Updates to documentation (#280)

Documentation updates (including moving of header files

lib_xua/doc/rst/arch.rst

@@ -0,0 +1,92 @@
+
+.. _usb_audio_sec_architecture:
+
+Software Architecture
+---------------------
+
+This section describes the required software architecture of a USB Audio device implemented using `lib_xua`, its dependencies and other supporting libraries.
+
+`lib_xua` provides fundamental building blocks for producing USB Audio products on XMOS devices. Every system is required to have the components from `lib_xua` listed in :ref:`usb_audio_shared_components`.
+
+.. tabularcolumns:: lp{5cm}l
+.. _usb_audio_shared_components:
+.. list-table:: Required XUA Components
+ :header-rows: 1
+ :widths: 40 60
+
+ * - Component
+   - Description
+ * - Endpoint 0
+   - Provides the logic for Endpoint 0 which handles
+     enumeration and control of the device including DFU related requests.
+ * - Endpoint buffer
+   - Buffers endpoint data packets to and from the host. Manages delivery of audio packets between the endpoint buffer
+     component and the audio components. It can also handle volume control processing. Note, this currently utilises two cores
+ * - AudioHub
+   - Handles audio I/O over I2S and manages audio data
+     to/from other digital audio I/O components.
+    
+In addition low-level USB I/0 is required and is provided by the external dependency `lib_xud`
+
+.. tabularcolumns:: lp{5cm}l
+.. list-table:: Additional Components Required
+ :header-rows: 1
+ :widths: 100 60
+
+ * - Component
+   - Description
+ * - XMOS USB Device Driver (XUD)
+   - Handles the low level USB I/O.
+
+In addition :ref:`usb_audio_optional_components` shows optional components that can be added/enabled from within `lib_xua`
+
+.. tabularcolumns:: lp{5cm}l
+.. _usb_audio_optional_components:
+.. list-table:: Optional Components
+ :header-rows: 1
+ :widths: 40 60
+
+ * - Component
+   - Description
+ * - Mixer
+   - Allows digital mixing of input and output channels.  It can also 
+     handle volume control instead of the decoupler.
+ * - Clockgen
+   - Drives an external frequency generator (PLL) and manages
+     changes between internal clocks and external clocks arising
+     from digital input.
+ * - MIDI
+   - Outputs and inputs MIDI over a serial UART interface.
+
+`lib_xua` also provides optional support for integrating with the following external dependencies listed in :ref:`usb_audio_external_components`
+
+.. tabularcolumns:: lp{5cm}l
+.. _usb_audio_external_components:
+.. list-table:: External Components
+ :header-rows: 1
+ :widths: 40 60
+
+ * - Component
+   - Description
+ * - S/PDIF Transmitter (lib_spdif)
+   - Outputs samples of an S/PDIF digital audio interface.
+ * - S/PDIF Receiver (lib_spdif)
+   - Inputs samples of an S/PDIF digital audio interface (requires the
+     clockgen component).
+ * - ADAT Receiver (lib_adat)
+   - Inputs samples of an ADAT digital audio interface (requires the
+     clockgen component).
+ * - PDM Microphones (lib_mic_array)
+   - Receives PDM data from microphones and performs PDM to PCM conversion
+
+.. _usb_audio_threads:
+
+.. figure:: images/threads-crop.*
+      :width: 100%
+ 
+      USB Audio Core Diagram
+
+:ref:`usb_audio_threads` shows how the components interact with each
+other in a typical system.  The green circles represent cores with arrows indicating inter-core communications.
+
+