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8-bit quasi-bidirectional I²C I/O expander

PCF8574 Working Principle

Learn how PCF8574 works with an illustrated explanation and practical readings.

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PCF8574 at a glance

Follow the labeled connections and functional stages

PCF8574 working principle: illustrated functional stages, basic circuit and connections
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How it works

3 stages · Explore one step at a time

Step 1 / 3

Choose an address

Tie A2–A0 to fixed levels. All LOW selects the 7-bit address 0x20.

Step 2 / 3

Write the output latch

Send a single port byte. A zero sinks a load, while one releases the line for input or a weak HIGH.

Step 3 / 3

Read and service changes

Read the physical port state and service INT. Debounce switches in firmware and preserve input bits in the output shadow.

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