PCF8574TSLASH3 datasheet(6/33 Pages) NXP

6 / 33 page

PCF8574_PCF8574A

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Product data sheet

Rev. 5 — 27 May 2013

6 of 33

NXP Semiconductors

PCF8574; PCF8574A

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

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 VDD, 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 VDD) is small, which saves

die size and therefore cost. LED drive only requires an internal transistor to ground,

while the LED is connected to VDD 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

A0 R/W

Write

Read

VSS

011

1000

70h

71h

38h

VSS

VDD

011

1001

72h

73h

39h

VSS

VDD

VSS

011

1010

74h

75h

3Ah

VSS

VDD

011

1011

76h

77h

3Bh

VDD

VSS

011

1100

78h

79h

3Ch

VDD

VSS

VDD

011

1101

7Ah

7Bh

3Dh

VDD

VSS

011

1110

7Ch

7Dh

3Eh

VDD

011

1111

7Eh

7Fh

3Fh

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