// Ersatz Reset Pin Demo
// For upload via Optiboot (for example) wire DTR or RTS to the Ersatz Reset pin

#ifdef __AVR_ATtinyxy2__
  #define ERSATZ_RESET_PIN PIN_PA3
  #define RESET_MASK 1<<3
  #define PINCTRL PORTA.PIN3CTRL
#else
  #define ERSATZ_RESET_PIN PIN_PA4
  // Always arduino pin 0
  #define RESET_MASK 1<<4
  #define PINCTRL PORTA.PIN4CTRL
#endif

// We will always use pin 0 as the ersatz reset pin, EXCEPT on the 8-pin parts.
// We can't use PA4 since it doesn't exist there, and pin 0 is one of the serial pins
// depending on the configuration of USART0, so that would be a bad choice. Instead we
// will use PA3, arduino pin 4, the pin between UPDI and geound on the chip.

void setup() {
  pinMode(LED_BUILTIN, OUTPUT);
  pinMode(ERSATZ_RESET_PIN, INPUT_PULLUP); //on 8-pin parts, this overrides the previous line! No LED blink on these; not enough pins to demo everything here...
  while ((!(VPORTA.IN & RESET_MASK)) && millis() < 10); //wait up to 10 milliseconds for pin to go high...
  // MUCH longer than we need to wait...
  if (!(VPORTA.IN & RESET_MASK)) { //if still not high, reset...
    _PROTECTED_WRITE(RSTCTRL.SWRR, 1);
  } else {
    PINCTRL = PORT_PULLUPEN_bm | PORT_ISC_LEVEL_gc;
  }
}

void loop() {
  digitalWrite(LED_BUILTIN, 1);
  delay(1000);
  digitalWrite(LED_BUILTIN, 0);
  delay(1000);
}

ISR(PORTA_PORT_vect) {
  byte flags = PORTA.INTFLAGS;
  PORTA.INTFLAGS = flags; //clear flags
  if (flags & RESET_MASK) {
    _PROTECTED_WRITE(RSTCTRL.SWRR, 1);
  }
}