PCF8574_PCF8574A

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Product data sheet

Rev. 5 — 27 May 2013

8 of 33

NXP Semiconductors

PCF8574; PCF8574A

8.2 Writing to the port (Output mode)

The master (microcontroller) sends the START condition and slave address setting the

last bit of the address byte to logic 0 for the write mode. The PCF8574/74A acknowledges

and the master then sends the data byte for P7 to P0 to the port register. As the clock line

goes HIGH, the 8-bit data is presented on the port lines after it has been acknowledged by

the PCF8574/74A. If a LOW is written, the strong pull-down turns on and stays on. If a

HIGH by the weak current source. The master can then send a STOP or ReSTART

condition or continue sending data. The number of data bytes that can be sent

successively is not limited and the previous data is overwritten every time a data byte has

been sent and acknowledged.

Ensure a logic 1 is written for any port that is being used as an input to ensure the strong

external pull-down is turned off.

Simple code WRITE mode:

<S> <slave address + write> <ACK> <data out> <ACK> <data out> <ACK> ...

<data out> <ACK> <P>

Remark: Bold type = generated by slave device.

Fig 8.

Write mode (output)

A5 A4 A3 A2 A1 A0

0

A

SA6

slave address

START condition

R/W

acknowledge

from slave

002aah349

P6

1

P7

data 1

A

acknowledge

from slave

12345678

SCL

9

SDA

A

acknowledge

from slave

write to port

data output from port

P5

data 2

DATA 2 VALID

P4 P3 P2 P1 P0

P7

P4 P3 P2 P1 P0

P6

P5

0

DATA 1 VALID

P5 output voltage

P5 pull-up output current

INT