1999 Apr 07 8
Philips Semiconductors Product specification
Remote 16-bit I/O expander for I
2
C-bus
PCF8575
7 FUNCTIONAL DESCRIPTION
7.1 Quasi-bidirectional I/Os
The PCF8575’s 16 ports (see Fig.7) are entirely independent and can be used either as input or output ports. Input data
is transferred from the ports to the microcontroller in the READ mode (see Fig.10). Output data is transmitted to the ports
in the WRITE mode (see Fig.9).
This 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 (I
OH
) to V
DD
is active. An additional strong pull-up to
V
DD
(I
OHt
) 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. After power-on
as all the I/Os are set HIGH all of them can be used as input. Any change in setting of the I/Os as either inputs or outputs
can be done with the write mode. Warning: If a HIGH is applied to an I/O which has been written earlier to LOW, a large
current (I
OL
) will flow to V
SS
. (see Characteristics note 3).
7.2 Addressing
Figures 8, 9 and 10 show the address and timing diagrams. Before any data is transmitted or received the master must
send the address of the receiver via the SDA line. The first byte transmitted after the START condition carries the address
of the slave device and the read/write bit. The address of the slave device must not be changed between the START and
the STOP conditions. The PCF8575 acts as a slave receiver or a slave transmitter.
Fig.7 Simplified schematic diagram of each I/O.
book, full pagewidth
MGL540
DQ
C
I
C
I
S
FF
D
I
OH
I
OL
I
OHt
Q
S
FF
100
µA
to interrupt
logic
V
SS
V
DD
P00 to P07
P10 to 17
write pulse
data from
shift register
power-on
reset
read pulse
data to
shift register
Fig.8 Byte containing the slave address and the R/W bits.
MGL541
handbook, halfpage
S 0 1 0 0 A2 A1 A0 R/W A
slave address
