Qi packet analyzer

QiPacketAnalyzer.ino

const int input_pin = 9;

void setup() {
  Serial.begin(115200);
  Serial.println("START");
  pinMode(input_pin, INPUT);
}

int count = 0;
int last = 0;
unsigned long start;

void loop() {
  int data[16], p, i;

  if (count < 10) {
    while (!digitalRead(input_pin));
  }
  start = micros();
  last = 1;
  int sig = detectSignal();
  if (count < 10 && sig != 1 || sig == -1) {
    count = 0;
    last = 0;
    return;
  }
  if (sig == 1) { // preamble
    count++;
    return;
  }
  // Serial.print("Preamble:");
  // Serial.println(count);

  for (p = 0; p < 16;) {
    data [p]= detectByte();
    if (data[p] < 0) break;
    p++;
    sig = detectSignal();
    if (sig < 0) break; // start byte
    if (sig == 1) { Serial.println("invalid start bit"); break; } 
  }
  byte check = 0;
  for (i = 0; i < p; i++) {
    Serial.print(data[i], HEX);
    Serial.print(" ");
    check ^= data[i];
  }
  if (check) {
    Serial.print("checksum error ");
    Serial.print(check, HEX);
  }
  Serial.println("");

  count = 0;
  last = 0;
}

int printLen() {
  for (;;) {
    unsigned long start = micros();
    while (digitalRead(input_pin) == last);
    unsigned long len = micros() - start;
    Serial.println(len);
    last = !last;
    if (len > 1500) break;
  }
}


int detectSignal() {
  unsigned long len, len2;
  while (digitalRead(input_pin) == last) {
    len = micros() - start;
    if (len > 1000) return -1; // no signal
  }
  if (len < 4) {
    if (count) {
      Serial.print("invalid t1 : ");
      Serial.println(len);
    }
    return -1;
  }

  last = !last;

  if (len > 350) {
    start = micros();
    return 0;
  }
  
  while (digitalRead(input_pin) == last) {
    len2 = micros() - start;
    if (len2 > 1500) {
      return -1;
    }
  }
  if (len2 < 100) {
    if (count) {
      Serial.print("invalid t2 : ");
      Serial.println(len2);
    }
    return -1;
  }

  last = !last;
  start = micros();
  return 1;
}

int detectByte() {
  int data = 0;
  for (int i = 0; i < 8; i++) {
    int sig = detectSignal();
    // Serial.print(sig);
    if (sig == -1) return -1;
    if (sig) data |= (1 << i);
  }

  int parity = detectSignal();
  // Serial.print(parity);
  if (parity == -1) return -1;
  // Serial.print(" ");

  int stop = detectSignal();
  if (stop == 1) return data;
  if (stop == 0) { Serial.println("invalid stop bit"); return data; }
  return -1;
}