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# Basic Usage

This section describes the basic usage of lib_xua and provides a guide on how to program USB Audio devices.

## Library Structure

The code is split into several directories.

Table 5 `lib_xua` structure
core | Common code for USB audio applications
midi | MIDI I/O code
dfu | Device Firmware Upgrade code
hid | Human Interface Device code

Note, the `midi` and `dfu` directories are potential candidates for separate libs in their own right.

## Using in an application

`lib_xua` is intended to be used with XCommon CMake , the XMOS application build and dependency management system.

To use `lib_xua` in an application, add `lib_xua`, to the list of dependent modules in the application’s CMakeLists.txt file.

> set(APP_DEPENDENT_MODULES “lib_xua”)

All lib_xua functions can be accessed via the `xua.h` header file:

    #include <xua.h>

## “Codeless” programming model

While it is possible to build a USB Audio device from the low-level components provided by `lib_xua`, this approach may not be suitable for all customers or product types.

For instance, some users may not have a large software development experience and simply want to customise some basic settings such as strings, sample-rates, channel-counts etc. Others may want to fully customise the implementation - adding additional functionality such as integrating DSP or possibly only using a subset of the functions provided - just `XUA_AudioHub`, for example.

In addition, the large number of supported features can lead to a large number of tasks, hardware resources, communication channels etc, requiring quite a lot of code to be authored for each product.

In order to cater for the former class of users, a “codeless” option is provided. Put simply, a file `main.xc` is provided which includes a pre-authored `main()` function along with all of the required hardware resource declarations. Code is generated based on the options provided by the developer in `xua_conf.h`.

Using this development model the user simply authors a `xua_conf.h` with their settings and optional implementations of any ‘user functions’ as desired. This, along with an XN file for their hardware platform, is all that is required to build a fully featured and functioning product. This XN file should contain definitions of the ports used for the various `lib_xua` functionality, see Options.

This development model also provides the benefit of a full and verified codebase as a basis for a product.

This behaviour described in this section is the default behaviour of `lib_xua`, to disable this set `EXCLUDE_USB_AUDIO_MAIN` to 1 in the application CMakeLists.txt or `xua_conf.h` and see Advanced usage.

## Configuring lib_xua

Configuration of the various build time options of `lib_xua` is done via the optional header `xua_conf.h`. To allow the build system to locate this file it should reside somewhere in the application src directory.

Such build time options include audio class version, sample rates, channel counts etc. See API reference for full listings.

The build system will automatically include the `xua_conf.h` header file as appropriate - the developer should continue to include `xua.h` as previously directed. A simple example `xua_conf.h` file is shown below:

    #ifndef _XUA_CONF_H_
    #define _XUA_CONF_H_

    /* Output channel count */
    #define XUA_NUM_USB_CHAN_OUT (2)

    /* Product string */
    #define XUA_PRODUCT_STR_A2 "My Product"

    #endif

## User functions

To enable custom functionality, such as configuring external audio hardware, bespoke behaviour on stream start/stop etc, various functions can be overridden by the user. (see API reference for full listings). The default implementations of these functions are empty.

## Extending the “Codeless” application

The `main.xc` function allows insertion of extra code using the preprocessor. For example, you may wish to add some control code to control buttons or LEDs or DSP tasks for audio enhancement.

### Adding Globals

An optional header file named `xua_conf_globals.h` can be added to the project.

If this file exists in the project’s source tree, its contents will be inserted into `main.xc` at global scope.

Example contents of `xua_conf_globals.h`:

    unsigned my_global_var = 42;

This file may also be used to include additional headers or declare function prototypes, for example:

    #include "my_header.h"
    void my_function(int x);

This allows these functions and variables to be available for use in custom code inserted by the following two sections (e.g. user-defined tasks or initialisation).

### Adding main function declarations

To add declarations to the application - such as channels or interfaces for communication between tasks — the define `USER_MAIN_DECLARATIONS` can be set in the `xua_conf.h` header file. This inserts code into `main.xc` after the `main()` definition but before the main `par` statement.

Alternatively, an optional header file may be added to the project called `xua_conf_declarations.h`. If this file exists in the project’s source tree, its contents will be inserted into `main.xc` before the main `par` statement.

Example contents of `xua_conf_declarations.h`:

    chan c_usb_to_user_interface;

### Adding main function tasks

To add extra tasks to the application, the define `USER_MAIN_TASKS` can be set in the `xua_conf.h` header file. This will insert code into `main.xc` after the main `par` statement, allowing the compiler to run these tasks in parallel — either on a dedicated hardware thread or combined with other tasks if marked as `[[combinable]]`.

Alternatively, an optional header file called `xua_conf_tasks.h` can be added to project. If this file exists anywhere in the project source tree, its contents will be inserted into `main.xc` after the main `par` statement.

Example contents of `xua_conf_tasks.h`:

    on tile[1]: my_user_interface_task(c_usb_to_user_interface);