Datasheet

PCF8574_PCF8574A All information provided in this document is subject to legal disclaimers. © NXP B.V. 2013. All rights reserved.
Product data sheet Rev. 5 — 27 May 2013 6 of 33
NXP Semiconductors
PCF8574; PCF8574A
Remote 8-bit I/O expander for I
2
C-bus with interrupt
8. I/O programming
8.1 Quasi-bidirectional I/Os
A quasi-bidirectional I/O is an input or output port without using a direction control register.
Whenever the master reads the register, the value returned to master depends on the
actual voltage or status of the pin. At power on, all the ports are HIGH with a weak 100 A
internal pull-up to V
DD
, but can be driven LOW by an internal transistor, or an external
signal. The I/O ports are entirely independent of each other, but each I/O octal is
controlled by the same read or write data byte.
Advantages of the quasi-bidirectional I/O over totem pole I/O include:
• Better for driving LEDs since the p-channel (transistor to V
DD
) is small, which saves
die size and therefore cost. LED drive only requires an internal transistor to ground,
while the LED is connected to V
DD
through a current-limiting resistor. Totem pole I/O
have both n-channel and p-channel transistors, which allow solid HIGH and LOW
output levels without a pull-up resistor — good for logic levels.
• Simpler architecture — only a single register and the I/O can be both input and output
at the same time. Totem pole I/O have a direction register that specifies the port pin
direction and it is always in that configuration unless the direction is explicitly
changed.
• Does not require a command byte. The simplicity of one register (no need for the
pointer register or, technically, the command byte) is an advantage in some
embedded systems where every byte counts because of memory or bandwidth
limitations.
Table 5. PCF8574A address map
Pin connectivity Address of PCF8574A Address byte value 7-bit
hexadecimal
address
without R/W
A2 A1 A0 A6 A5 A4 A3 A2 A1 A0 R/W Write Read
V
SS
V
SS
V
SS
0111000 - 70h 71h 38h
V
SS
V
SS
V
DD
0111001 - 72h 73h 39h
V
SS
V
DD
V
SS
0111010 - 74h 75h 3Ah
V
SS
V
DD
V
DD
0111011 - 76h 77h 3Bh
V
DD
V
SS
V
SS
0111100 - 78h 79h 3Ch
V
DD
V
SS
V
DD
0111101 - 7Ah 7Bh 3Dh
V
DD
V
DD
V
SS
0111110 - 7Ch 7Dh 3Eh
V
DD
V
DD
V
DD
0111111 - 7Eh 7Fh 3Fh