diff --git a/mozc-mageru/README.md b/mozc-mageru/README.md
index 675b76b..74ad84c 100644
--- a/mozc-mageru/README.md
+++ b/mozc-mageru/README.md
@@ -54,13 +54,15 @@ We have tried it with PLA, but it will probably work with ABS as well.
 ## Sensor calibration
 
 1. Open Serial Monitor.
-2. (the home position, which decides when to commit a character)
+2. Send '!' once to disable key output.
+3. (the home position, which decides when to commit a character)
    Bend the sensor slightly and send '0'. Make sure the LED turns off when the spoon is not bent by external force.
-3. (the position for the first letter)
+4. (the position for the first letter)
    Bend the sensor slightly a bit more and send '1'.
-4. (the position for the last letter)
+5. (the position for the last letter)
    Bend the sensor to the end and send '2'.
-5. Send 'w' to save the configuration to EEPROM.
+6. Send 'w' to save the configuration to EEPROM.
+7. Send '!' again to enable key output, or reset the device.
 
 # Wireless Version
 
diff --git a/mozc-mageru/wired/firmware/firmware.ino b/mozc-mageru/wired/firmware/firmware.ino
index 0b8ec15..0c252b9 100644
--- a/mozc-mageru/wired/firmware/firmware.ino
+++ b/mozc-mageru/wired/firmware/firmware.ino
@@ -77,16 +77,12 @@ void LoadDefaultConfig() {
 
 void setup() {
   Serial.begin(115200);
+  Keyboard.begin();
   LoadConfig();
   pinMode(kLedPin, OUTPUT);
   digitalWrite(kLedPin, LOW);
 }
 
-void SetupKeyboard() {
-  enable_key_output = true;
-  Keyboard.begin();
-}
-
 void OutputKey(const char* str) {
   if (!enable_key_output) {
     Serial.print(str);
@@ -152,7 +148,7 @@ void loop() {
 
       // Other debug commands.
       case '!':
-        SetupKeyboard();
+        enable_key_output = !enable_key_output;
         break;
       case ' ':
         // Show sensor status for debugging.