Nano dial

// Dial: a potentiometer on A7, the pin an Uno does not have, fading an LED.
//
// A7 is ADC channel 7. On the Nano's TQFP package it is bonded out as an
// analogue input with no port pin behind it, so it can be read with
// analogRead and nothing else: pinMode, digitalWrite and digitalRead ignore
// it, and there is no internal pull-up to switch on. That is the one thing
// this board has that an Uno does not, so it is worth a sketch of its own.
//
// Wire the potentiometer's two ends to 5V and GND and its wiper to A7. The
// LED goes on pin 9 through a 220 ohm resistor to GND; pin 9 is OC1A, so
// analogWrite gives it hardware PWM at 490 Hz rather than a software loop.

const int DIAL_PIN = A7;
const int LAMP_PIN = 9;

unsigned long lastReport = 0;

void setup() {
  Serial.begin(115200);
  pinMode(LAMP_PIN, OUTPUT);
  Serial.println("turn the dial on A7");
}

void loop() {
  // 0..1023 from the ten-bit conversion, 0..255 for the eight-bit PWM.
  int raw = analogRead(DIAL_PIN);
  analogWrite(LAMP_PIN, raw >> 2);

  unsigned long now = millis();
  if (now - lastReport >= 250) {
    lastReport = now;
    Serial.print("A7 ");
    Serial.print(raw);
    Serial.print("  duty ");
    Serial.println(raw >> 2);
  }
}