// uart-master.ino — Node 0 (master), XIAO RP2040 on my own milled board
// Week 11 — three-node UART bus with software addressing (NID).
// The master only transmits. It runs a fixed 2 s traffic-light cycle and
// sends 6-byte frames to two slaves on a shared TX line. Each slave filters
// by DEST_NID in firmware. Serial Monitor (USB) prints every frame as evidence.
 
// --- Node IDs (NID): application-layer addresses, decimal, NOT I2C addresses ---
#define NID_MASTER  0
#define NID_RED     1   // slave A, red LED
#define NID_GREEN   2   // slave B, green LED
 
// --- Frame ---
#define FRAME_START 0xFF
#define CMD_LED     0x03
 
const unsigned long PHASE_MS = 2000;   // 2 s per phase
unsigned long lastSwitch = 0;
bool phaseA = true;
 
void sendFrame(uint8_t dest, uint8_t cmd, uint8_t payload) {
  uint8_t checksum = dest ^ NID_MASTER ^ cmd ^ payload;
  Serial1.write(FRAME_START);
  Serial1.write(dest);
  Serial1.write((uint8_t)NID_MASTER);
  Serial1.write(cmd);
  Serial1.write(payload);
  Serial1.write(checksum);
 
  // Evidence on the USB Serial Monitor
  Serial.print("TX  DEST_NID=");
  Serial.print(dest);
  Serial.print("  CMD=0x");
  Serial.print(cmd, HEX);
  Serial.print("  PAYLOAD=");
  Serial.println(payload);
}
 
void setup() {
  Serial.begin(9600);    // USB, monitor
  Serial1.begin(9600);   // UART bus, TX = GP0 (D6), RX = GP1 (D7, unused here)
  unsigned long t0 = millis();
  while (!Serial && millis() - t0 < 3000) {}
  Serial.println("Master (NID 0) ready. Traffic-light loop on the UART bus.");
}
 
void loop() {
  unsigned long now = millis();
  if (now - lastSwitch >= PHASE_MS) {
    lastSwitch = now;
    if (phaseA) {
      Serial.println("-- phase A: RED on, GREEN off --");
      sendFrame(NID_RED,   CMD_LED, 1);
      sendFrame(NID_GREEN, CMD_LED, 0);
    } else {
      Serial.println("-- phase B: RED off, GREEN on --");
      sendFrame(NID_RED,   CMD_LED, 0);
      sendFrame(NID_GREEN, CMD_LED, 1);
    }
    phaseA = !phaseA;
  }
}