Philips Semiconductors
Product specification
Remote 8-bit I/O expander for I
2
C-bus
7.2
Interrupt
(see Figs 12 and 13)
PCF8574
•
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)
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 t
iv
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
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 V
DD
is active. An additional strong pull-up to V
DD
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
V DD
PCF8574
(1)
PCF8574
(2)
PCF8574
(16)
MICROCOMPUTER
INT
INT
INT
INT
MBD976
Fig.12 Application of multiple PCF8574s with interrupt.
handbook, full pagewidth
slave address (PCF8574)
data from port
SDA
S
0
1
0
0
A2
A1
A0
1
A
1
1
P
start condition
R/W
acknowledge P5
from slave
stop
condition
SCL
1
2
3
4
5
6
7
8
DATA INTO
P5
INT
t iv
t ir
MBD972
Fig.13 Interrupt generated by a change of input to I/O P5.
1997 Apr 02
11
PHILIPS [ NXP SEMICONDUCTORS ]
