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DIP-16 / SOP-16 · PCF8574
ActiveI2C GPIO Expander (8-bit)
PCF8574
Texas Instruments · DIP-16 / SOP-16 · I2C GPIO Expander (8-bit)
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Get a competitive quote within 24 hours. Volume discounts available. Shenzhen stock ships in 1-2 business days.
Overview
The PCF8574 is the classic remote 8-bit I/O expander, originally from Philips and today offered by NXP and Texas Instruments as a pin- and bus-compatible second source. It gives any I2C master eight extra quasi-bidirectional pins through just two bus wires plus power, from a supply anywhere between 2.5 V and 6.0 V — unusually wide, letting it live happily on 3.3 V and 5 V rails alike without level shifting.
The quasi-bidirectional architecture has a practical consequence every designer should know: each output actively pulls low with up to 25 mA sink capability (enough to drive LEDs directly) but only sources about 100 µA when high, so inputs are simply written high first and then read — no direction register is needed. A pin change on any input asserts the open-drain INT line, sparing the master from polling, and the RESET pin restores all pins to the default input state without an I2C transaction. Three address pins (A0-A2) allow eight PCF8574 devices at addresses 0x20-0x27; the PCF8574A occupies 0x38-0x3F, so up to 16 expanders can share one bus.
Current consumption is minimal — 2.5 µA typical — and the 16-pin DIP, SOIC and TSSOP footprints suit everything from LCD backpacks (the ubiquitous I2C character-LCD adapter is literally a PCF8574T) to industrial relay racks. Sourcing is straightforward but watch for remarked parts: the PCF8574 is frequently relabeled from lower-grade die or from the interrupt-lacking PCF8575 variants. Genuine parts respond at the correct address range and sink the full 25 mA at 0.4 V. Shenzhen stock is deep; MOQ from 1 reel or 100 pcs loose.
Key Features
- 8 quasi-bidirectional I/O over I2C (Fast-mode 400 kHz), 2.5-6.0 V supply
- Sinks 25 mA per pin for direct LED and relay-driver interfacing
- Open-drain INT output on any input change; hardware RESET pin
- Eight addresses 0x20-0x27; PCF8574A adds 0x38-0x3F for 16 devices per bus
Typical Applications
- I2C backpacks for HD44780 character LCDs and keypads
- Server and telecom board housekeeping (LEDs, fans, resets)
- Industrial relay and opto-isolated output racks
- Button/switch matrix expansion on Arduino and Raspberry Pi projects
- Seven-segment displays and simple test fixtures
Specifications
- Number of I/O
- 8
- Interface
- I2C (Fast-mode, up to 400 kHz)
- Supply Voltage
- 2.5 to 6 V
- I2C Address Range
- 0x20 to 0x27 (A0-A2)
- Output Type
- Quasi-Bidirectional
- Sink Current (VOL = 0.4 V)
- 25 mA
- Interrupt Output
- Open-Drain INT
- Operating Temperature
- -40 to 85 °C
- Pins
- 16
- RoHS
- Compliant
Procurement Tips
If your panel drives LEDs, remember the PCF8574 is active-low for sinking — design the LED common to VCC. For mixed-voltage systems, buy the same expander family in one voltage domain to avoid level-shifter BOM creep.
Frequently Asked Questions
What is the difference between PCF8574 and PCF8574A?
Electrically identical; only the I2C slave address block differs. PCF8574 responds at 0x20-0x27, PCF8574A at 0x38-0x3F. You can run both on the same bus for up to 16 extra ports without address conflict.
Why can the PCF8574 sink 25 mA but only source 100 µA?
That is the quasi-bidirectional design: a strong NMOS pull-down plus a weak PMOS pull-up (100 µA current source) that only holds the line high. LEDs should be wired anode-to-VCC, cathode-to-pin (active-low, sunk current). If you need strong sourcing, choose a push-pull expander like the TCA6408 or MCP23008.
PCF8574 versus MCP23008 — which is better?
The MCP23008 offers configurable direction registers, internal pull-ups, and true push-pull outputs, but runs only to 5.5 V and needs more register setup. The PCF8574 is simpler (read/write bytes directly), has the wider 6 V supply and stronger sink. For buttons and LEDs the PCF8574 wins on simplicity; for bidirectional logic with pull-ups the MCP23008 is more flexible.
How fast can I read inputs on the PCF8574?
The I2C bus runs to 400 kHz (Fast-mode), so a full port read takes roughly 100 µs including addressing. For faster sampling use the interrupt line to react only on changes, or move to an SPI expander such as the MCP23S08.
Does the PCF8574 work at 3.3 V with a 5 V MCU bus?
Yes, within limits. The PCF8574 I2C pins tolerate up to 6 V, so a 5 V SDA/SCL into a 3.3 V-powered PCF8574 is not recommended (it can power the chip through the ESD diodes), but 3.3 V bus lines into a 5 V-powered device work fine. Keep bus and device voltages consistent or add a proper level shifter.
How do I spot counterfeit PCF8574 parts?
Clones frequently fail the 25 mA sink spec (VOL rises above 0.4 V under load) or respond at wrong addresses. Scan the whole bus with an I2C scanner, then load each output with 10 mA and measure VOL. Genuine parts also carry clean laser-etched TI/NXP marking with traceable date codes.
What is the MOQ and lead time?
MOQ 1 reel (2,000 pcs SO-16) or 100 pcs loose; DIP-16 available by tube. Shenzhen stock ships in 1-2 business days; TI authorized channel replenishment runs 4-8 weeks.
Alternatives & Equivalents
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