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diff --git a/os/hal/dox/hal_serial_usb.dox b/os/hal/dox/hal_serial_usb.dox
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+/*
+    ChibiOS - Copyright (C) 2006..2018 Giovanni Di Sirio
+
+    Licensed under the Apache License, Version 2.0 (the "License");
+    you may not use this file except in compliance with the License.
+    You may obtain a copy of the License at
+
+        http://www.apache.org/licenses/LICENSE-2.0
+
+    Unless required by applicable law or agreed to in writing, software
+    distributed under the License is distributed on an "AS IS" BASIS,
+    WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+    See the License for the specific language governing permissions and
+    limitations under the License.
+*/
+
+/**
+ * @defgroup SERIAL_USB Serial over USB Driver
+ * @brief   Serial over USB Driver.
+ * @details This module implements an USB Communication Device Class
+ *          (CDC) as a normal serial communication port accessible from
+ *          the device application.
+ * @pre     In order to use the USB over Serial driver the
+ *          @p HAL_USE_SERIAL_USB option must be enabled in @p halconf.h.
+ *
+ * @section usb_serial_1 Driver State Machine
+ * The driver implements a state machine internally, not all the driver
+ * functionalities can be used in any moment, any transition not explicitly
+ * shown in the following diagram has to be considered an error and shall
+ * be captured by an assertion (if enabled).
+ * @dot
+  digraph example {
+    rankdir="LR";
+    node [shape=circle, fontname=Helvetica, fontsize=8, fixedsize="true",
+          width="0.9", height="0.9"];
+    edge [fontname=Helvetica, fontsize=8];
+
+    uninit [label="SDU_UNINIT", style="bold"];
+    stop [label="SDU_STOP\nLow Power"];
+    ready [label="SDU_READY\nClock Enabled"];
+
+    uninit -> stop [label=" sduInit()"];
+    stop -> stop [label="\nsduStop()"];
+    stop -> ready [label="\nsduStart()"];
+    ready -> stop [label="\nsduStop()"];
+    ready -> ready [label="\nsduStart()"];
+    ready -> ready [label="\nAny I/O operation"];
+  }
+ * @enddot
+ *
+ * @ingroup HAL_COMPLEX_DRIVERS
+ */