T1 and Time-Division Multiplexing

// Four channels onto one wire, and the receiver has to FIND THE FRAME.
//
// This is T1 at a hundred-thousandth of the rate. The transmitter sends, forever:
//     [0x7E framing byte][ch0][ch1][ch2][ch3]
// and the receiver has no idea where in that sequence it joined. Until it locks onto the
// framing byte, every channel is showing somebody else's potentiometer -- which is
// exactly why T1 spends one bit in every 193 on saying "the frame starts here".
//
// Set ROLE to TX or RX and flash one of each.

#define ROLE_TX 1          // 1 on the transmitter, 0 on the receiver

const uint8_t FRAME = 0x7E;
const int N_CH = 4;
const int PIN_ADC[N_CH] = {34, 35, 32, 33};
const int PIN_TX = 17, PIN_RX = 16;

HardwareSerial Link(1);

void setup() {
  Serial.begin(115200);
  delay(300);
  Link.begin(9600, SERIAL_8N1, PIN_RX, PIN_TX);
  analogReadResolution(12);
#if ROLE_TX
  Serial.println("# TDM transmitter: [0x7E][ch0][ch1][ch2][ch3]");
#else
  Serial.println("# TDM receiver: hunting for frame alignment");
  Serial.println("locked,ch0,ch1,ch2,ch3");
#endif
}

#if ROLE_TX
void loop() {
  Link.write(FRAME);
  for (int c = 0; c < N_CH; c++) {
    uint8_t v = analogRead(PIN_ADC[c]) >> 4;      // 12 bits -> 8
    if (v == FRAME) v = FRAME - 1;                // never counterfeit the framing byte
    Link.write(v);
  }
  delay(50);                                       // 20 frames per second
}
#else
// Frame hunting. Seeing 0x7E ONCE proves nothing -- data can look like anything. A real
// receiver requires the candidate to recur at exactly the frame period several times
// before it declares lock, and drops lock if it stops. That is the whole of frame sync.
int  candidate = -1, confirms = 0;
bool locked = false;
uint8_t buf[1 + N_CH];
int fill = 0;

void loop() {
  while (Link.available()) {
    uint8_t b = Link.read();

    if (!locked) {
      if (b == FRAME) {
        if (candidate == fill) { if (++confirms >= 3) { locked = true; fill = 0; continue; } }
        else                   { candidate = fill; confirms = 1; }
      }
      if (++fill > N_CH) fill = 0;
      continue;
    }

    buf[fill++] = b;
    if (fill == 1 + N_CH) {
      fill = 0;
      if (buf[0] != FRAME) {                       // lost alignment: start hunting again
        locked = false; candidate = -1; confirms = 0;
        Serial.println("0,-,-,-,-      # LOST LOCK");
      } else {
        Serial.printf("1,%u,%u,%u,%u\n", buf[1], buf[2], buf[3], buf[4]);
      }
    }
  }
}
#endif