diff --git a/XtKeyboard.ino b/XtKeyboard.ino
index 9e7ea18..758524f 100644
--- a/XtKeyboard.ino
+++ b/XtKeyboard.ino
@@ -5,21 +5,36 @@
 
 int clockPin = 2;
 int dataPin = 3;
-byte data = B0;
+volatile byte data = B0;
 int test = 0;
-int counter = 0;
+volatile int counter = 0;
 int numbits = 10;
 int speakerPin = 5;
-int keyReleaseBit = 0x80;
+int keysPressed = 0;
+unsigned long repeatTimeout = 0;
+unsigned long repeatTimeoutValue = 1000;
+const int keyReleaseBit = 0x80;
 
 char m[255];
+bool pressed[255];
 
-// MODS
-// play speaker sounds of keys
-const bool modKeySounds = true;
-// replace CapsLock with Super key (aka GUI or Windows key)
+// MODS >>>
+// [1] send debug scancode information to serial port
+bool modConsoleLog = true;
+const int modConsoleLogKey = 59; // F1
+// [2] play speaker sounds of keys
+bool modKeySounds = false;
+const int modKeySoundsKey = 60; // F2
+// [3] replace CapsLock with Super key (aka GUI or Windows key)
 const bool modSuperCapslock = true;
+// [4] stuck key guard: release all the pressed keys if no data comes from the keyboard in a while
+bool modStuckKeyGuard = false;
+// [13] mod switcher: enable changing of active mods!
+const bool modModSwitcher = true;
+const int modSwitcherKey = 55; // PRTSC *
+// <<< MODS
 
+// https://www.arduino.cc/en/Reference/KeyboardModifiers
 void setupKeyMapping() {
   m[1]=KEY_ESC;
   m[2]='1';
@@ -128,24 +143,62 @@ char translateKeyToChar(int key) {
 }
 
 void printChar(char keyChar) {
-  Serial.print("'"); Serial.print(keyChar); Serial.print("' (");  Serial.print(int(keyChar)); Serial.println(")");
+  Serial.print("'"); Serial.print(keyChar); Serial.print("' (");  Serial.print(int(keyChar)); Serial.print("), Pressed keys: ["); Serial.print(keysPressed); Serial.println("]");
 }
 
 void processKbdByte(byte data) {
   bool isRelease = data & keyReleaseBit;
   int key = data - (isRelease ? keyReleaseBit : 0);
+  if (modConsoleLog) {
    Serial.print("Scancode: <"); Serial.print(int(data)); Serial.print("> ");
    Serial.print("Key: <"); Serial.print(int(key)); Serial.print("> ");
+  }
   char keyChar = translateKeyToChar(key);
   
   if (isRelease) {
     Keyboard.release(keyChar);
-    Serial.print("Release: ");
-    printChar(keyChar);
+    if (pressed[key]) {
+      pressed[key] = false;
+      keysPressed--;
+    }
+    if (modConsoleLog) {
+      Serial.print("Release: ");
+      printChar(keyChar);
+    }
   } else {
     Keyboard.press(keyChar);
-    Serial.print("Press: ");
-    printChar(keyChar);
+    if (!pressed[key]) {
+      pressed[key] = true;
+      keysPressed++;
+    }
+    if (modConsoleLog) {
+      Serial.print("Press: ");
+      printChar(keyChar);
+    }
+  }
+}
+
+void clearPressedKeys() {
+  Keyboard.releaseAll();
+  keysPressed = 0;
+  for (int i = 0; i < sizeof(pressed); i++) {
+    pressed[i] = false;
+  }
+  Serial.println("Pressed keys list cleared."); 
+}
+
+void clockInterrupt() {
+  int clockValue = digitalRead(clockPin);
+  if (clockValue == LOW && test == 0 && counter < numbits) {
+    test = 1;
+    data = data >> 1;
+    if (digitalRead(dataPin) == HIGH) {
+      bitSet(data, 7);
+    }
+    counter++;
+  }
+  if (clockValue == HIGH && test == 1) {
+    test = 0;
   }
 }
 
@@ -153,28 +206,46 @@ void setup() {
  setupKeyMapping();
 
  pinMode(dataPin, INPUT);
- pinMode(clockPin, INPUT);
+ pinMode(clockPin, INPUT_PULLUP);
+ 
  Serial.begin(9600);
+ 
  Keyboard.begin();
+ clearPressedKeys();
+
+ attachInterrupt(digitalPinToInterrupt(clockPin), clockInterrupt, CHANGE);
 }
 
 void loop() {
- if(digitalRead(clockPin) == LOW && test == 0 && counter < numbits) {
-   test = 1;
-   data = data >> 1;
-   if(digitalRead(dataPin) == HIGH) {
-     bitSet(data, 7);
-   }
-   counter++;
- }
- if(digitalRead(clockPin) == HIGH && test == 1) {
-   test = 0;
- }
  if(counter >= numbits) {
-   //Serial.println(int(data));
    processKbdByte(data);
    if (modKeySounds) tone(speakerPin, 60 + int(data), 25);
    data = B0;
    counter = 0;
+   repeatTimeout = millis() + repeatTimeoutValue;
+ }
+ if (modStuckKeyGuard && keysPressed > 0) {
+   unsigned long timeNow = millis();
+   if (repeatTimeout < timeNow) {
+    if (modConsoleLog) Serial.println("Stuck key detected!");
+    clearPressedKeys();
+   }
+ }
+ if (modModSwitcher && pressed[modSwitcherKey]) {
+  int fired = 0;
+  if (pressed[modConsoleLogKey]) {
+    modConsoleLog = !modConsoleLog;
+    fired = 1;
+  }
+  if (pressed[modKeySoundsKey]) {
+    modKeySounds = !modKeySounds;
+    fired = 2;
+  }
+  if (fired > 0) {
+    tone(speakerPin, 400 + fired * 20, 300);
+    Serial.print("Metakey fired: "); Serial.println(fired);
+    clearPressedKeys();
+    delay(300);
+  }
  }
 }