Adds eight general-purpose I/O pins to a host over I2C. Each of the eight pins (GP0-GP7) is independently set as input or output, with optional pull-ups and per-pin interrupt-on-change that flags a change on the INT output, so the host is told which pin moved without polling. Three address pins (A0-A2) put up to eight devices on one bus. The 8-bit sibling of the MCP23017, at half the pins.

Pinout

Pin	Name	Type	Function
1	SCL	input	I2C serial clock input. Supports Standard-mode (100 kHz), Fast-mode (400 kHz) and High-speed mode (1.7 MHz). Requires an external pull-up resistor to VDD.
2	SDA	bidirectional	I2C serial data (open-drain, bidirectional). Requires an external pull-up resistor to VDD.
3	A2	input	Hardware address input bit 2. Sets the upper I2C slave-address bit of the MCP23008 (base address 0100 A2 A1 A0). Must be biased externally to VDD or VSS.
4	A1	input	Hardware address input bit 1. Must be biased externally to VDD or VSS.
5	A0	input	Hardware address input bit 0. Must be biased externally to VDD or VSS.
6	RESET	input	External active-low reset input. Drive low to reset the device; hold high (or tie to VDD) for normal operation.
7	NC	unconnected	No connect on the MCP23008 (this pin is the SPI chip-select CS on the MCP23S08 variant). Leave floating or tie to VDD/VSS.
8	INT	output	Interrupt output. Configurable as active-high, active-low, or open-drain via the IOCON register. Asserted on GPIO input change or mismatch vs DEFVAL.
9	VSS	power_in	Ground reference (0 V).
10	GP0	bidirectional	General-purpose bidirectional I/O bit 0. Individually configurable as input or output; supports interrupt-on-change and an internal weak pull-up to VDD.
11	GP1	bidirectional	General-purpose bidirectional I/O bit 1. Interrupt-on-change and internal weak pull-up available.
12	GP2	bidirectional	General-purpose bidirectional I/O bit 2. Interrupt-on-change and internal weak pull-up available.
13	GP3	bidirectional	General-purpose bidirectional I/O bit 3. Interrupt-on-change and internal weak pull-up available.
14	GP4	bidirectional	General-purpose bidirectional I/O bit 4. Interrupt-on-change and internal weak pull-up available.
15	GP5	bidirectional	General-purpose bidirectional I/O bit 5. Interrupt-on-change and internal weak pull-up available.
16	GP6	bidirectional	General-purpose bidirectional I/O bit 6. Interrupt-on-change and internal weak pull-up available.
17	GP7	bidirectional	General-purpose bidirectional I/O bit 7. Interrupt-on-change and internal weak pull-up available.
18	VDD	power_in	Supply voltage. Operating range 1.8 V to 5.5 V (I2C speed derated at lower voltages: 100 kHz min at 1.8 V, 400 kHz at 2.7 V, 1.7 MHz at 4.5 V). Decouple to VSS with a 0.1 µF ceramic near the pin.

Key specifications

Parameter	Value
Type	8-bit I2C I/O expander
Interface	I2C (up to 1.7 MHz)
I/O	8 pins (GP0-GP7), input or output
Interrupt	INT output, interrupt-on-change per pin
Addresses	8 (A0-A2)
Pull-ups	Software-enabled per pin (100 kΩ)
Supply	1.8-5.5 V
Package	SOIC-18W (300 mil)

Ordering variants

Same part, different packaging or reel option.

MCP23008-E/SO (SOIC-18W)
MCP23008-E/P (PDIP-18)
MCP23008-E/SS (SSOP-20)
MCP23008-E/ML (QFN-20)

CAD (KiCad official)

Symbol: Interface_Expansion:MCP23008-xSO (KiCad official)
Footprint: Package_SO:SOIC-18W_7.5x11.6mm_P1.27mm
3D model: SOIC-18W (KiCad packages3D, STEP)

Provenance

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

Datasheet: Microchip datasheet (43 pp, sha256 3516fba5df6da78c, 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