A 16-bit general-purpose I/O expander with an I2C interface, in SO-24, NXP's take on the port-expander and a peer of the MCP23017. It gives a microcontroller sixteen more pins across two 8-bit ports (IO0 and IO1), each pin independently input or output, with a per-pin polarity-inversion register and an open-drain interrupt (INT) that fires on an input change so the host need not poll. Three address pins (A0-A2) put up to eight on one bus for 128 I/O. It runs I2C at up to 400 kHz over a 1.65-5.5 V supply, and unlike the PCF8574 it has real direction control per pin.

Pinout

Pin	Name	Type	Function
1	INT	output	Active-LOW open-drain interrupt output. Asserts when any input port pin state differs from its input port register value; requires an external pull-up to VDD.
2	A1	input	I2C-bus slave address input bit 1. Tie to VDD or VSS to set the programmable portion of the 7-bit slave address (0100 A2 A1 A0).
3	A2	input	I2C-bus slave address input bit 2. Tie to VDD or VSS to set the programmable portion of the 7-bit slave address.
4	IO0_0	bidirectional	Port 0 bit 0 general-purpose I/O. Configurable per bit as input (default, high-Z with weak 100 kΩ pull-up to VDD) or push-pull output. 5 V-tolerant input.
5	IO0_1	bidirectional	Port 0 bit 1 general-purpose I/O. Configurable per bit as input (default) or push-pull output. 5 V-tolerant input.
6	IO0_2	bidirectional	Port 0 bit 2 general-purpose I/O. Configurable per bit as input (default) or push-pull output. 5 V-tolerant input.
7	IO0_3	bidirectional	Port 0 bit 3 general-purpose I/O. Configurable per bit as input (default) or push-pull output. 5 V-tolerant input.
8	IO0_4	bidirectional	Port 0 bit 4 general-purpose I/O. Configurable per bit as input (default) or push-pull output. 5 V-tolerant input.
9	IO0_5	bidirectional	Port 0 bit 5 general-purpose I/O. Configurable per bit as input (default) or push-pull output. 5 V-tolerant input.
10	IO0_6	bidirectional	Port 0 bit 6 general-purpose I/O. Configurable per bit as input (default) or push-pull output. 5 V-tolerant input.
11	IO0_7	bidirectional	Port 0 bit 7 general-purpose I/O. Configurable per bit as input (default) or push-pull output. 5 V-tolerant input.
12	VSS	power_in	Supply ground (0 V) reference for the device.
13	IO1_0	bidirectional	Port 1 bit 0 general-purpose I/O. Configurable per bit as input (default) or push-pull output. 5 V-tolerant input.
14	IO1_1	bidirectional	Port 1 bit 1 general-purpose I/O. Configurable per bit as input (default) or push-pull output. 5 V-tolerant input.
15	IO1_2	bidirectional	Port 1 bit 2 general-purpose I/O. Configurable per bit as input (default) or push-pull output. 5 V-tolerant input.
16	IO1_3	bidirectional	Port 1 bit 3 general-purpose I/O. Configurable per bit as input (default) or push-pull output. 5 V-tolerant input.
17	IO1_4	bidirectional	Port 1 bit 4 general-purpose I/O. Configurable per bit as input (default) or push-pull output. 5 V-tolerant input.
18	IO1_5	bidirectional	Port 1 bit 5 general-purpose I/O. Configurable per bit as input (default) or push-pull output. 5 V-tolerant input.
19	IO1_6	bidirectional	Port 1 bit 6 general-purpose I/O. Configurable per bit as input (default) or push-pull output. 5 V-tolerant input.
20	IO1_7	bidirectional	Port 1 bit 7 general-purpose I/O. Configurable per bit as input (default) or push-pull output. 5 V-tolerant input.
21	A0	input	I2C-bus slave address input bit 0. Tie to VDD or VSS to set the programmable portion of the 7-bit slave address (0100 A2 A1 A0).
22	SCL	input	I2C-bus serial clock input line. 0 Hz to 400 kHz clock frequency; requires external pull-up to VDD.
23	SDA	bidirectional	I2C-bus serial data line (open-drain). Requires external pull-up to VDD.
24	VDD	power_in	Positive supply voltage input. Operating range 2.3 V to 5.5 V; bypass locally with a 100 nF ceramic capacitor to VSS.

Key specifications

Parameter	Value
Type	I2C 16-bit I/O expander
I/O lines	16 (ports IO0, IO1)
Bus	I2C up to 400 kHz
Per-pin	direction + polarity inversion
Interrupt	open-drain INT on change
Address pins	A0-A2 (8 per bus)
Supply voltage	1.65-5.5 V
Package	SO-24

Ordering variants

Same part, different packaging or reel option.

PCA9555D (SO-24)
PCA9555PW (TSSOP-24)

CAD (KiCad official)

Symbol: Interface_Expansion:PCA9555D
Footprint: Package_SO:SOIC-24W_7.5x15.4mm_P1.27mm
3D model: SOIC-24W (KiCad packages3D)

Provenance

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

Datasheet: NXP Semiconductors datasheet (34 pp, sha256 9f4f9c0ce4c461d7, 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