DHT11 on A0, read with no library, on an Arduino Uno

// DHT11 on A0, read with no library: the whole conversation by hand.
//
//   DHT11 module  S (data) -> A0   + -> 5V   - -> GND
//
// An analog pin is a digital pin too, so A0 can talk to the sensor. The data
// line idles high on the module's pull-up, and the board speaks first:
//
//   1. hold the line low for 20 ms (the sensor's "wake up"), then let go;
//   2. the sensor answers low for 80 us, then high for 80 us;
//   3. it sends 40 bits, each a 50 us low and then a high: about 27 us high
//      is a 0 and about 70 us is a 1;
//   4. the five bytes are humidity, humidity tenths, temperature,
//      temperature tenths and a checksum, the low byte of the other four's sum.
//
// So the sketch only has to time how long each high lasts. A DHT11 sends
// whole numbers, so the tenths bytes are 0.
//
// Click the sensor and change the temperature or the humidity.

const int DHT_PIN = A0;

// Wait while the line stays at `level`. Returns the microseconds it took,
// or 0 when it took longer than `limit` (no sensor, or a lost bit).
unsigned int waitWhile(int level, unsigned int limit) {
  unsigned long start = micros();
  while (digitalRead(DHT_PIN) == level) {
    if (micros() - start > limit) return 0;
  }
  unsigned int took = micros() - start;
  return took == 0 ? 1 : took;
}

// Read the five bytes into `data`. Returns an error message, or 0.
const char *readDht11(byte data[5]) {
  // The wake-up: low for 20 ms, then let the pull-up have the line.
  pinMode(DHT_PIN, OUTPUT);
  digitalWrite(DHT_PIN, LOW);
  delay(20);
  pinMode(DHT_PIN, INPUT);

  // The answer: the line goes low, then high, then low for the first bit.
  if (!waitWhile(HIGH, 200)) return "no answer";
  if (!waitWhile(LOW, 200)) return "answer too long (low)";
  if (!waitWhile(HIGH, 200)) return "answer too long (high)";

  for (int i = 0; i < 40; i++) {
    if (!waitWhile(LOW, 200)) return "lost a bit (low)";
    unsigned int high = waitWhile(HIGH, 200);
    if (!high) return "lost a bit (high)";
    data[i / 8] <<= 1;
    if (high > 45) data[i / 8] |= 1;
  }
  byte sum = data[0] + data[1] + data[2] + data[3];
  if (sum != data[4]) return "checksum error";
  return 0;
}

void setup() {
  Serial.begin(9600);
  Serial.println("Mokxi Uno: DHT11 on A0, no library");
  // The sensor ignores anything in its first second after power comes up.
  delay(1500);
}

void loop() {
  byte data[5] = {0, 0, 0, 0, 0};
  const char *error = readDht11(data);
  if (error) {
    Serial.print("DHT11: ");
    Serial.println(error);
  } else {
    Serial.print("Humidity ");
    Serial.print(data[0]);
    Serial.print(".");
    Serial.print(data[1]);
    Serial.print(" %  Temperature ");
    Serial.print(data[2]);
    Serial.print(".");
    Serial.print(data[3]);
    Serial.println(" C");
  }
  delay(2000);
}