.

At power-on the I/Os are HIGH. In this mode only a current

source to VDD is active. An additional strong pull-up to VDD

allows fast rising edges into heavily loaded outputs. These

devices turn on when an output is written HIGH, and are

switched off by the negative edge of SCL. The I/Os should

be HIGH before being used as inputs.

handbook, full pagewidth

MBD976

MICROCOMPUTER

INT

PCF8574

(1)

PCF8574

(2)

V DD

INT

PCF8574

(16)

Fig.12 Application of multiple PCF8574s with interrupt.

Fig.13 Interrupt generated by a change of input to I/O P5.

handbook, full pagewidth

MBD972

start condition

R/W

acknowledge

from slave

SDA

SCL

DATA INTO

t ir

INT

slave address (PCF8574)

data from port

234

567

stop

condition

t iv

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