PCF8574 — Remote 8-Bit I/O Expander for I2C Bus

Overview

The PCF8574 (and its companion variant PCF8574A) is an 8-bit quasi-bidirectional I/O expander for the I2C bus manufactured by NXP Semiconductors and Texas Instruments. It provides general-purpose remote I/O expansion for microcontrollers via a two-line bidirectional I2C bus (SDA and SCL).

It features 8 quasi-bidirectional I/O pins (P0–P7), three hardware address input pins (A0, A1, A2), and an active-LOW open-drain interrupt output (INT). The PCF8574 is widely integrated into I2C backpack boards for HD44780 character LCD displays (LCD1602 / LCD2004) and used for button matrix scanning or driving LEDs.

Quick reference


Operating voltage (`VCC`)	2.5 V to 6.0 V DC
I2C bus clock speed	Up to 100 kHz (Standard-mode I2C)
I/O ports	8 Quasi-bidirectional pins (`P0`–`P7`)
I2C address (PCF8574)	`0x20` to `0x27` (determined by `A0`–`A2`)
I2C address (PCF8574A)	`0x38` to `0x3F` (determined by `A0`–`A2`)
Interrupt output	Active-LOW open-drain (`INT`)
Standby current	10 µA max

Pinout

16-Pin DIP / SOIC Package

Pin	Name	Type	Description
1	`A0`	Digital Input	Address input 0
2	`A1`	Digital Input	Address input 1
3	`A2`	Digital Input	Address input 2
4	`P0`	Quasi-I/O	Quasi-bidirectional I/O port 0
5	`P1`	Quasi-I/O	Quasi-bidirectional I/O port 1
6	`P2`	Quasi-I/O	Quasi-bidirectional I/O port 2
7	`P3`	Quasi-I/O	Quasi-bidirectional I/O port 3
8	`VSS`	Power	Ground (0 V)
9	`P4`	Quasi-I/O	Quasi-bidirectional I/O port 4
10	`P5`	Quasi-I/O	Quasi-bidirectional I/O port 5
11	`P6`	Quasi-I/O	Quasi-bidirectional I/O port 6
12	`P7`	Quasi-I/O	Quasi-bidirectional I/O port 7
13	`INT`	Digital Output	Interrupt output (Open-drain, Active-LOW)
14	`SCL`	Digital Input	I2C Serial Clock line
15	`SDA`	Digital I/O	I2C Serial Data line (Open-drain)
16	`VDD`	Power	Positive supply voltage (+2.5 V to +6.0 V)

Specifications

Parameter	Symbol	Min	Typ	Max	Unit	Conditions
Supply Voltage	$V_{DD}$	2.5	5.0	6.0	V
Supply Current	$I_{DD}$	—	40	100	µA	$V_{DD} = 5.0\text{ V}$ , no load, $f_{SCL} = 100\text{ kHz}$
Standby Current	$I_{stb}$	—	2.5	10	µA	$V_{DD} = 5.0\text{ V}$ , $I_{IO} = 0$
LOW-level Input Voltage	$V_{IL}$	-0.5	—	$0.3 V_{DD}$	V
HIGH-level Input Voltage	$V_{IH}$	$0.7 V_{DD}$	—	$V_{DD} + 0.5$	V
LOW-level Output Current	$I_{OL}$	10	25	—	mA	$V_{DD} = 5.0\text{ V}$ , $V_{OL} = 1.0\text{ V}$ (Sinking)
HIGH-level Output Current	$I_{OH}$	-30	-100	-300	µA	$V_{DD} = 5.0\text{ V}$ , $V_{OH} = V_{SS}$ (Source)
SCL/SDA Clock Frequency	$f_{SCL}$	0	—	100	kHz	Standard Mode

Communication & Addressing

The PCF8574 uses a 7-bit slave address consisting of a 4-bit fixed control code (0100 for PCF8574, 0111 for PCF8574A) followed by 3 programmable address bits (A2, A1, A0).

Address Map

Part Number	Slave Address Byte Format (R/W = 0)	I2C Address Range (Hex)
PCF8574	`0 1 0 0 A2 A1 A0 R/W`	`0x20` – `0x27`
PCF8574A	`0 1 1 1 A2 A1 A0 R/W`	`0x38` – `0x3F`

Quasi-Bidirectional I/O Architecture

Each pin can be used as an input or output without a direction control register.

As an Output: Sinks up to 25 mA (LOW). Sourcing current (HIGH) is limited to ~100 µA via an internal pull-up.
As an Input: Software must first write a HIGH (1) to the port pin. When configured HIGH, an external driver can pull the pin LOW.

Wiring

PCF8574 Breakout	Microcontroller (Arduino / ESP32)	Notes
`VCC` / `VDD`	`5V` (or `3.3V`)	Power supply
`GND` / `VSS`	`GND`	Common ground
`SDA`	`SDA` (A4 on Uno, GPIO21 on ESP32)	I2C Data (requires pull-up resistors)
`SCL`	`SCL` (A5 on Uno, GPIO22 on ESP32)	I2C Clock (requires pull-up resistors)
`INT`	GPIO Pin (e.g. GPIO2)	Optional interrupt line

Note

When using PCF8574 as an LCD backpack board, solder jumpers A0, A1, and A2 configure the address (0x27 is common for PCF8574, 0x3F for PCF8574A).

Common mistakes

Attempting to read inputs without writing HIGH first: Because ports are quasi-bidirectional, reading a pin that was previously written LOW will always return 0. Always write 1 (HIGH) to pins intended as inputs before reading.
Relying on strong HIGH output drive: The chip can sink up to 25 mA (LOW) but can only source ~100 µA (HIGH). Active-HIGH LED circuits will be very dim; connect LEDs in active-LOW (sinking) configuration instead.
Confusing PCF8574 and PCF8574A I2C addresses: PCF8574 uses base 0x20, whereas PCF8574A uses base 0x38. Running an I2C scanner script is recommended if communication fails.

Notes

Up to 8 PCF8574 and 8 PCF8574A devices can co-exist on the same I2C bus for up to 128 total I/O pins.

Interface	I2C
Supply	2.5–6 V
Package	DIP16, SO16, SSOP20