#define LED_PIN PA3
#define BTN_PIN PA1

bool ledState = LOW;

void setup() {
  pinMode(LED_PIN, OUTPUT);
  pinMode(BTN_PIN, INPUT_PULLUP);   // button to GND

  digitalWrite(LED_PIN, ledState);
}

void loop() {

  if (digitalRead(BTN_PIN) == LOW) {  // pressed
    ledState = !ledState;
    digitalWrite(LED_PIN, ledState);

    delay(50);  // debounce

    // wait till released
    while (digitalRead(BTN_PIN) == LOW) {
      delay(10);
    }
  }
}