#define PIN_MOSFET 4
#define PWM_MAX 255

char rcv;
char cmd;
char s_enable = false;
char s_endstop = false;
int s_level = 0;

void setup() {
  // disable prescaler
   TCA0.SPLIT.CTRLA = TCA_SINGLE_ENABLE_bm; 
   pinMode(PIN_MOSFET, OUTPUT);
   digitalWrite(PIN_MOSFET, LOW);
   delay(500);
   Serial.begin(250000);
   // initial flush
   while (Serial.available()) {
      Serial.read();
   }
}

void loop() {
  char cmd;
  char snd;
  if (Serial.available()) {
    rcv = Serial.read();
    cmd = rcv & 0b11;
    if (cmd != 0b00) {
      if (cmd == 0b11) {
        s_enable = !s_enable;
      } else {
        s_level += cmd & 0b01 ? 1 : -1;
        s_level = min(max(s_level, 0), PWM_MAX);
        s_endstop = s_level == PWM_MAX;
      }
      if (s_enable) {
        analogWrite(PIN_MOSFET, s_level);
      } else {
        digitalWrite(PIN_MOSFET, 0);
      }
    }
    snd = ((rcv >> 2) & 0b00111111) | (s_endstop << 6) | (s_enable << 7);
    Serial.write(snd);
  }
}