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8-bit quasi-bidirectional I²C I/O expander

PCF8574 Internal Circuitry

Explore the internal circuitry and functional stages of PCF8574.

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Inside PCF8574

I²C control, an 8-bit latch and quasi-bidirectional ports.

PCF8574 cutaway containing address decode I2C shift register output latch weak source sink driver input sampling and interrupt logic.
Functional architecture; all port latches start HIGH after power-on.

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Address and transfer

SDA and SCL move a port byte after the matched address.

Latch and release

LOW activates the sink. HIGH releases a port with weak sourcing for input use.

Sample and notify

Reads sample terminal levels; input changes can assert the open-drain interrupt.

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