HCS361 Trigger+Read

fob_trigger_and_read.cpp

#define SAMPLE_OFFSET 188
#define NSAMPLES 68
#define TRACES_TO_CAPTURE 200

#define FOB_PIN 5
#define BUTTON_PIN 8
#define TRIGGER_PIN 9

unsigned int timeDiffs[NSAMPLES];               // the diffs (us) between all the codeword pulses received
unsigned char bits[NSAMPLES - 1];               // timeDiffs converted into bits according to the VPWM modulation in the datasheet
unsigned long lastTime = 0, currTime;           // the time of the last and current pulses respectively

unsigned char samplesTilRecord = SAMPLE_OFFSET, // number of pulses received until the codeword is being transmitted
              sampleNo = 0;                     // the current number of pulses received

unsigned int traceNo = 0;                       // the current number codewords read

void setup() {        
  Serial.begin(115200);  
  
  pinMode(FOB_PIN, INPUT);
  pinMode(BUTTON_PIN, OUTPUT);
  pinMode(TRIGGER_PIN, OUTPUT);

  digitalWrite(BUTTON_PIN, LOW);
  digitalWrite(TRIGGER_PIN, LOW);

  cli();
  PCICR |= 4; // turn on PC interrupts for port d
  PCMSK2 = 1 << FOB_PIN; // enable PC interrupts for the fob pin
  sei();

  while (!Serial);
  
  delay(5000);
}

ISR(PCINT2_vect) {
  // Early exit until the codeword is being transmitted
  if (samplesTilRecord) {
    samplesTilRecord -= 1;
    return;
  }

  // Don't overwrite any samples
  if (sampleNo == NSAMPLES)
    return;

  // Get the time diff
  currTime = micros();
  timeDiffs[sampleNo++] = currTime - lastTime;
  lastTime = currTime;
}

void cvt_to_bits() {
  unsigned char i;
  
  for (i=1; i<NSAMPLES; i++)
    bits[i - 1] = (1 - ((i - 1) & 1)) ^ (abs(timeDiffs[i] - 200) / 200);
}

unsigned long decode_bits(unsigned char s, unsigned char e) {
  unsigned long decoded = 0;
  unsigned char i;

  for (i=s; i<e; i++)
    decoded |= ((unsigned long)bits[i]) << (i - s);
    
  return decoded;
}

unsigned char calculate_crc() {
  unsigned char i, crc0 = 0, crc1 = 0, tmp_crc0;

  for (i=0; i<NSAMPLES-3; i++) {
    tmp_crc0 = (crc0 ^ bits[i]) ^ crc1;
    crc1 = crc0 ^ bits[i];
    crc0 = tmp_crc0;
  }

  return (crc1 << 1) | crc0;
}

void loop() {
  // Halt once we've collected the desired number of traces
  while (traceNo == TRACES_TO_CAPTURE);
  traceNo += 1;

  // Send a button press and trigger the oscilloscope
  digitalWrite(BUTTON_PIN, HIGH);
  digitalWrite(TRIGGER_PIN, HIGH);
  digitalWrite(BUTTON_PIN, LOW);

  // Wait until the transmission has completed
  while (sampleNo < NSAMPLES)
    delay(1);

  // Decode the VPWM and print the ciphertext
  cvt_to_bits();
  Serial.println(decode_bits(0, 32));
  
  // Reset the trigger and sample reader
  digitalWrite(TRIGGER_PIN, LOW);
  samplesTilRecord = SAMPLE_OFFSET;
  sampleNo = 0;

  // Guestimated HCS cooldown time
  delay(1000);
}