diff --git a/core/HardwareSerial.cpp b/core/HardwareSerial.cpp
index 7855472..bd5d935 100644
--- a/core/HardwareSerial.cpp
+++ b/core/HardwareSerial.cpp
@@ -62,7 +62,7 @@ size_t HardwareSerial::write(const uint8_t *buffer, size_t size)
 
 void HardwareSerial::begin(void)
 {
-    /* Do nothing. Use RT-Thread default serial configurations. */
+    /* Do nothing. Use RT-Thread default configurations: typically 115200, 8N1 */
 }
 
 void HardwareSerial::begin(uint32_t baud)
diff --git a/libraries/buildin/USBSerial/USBSerial.cpp b/libraries/buildin/USBSerial/USBSerial.cpp
index 02f2150..f5a9f27 100644
--- a/libraries/buildin/USBSerial/USBSerial.cpp
+++ b/libraries/buildin/USBSerial/USBSerial.cpp
@@ -13,19 +13,26 @@
 #include <Arduino.h>
 #include "USBSerial.h"
 
-void USBSerial::begin(uint32_t baud)
+void USBSerial::begin(void)
 {
+    /* Do nothing. Use RT-Thread default configurations: typically 115200, 8N1 */
+}
 
+void USBSerial::begin(uint32_t baud)
+{
+    /* TODO */
 }
 
 void USBSerial::begin(uint32_t baud, uint8_t config)
 {
-
+    /* TODO */
 }
 
 void USBSerial::end(void)
 {
-
+    // Nothing to do.
+    // RTduino has not right to close RT-Thread devices.
+    // This could cause other threads make errors.
 }
 
 /*Code to display letter when given the ASCII code for it*/
diff --git a/libraries/buildin/USBSerial/USBSerial.h b/libraries/buildin/USBSerial/USBSerial.h
index 32d3386..0558f86 100644
--- a/libraries/buildin/USBSerial/USBSerial.h
+++ b/libraries/buildin/USBSerial/USBSerial.h
@@ -16,6 +16,7 @@
 class USBSerial: public Stream
 {
 public:
+    void begin(void);
     void begin(uint32_t baud);
     void begin(uint32_t baud, uint8_t config);
     void end(void);