Adafruit MCP23017: sixteen extra pins on two I2C wires

// Two port expanders on one I2C bus: a PCF8574 at 0x20 and an MCP23017 at 0x27.
//
// The PCF8574 has no registers at all. Every byte written is its eight pins,
// and a pin written 1 is only weakly high, so the LEDs on it run from 5 V
// through a resistor *into* the pin and light when their bit is 0. The sketch
// walks a single dark bit along four of them.
//
// The MCP23017 is a proper pair of ports: a direction register, pull-ups and
// push-pull outputs. GPB0 is an input with its pull-up on and a button to
// ground; GPA0 drives an LED to ground, lit while the button is held. Its
// RESET is tied high and A0 to A2 high too, which is address 0x27: none of the
// four pins is pulled inside the chip, and left floating the chip is simply
// not on the bus.

#include <Wire.h>
#include <Adafruit_MCP23X17.h>

const uint8_t PCF8574 = 0x20;
Adafruit_MCP23X17 mcp;
uint8_t step = 0;

void setup() {
  Serial.begin(9600);
  Wire.begin();
  Wire.beginTransmission(PCF8574);
  Serial.println(Wire.endTransmission() == 0 ? "PCF8574 at 0x20" : "no PCF8574 at 0x20");
  if (!mcp.begin_I2C(0x27)) {
    Serial.println("no MCP23017 at 0x27");
  } else {
    Serial.println("MCP23017 at 0x27");
  }
  mcp.pinMode(0, OUTPUT);        // GPA0: the LED
  mcp.pinMode(8, INPUT_PULLUP);  // GPB0: the button
}

void loop() {
  // One dark bit walking along P0..P3; the other four stay high, which is
  // also how they would be read as inputs.
  Wire.beginTransmission(PCF8574);
  Wire.write((uint8_t)~(1 << step));
  Wire.endTransmission();
  step = (step + 1) % 4;

  bool pressed = mcp.digitalRead(8) == LOW;
  mcp.digitalWrite(0, pressed ? HIGH : LOW);
  Serial.println(pressed ? "button held" : "button up");
  delay(250);
}