> For the complete documentation index, see [llms.txt](https://mwx.twelite.info/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://mwx.twelite.info/latest_en/api-reference/sys_callbacks/begin.md).

# begin()

It is called only once before the first call to the `loop()` function; since TWENET is already initialized, there is no need to consider constraints such as `setup()`.

The main usage is to

* Displaying startup messages
* Writing code for testing
* Sleep transition immediately after startup
* Processing that is inconvenient for `setup()` (radio packet processing, timer operation, etc.)

{% hint style="info" %}
This callback function definition is optional.
{% endhint %}