NXP Semiconductors
PCF8574; PCF8574A
Remote 8-bit I/O expander for I
2
C-bus with interrupt
PCF8574A address map
Address of PCF8574A
A6
A5
A4
A3
A2
A1
A0 R/W
Address byte value
Write
Read
A0
7-bit
hexadecimal
address
without R/W
38h
39h
3Ah
3Bh
3Ch
3Dh
3Eh
3Fh
Table 5.
A2
A1
Pin connectivity
V
SS
V
SS
V
SS
V
SS
V
DD
V
DD
V
DD
V
DD
V
SS
V
SS
V
DD
V
DD
V
SS
V
SS
V
DD
V
DD
V
SS
V
DD
V
SS
V
DD
V
SS
V
DD
V
SS
V
DD
0
0
0
0
0
0
0
0
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
0
0
0
0
1
1
1
1
0
0
1
1
0
0
1
1
0
1
0
1
0
1
0
1
-
-
-
-
-
-
-
-
70h
72h
74h
76h
78h
7Ah
7Ch
7Eh
71h
73h
75h
77h
79h
7Bh
7Dh
7Fh
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.
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
PHILIPS [ NXP SEMICONDUCTORS ]
