Adds sixteen general-purpose I/O to a microcontroller over I2C, in a SOIC-28, the 16-bit big brother of the PCF8574. The pins are split into two 8-bit ports (GPA and GPB), each with per-pin direction, optional internal pull-ups, input polarity inversion, and interrupt-on-change that drives the INTA and INTB pins so the host need not poll. Three address pins (A0-A2) put up to eight of them on one bus for 128 I/O total. It speaks I2C up to 1.7 MHz (a sibling, the MCP23S17, uses SPI). Runs 1.8-5.5 V.

Pinout

Pin	Name	Type	Function
1	GPB0	bidirectional	PORTB bit 0 bidirectional I/O. Configurable per-pin as input or output; supports interrupt-on-change and internal ~75 µA weak pull-up to VDD. IOL up to 8.0 mA @ VDD=4.5V.
2	GPB1	bidirectional	PORTB bit 1 bidirectional I/O. Same characteristics as GPB0 - configurable direction, interrupt-on-change, optional weak pull-up.
3	GPB2	bidirectional	PORTB bit 2 bidirectional I/O with optional interrupt-on-change and weak pull-up.
4	GPB3	bidirectional	PORTB bit 3 bidirectional I/O with optional interrupt-on-change and weak pull-up.
5	GPB4	bidirectional	PORTB bit 4 bidirectional I/O with optional interrupt-on-change and weak pull-up.
6	GPB5	bidirectional	PORTB bit 5 bidirectional I/O with optional interrupt-on-change and weak pull-up.
7	GPB6	bidirectional	PORTB bit 6 bidirectional I/O with optional interrupt-on-change and weak pull-up.
8	GPB7	bidirectional	PORTB bit 7 bidirectional I/O with optional interrupt-on-change and weak pull-up.
9	VDD	power_in	Positive supply. Operating range 1.8V-5.5V (industrial), 2.7V-5.5V and 4.5V-5.5V for wider temperature grades. Bypass with a 0.1 µF ceramic close to the pin.
10	VSS	power_in	Ground reference for supply and all I/O.
11	NC	unconnected	No connect on MCP23017 (this pin is Chip Select /CS on the SPI-variant MCP23S17). Leave floating or tie to ground.
12	SCL	input	I2C serial clock input (Schmitt-trigger). Standard 100 kHz, Fast 400 kHz, and Fast-mode Plus 1.7 MHz supported. Requires an external pull-up resistor to VDD.
13	SDA	bidirectional	I2C serial data (open-drain, Schmitt-trigger input). Requires an external pull-up resistor to VDD. VOL ≤ 0.6V @ 3 mA sink (VDD=1.8V), ≤ 0.8V @ 3 mA sink (VDD=4.5V).
14	NC	unconnected	No connect on MCP23017 (this pin is SPI Serial Out on MCP23S17). Leave floating or tie to ground.
15	A0	input	Hardware slave address bit 0 (TTL buffer). Must be externally biased to VDD or VSS - do not leave floating. Sets bit A0 of the 7-bit I2C address 0100 A2 A1 A0.
16	A1	input	Hardware slave address bit 1 (TTL buffer). Must be externally biased to VDD or VSS.
17	A2	input	Hardware slave address bit 2 (TTL buffer). Must be externally biased to VDD or VSS. Combined with A1/A0 allows up to eight MCP23017 devices on one I2C bus.
18	RESET	input	Active-low hardware reset (Schmitt-trigger). Must be externally biased (tie to VDD directly or via a pull-up if unused). Minimum low pulse 1 µs; device is active 0 ns after RESET returns high; outputs go Hi-Z within 1 µs of RESET going low.
19	INTB	output	Interrupt output for PORTB. Configurable via IOCON as active-high, active-low, or open-drain. Can be mirrored with INTA when IOCON.MIRROR=1. VOL ≤ 0.6V @ 1.6 mA sink (VDD=4.5V).
20	INTA	output	Interrupt output for PORTA. Configurable via IOCON as active-high, active-low, or open-drain. Can be mirrored with INTB when IOCON.MIRROR=1.
21	GPA0	bidirectional	PORTA bit 0 bidirectional I/O. Configurable per-pin as input or output; supports interrupt-on-change and internal ~75 µA weak pull-up to VDD. IOL up to 8.0 mA @ VDD=4.5V.
22	GPA1	bidirectional	PORTA bit 1 bidirectional I/O with optional interrupt-on-change and weak pull-up.
23	GPA2	bidirectional	PORTA bit 2 bidirectional I/O with optional interrupt-on-change and weak pull-up.
24	GPA3	bidirectional	PORTA bit 3 bidirectional I/O with optional interrupt-on-change and weak pull-up.
25	GPA4	bidirectional	PORTA bit 4 bidirectional I/O with optional interrupt-on-change and weak pull-up.
26	GPA5	bidirectional	PORTA bit 5 bidirectional I/O with optional interrupt-on-change and weak pull-up.
27	GPA6	bidirectional	PORTA bit 6 bidirectional I/O with optional interrupt-on-change and weak pull-up.
28	GPA7	bidirectional	PORTA bit 7 bidirectional I/O with optional interrupt-on-change and weak pull-up.

Key specifications

Parameter	Value
Type	I2C 16-bit I/O expander
I/O lines	16 (ports GPA0-7, GPB0-7)
Bus	I2C up to 1.7 MHz
Address pins	A0-A2 (8 per bus)
Per-pin features	direction, pull-up, polarity, interrupt
Interrupts	INTA + INTB (configurable)
Supply voltage	1.8-5.5 V
Package	SOIC-28

Ordering variants

Same part, different packaging or reel option.

MCP23017-E/SO (SOIC-28)
MCP23017-E/SP (SPDIP-28)

CAD (KiCad official)

Symbol: Interface_Expansion:MCP23017x-x-SO
Footprint: Package_SO:SOIC-28W_7.5x17.9mm_P1.27mm
3D model: SOIC-28W (KiCad packages3D)

Provenance

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

Datasheet: Microchip Technology datasheet (42 pp, sha256 14159c6f5655e943, 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