Adds eight general-purpose I/O lines to any microcontroller over the two-wire I2C bus, in a SOIC-16. Each of the eight quasi-bidirectional pins (P0-P7) can be written as an output or read as an input with no direction register, and an open-drain INT pin flags an input change so the host need not poll. Three address pins (A0-A2) set the low bits of the I2C address, so up to eight PCF8574s (and eight more PCF8574As) share one bus. It is the chip behind most I2C LCD backpacks and cheap port expanders. Runs 2.5-6 V.

Pinout

Pin	Name	Type	Function
1	A0	input	Hardware I2C slave-address input bit 0. Tie directly to VCC or GND; no pullup required. Sets the lower address bits so up to eight PCF8574 devices can share the same bus.
2	A1	input	Hardware I2C slave-address input bit 1. Tie directly to VCC or GND; no pullup required.
3	A2	input	Hardware I2C slave-address input bit 2. Tie directly to VCC or GND; no pullup required.
4	P0	bidirectional	Quasi-bidirectional I/O port bit 0. Push-pull structure with a latched output; can drive LEDs directly (100 mA/pin, JESD 78 Class II latch-up). High at power-up (input mode with weak current source to VCC).
5	P1	bidirectional	Quasi-bidirectional I/O port bit 1. See P0 for drive characteristics.
6	P2	bidirectional	Quasi-bidirectional I/O port bit 2. See P0 for drive characteristics.
7	P3	bidirectional	Quasi-bidirectional I/O port bit 3. See P0 for drive characteristics.
8	GND	power_in	Ground reference. 0 V return for the supply and I/O.
9	P4	bidirectional	Quasi-bidirectional I/O port bit 4. See P0 for drive characteristics.
10	P5	bidirectional	Quasi-bidirectional I/O port bit 5. See P0 for drive characteristics.
11	P6	bidirectional	Quasi-bidirectional I/O port bit 6. See P0 for drive characteristics.
12	P7	bidirectional	Quasi-bidirectional I/O port bit 7. See P0 for drive characteristics.
13	INT	output	Open-drain interrupt output, active-low. Asserts when a P-port input changes state; requires an external pullup resistor to VCC. Interrupt reset delay (t_ir) is 4 µs max after SCL toggle.
14	SCL	input	I2C serial clock line. Requires an external pullup resistor to VCC. Supports 100 kHz standard-mode I2C timing.
15	SDA	bidirectional	I2C serial data line, open-drain. Requires an external pullup resistor to VCC. Carries device address and P-port read/write data.
16	VCC	power_in	Positive supply voltage, 2.5 V to 6 V (recommended). Standby ICC = 2.5 µA typ; operating ICC = 40 µA typ at 100 kHz SCL.

Key specifications

Parameter	Value
Type	I2C 8-bit I/O expander
I/O lines	8 quasi-bidirectional (P0-P7)
Bus	I2C (two-wire), up to 100 kHz
Address pins	A0-A2 (8 per bus)
Interrupt	open-drain INT on change
Direction register	none (write-high to read)
Supply voltage	2.5-6 V
Package	SOIC-16

Ordering variants

Same part, different packaging or reel option.

PCF8574T (SOIC-16, tube)
PCF8574TR (SOIC-16, tape & reel)

CAD (KiCad official)

Symbol: Interface_Expansion:PCF8574T
Footprint: Package_SO:SOIC-16_3.9x9.9mm_P1.27mm
3D model: SOIC-16 (KiCad packages3D)

Provenance

Data extracted from the manufacturer datasheet, not a distributor listing:

Datasheet: Texas Instruments datasheet (42 pp, sha256 4e4cb9144f852941, retrieved 2026-08-17, tier: manufacturer)
Extractor: ds2sf (Claude). Every pin and dimension is cited to a datasheet page; it refuses to fabricate.
CAD: KiCad official libraries (CC-BY-SA 4.0).

Datasheet PDF