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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 PAL PAL Driver
+ * @brief   I/O Ports Abstraction Layer
+ * @details This module defines an abstract interface for digital I/O ports.
+ *          Note that most I/O ports functions are just macros. The macros
+ *          have default software implementations that can be redefined in a
+ *          PAL Low Level Driver if the target hardware supports special
+ *          features like, for example, atomic bit set/reset/masking. Please
+ *          refer to the ports specific documentation for details.<br>
+ *          The @ref PAL driver has the advantage to make the access to the I/O
+ *          ports platform independent and still be optimized for the specific
+ *          architectures.<br>
+ *          Note that the PAL Low Level Driver may also offer non standard
+ *          macro and functions in order to support specific features but,
+ *          of course, the use of such interfaces would not be portable.
+ *          Such interfaces shall be marked with the architecture name inside
+ *          the function names.
+ * @pre     In order to use the PAL driver the @p HAL_USE_PAL option
+ *          must be enabled in @p halconf.h.
+ *
+ * @section pal_1 Implementation Rules
+ * In implementing a PAL Low Level Driver there are some rules/behaviors that
+ * should be respected.
+ *
+ * @subsection pal_1_1 Writing on input pads
+ * The behavior is not specified but there are implementations better than
+ * others, this is the list of possible implementations, preferred options
+ * are on top:
+ * -# The written value is not actually output but latched, should the pads
+ *    be reprogrammed as outputs the value would be in effect.
+ * -# The write operation is ignored.
+ * -# The write operation has side effects, as example disabling/enabling
+ *    pull up/down resistors or changing the pad direction. This scenario is
+ *    discouraged, please try to avoid this scenario.
+ * .
+ * @subsection pal_1_2 Reading from output pads
+ * The behavior is not specified but there are implementations better than
+ * others, this is the list of possible implementations, preferred options
+ * are on top:
+ * -# The actual pads states are read (not the output latch).
+ * -# The output latch value is read (regardless of the actual pads states).
+ * -# Unspecified, please try to avoid this scenario.
+ * .
+ * @subsection pal_1_3 Writing unused or unimplemented port bits
+ * The behavior is not specified.
+ *
+ * @subsection pal_1_4 Reading from unused or unimplemented port bits
+ * The behavior is not specified.
+ *
+ * @subsection pal_1_5 Reading or writing on pins associated to other functionalities
+ * The behavior is not specified.
+ *
+ * @ingroup HAL_NORMAL_DRIVERS
+ */