Source: https://mokxi.com/parts/mcp23017 Updated: 2026-11-10 Part # I/O expander, MCP23017 Sixteen more pins, two ports of registers, and interrupts. Tie RESET and the address pins or it is not there. See every part - 28 pins - In 1 project - In 1 lesson Drawn live by the editor's own code, at the size you see it. At a glance ## The I/O expander, MCP23017 in brief What it is, and where it sits on the bench. The MCP23017 adds sixteen pins over I2C as two eight-bit ports, A and B, in a 28-pin DIP. Unlike the PCF8574 it has a register for everything: each pin’s direction, an optional 100 kilohm pull-up, inverted reading, push-pull outputs, and interrupts on change with the port captured at that moment. Three address pins choose 0x20 to 0x27, so eight chips give a board 128 extra pins. It is a bare chip with no pull-ups or pull-downs on its control pins, and Mokxi models that faithfully: RESET and A0 to A2 have to be wired, or the part is simply absent from the bus, as a real one would be. The model covers both register layouts selected by IOCON.BANK, the register pointer’s three stepping behaviors, pull-ups, polarity and both interrupt modes with INTF and INTCAP, and the INTA and INTB outputs. Family Logic Pins 28 Pins ## 28 pins Named as the editor names them, so a wire lands where you expect it. ### GPB8 - GPB0 - GPB1 - GPB2 - GPB3 - GPB4 - GPB5 - GPB6 - GPB7 ### NC2 - NC1 - NC2 ### A3 - A0 - A1 - A2 ### GPA8 - GPA0 - GPA1 - GPA2 - GPA3 - GPA4 - GPA5 - GPA6 - GPA7 ### Other7 - VDD - VSS - SCL - SDA - RESET - INTB - INTA Show all 28 pins Pin What it does GPB0 GPB1 GPB2 GPB3 GPB4 GPB5 GPB6 GPB7 VDD Supply, 1.8 to 5.5 V, and ground. VSS Ground. NC1 Not connected. SCL I2C. SDA I2C. NC2 Not connected. A0 The low three bits of the address, 0x20 to 0x27. A1 The low three bits of the address, 0x20 to 0x27. A2 The low three bits of the address, 0x20 to 0x27. RESET Active low. INTB Interrupt outputs for port A and port B. INTA Interrupt outputs for port A and port B. GPA0 GPA1 GPA2 GPA3 GPA4 GPA5 GPA6 GPA7 How to use it ## How to use the MCP23017 I/O expander with an Arduino Power on VDD (5 V) and VSS (GND), and the bus on SDA (A4) and SCL (A5). Then the three pins people forget: RESET to VDD, and each of A0, A1 and A2 to VDD or VSS. In the More pins project all three address pins go to 5 V, which makes the address 0x27. A pushbutton runs from GPB0 to ground. With Adafruit’s library, begin_I2C(0x27) finds the chip, and the pins are numbered 0 to 15: 0 to 7 are GPA0 to GPA7, and 8 to 15 are GPB0 to GPB7. The project sets pin 8 to INPUT_PULLUP, so GPB0 reads LOW while the button is held, and sets pin 0 as an output that goes HIGH while it is held; wire an LED and resistor from GPA0 to ground to see it. The serial monitor prints "button held" or "button up" every 250 ms. Part pin Board pin VDD 5 V VSS GND SDA A4 SCL A5 RESET 5 V A0, A1, A2 5 V each: address 0x27 GPB0 Pushbutton to GND The chip runs from 1.8 to 5.5 V, so a 3.3 V board can power it directly; tie RESET and the address pins to that same 3.3 V. Hold the button This is the simulator itself, running here. Click anything to open it in the editor. Open this project in the editor Read the full tutorial A button in and an output out, from the More pins sketch Common errors ## Common I/O expander, MCP23017 errors and fixes What goes wrong The fix The chip does not answer at any address. RESET or an address pin is floating. RESET has no internal pull-up and must go to VDD; A0, A1 and A2 must each go to VDD or VSS. Code from an old tutorial calls mcp.begin(0) and fails. The current Adafruit library takes the full address: begin_I2C(0x20) for all address pins low, up to 0x27 for all high. After setting IOCON, later writes land in the wrong registers. Setting the BANK bit moves port B’s registers at once. Leave BANK at 0 unless you have rewritten every address for the other layout. An input reads random values. Pins power up as inputs with no pull-up. Turn the pull-up on with INPUT_PULLUP, or add your own resistor. Questions ## Questions and answers ### How are the MCP23017 pins numbered in the Adafruit library? Pins 0 to 7 are GPA0 to GPA7 and pins 8 to 15 are GPB0 to GPB7, in the order of the chip’s two ports. ### Does the MCP23017 need pull-up resistors on SDA and SCL? A bare chip has none, so a real bus wants 4.7 k pull-ups. On a short breadboard bus the board’s own weak pull-ups, which Wire turns on, often carry it at 100 kHz. ### Can the MCP23017 tell the Arduino when a button is pressed? Yes. Enable interrupt-on-change for the pin and INTA or INTB goes low; INTF names the pin and INTCAP holds the port as it was at that moment. ### Should GPA7 and GPB7 be used as inputs? Microchip’s current datasheet says to use them as outputs only on real chips. The simulation lets them work as inputs, so keep inputs on the other fourteen pins if the design is going to hardware. How it is modeled ## What is true about the I/O expander, MCP23017, here In its help article’s own words: what the simulation gets right, and where it stops. ### Modeled Both register maps and the pointer's three behaviors. Outputs from OLAT, 100 kΩ pull-ups from GPPU, and IPOL inverting what GPIO reads. Interrupt-on-change: compare against the last value (INTCON 0) or against DEFVAL (INTCON 1), INTF naming the pin, INTCAP catching the port at that moment, and a read of INTCAP or GPIO clearing it. With DEFVAL, a pin that is still different fires again at once. INTA and INTB are push-pull active low at power-up; INTPOL, ODR and MIRROR change that as the datasheet says. ### Not modeled SPI, which is the MCP23S17. Output current limits. The slew-rate control. Microchip's current datasheet says GPA7 and GPB7 should be outputs only, because a change on either during a read can corrupt SDA on real silicon; here they work as inputs like the rest, so a circuit that relies on that works in Mokxi and may not on the bench. From MCP23017 I/O expander, in full. Projects ## See the I/O expander, MCP23017 in a project One built-in project uses it. It opens in the editor, already wired. Lessons ## Where it turns up in a lesson One lesson places it in its own circuit. ### Learn articles 1 lesson - Arduino PCF8574 and MCP23017 I/O Expanders Boards ## The boards it is shown on This board has a built-in project with the I/O expander, MCP23017 in it. - Arduino Uno R3 Libraries ## Arduino libraries for the I/O expander, MCP23017 Built in, so the sketch compiles with nothing to install. Each page has the calls, the wiring and an example to run. - Adafruit MCP23017 Arduino Library Keep going ## Keep going - I2C Explained: Addresses, Acknowledges and Pull-Ups - I2C Scanner for Arduino and ESP32, and Fixes - I/O expander, PCF8574 - Keypad, 4x4 - Pushbutton More parts ## The rest of the bench Every one of these is drawn and simulated the same way. Full-size breadboard A real 0.1 inch grid with the rails and the center channel, 63 columns wide. Jumper wires Drag from any pin or hole to any other. Corners snap, and you can drag them. Power A supply rail at the voltage you choose. Ground The other end of every circuit. LED Lights when current flows. Five colors, and the brightness is what your eye would see. Filament lamp A tungsten bulb in a holder: an inrush when it is switched on cold, tens of milliseconds to warm up, and dim on half its voltage. Resistor Any value you like, with the color bands drawn to match. ESP32-C3-DevKitM-1 A RISC-V board that runs your firmware at 160 MHz on the real memory map. Pushbutton A 12 mm tactile switch. Hold it while the simulation runs. Raspberry Pi Pico The RP2040 board on our own Cortex-M0+ core. Pick a program and press Run. Raspberry Pi Pico W Raspberry Pi Pico W (RP2040). The same board and the same pinout as a Pico, with the CYW43439 on it. The WiFi is simulated (no radio; real internet through the Cloud panel’s switch), and the on-board LED, which hangs off that chip rather than off GP25, is driven through the same path. Everything else is the Pico. Raspberry Pi 4 Model B Raspberry Pi 4 Model B: the 40-pin header, driven by main.py with RPi.GPIO or gpiozero. See every part ## Wire up the I/O expander, MCP23017 Open the editor and push it into the breadboard. It is free, and it runs on your own machine. Start building free Open the editor