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Configuration and application threads

The Control Thread

As well as the components described in previous sections, an AVB endpoint application needs a thread to control and configure the system. This control thread varies across application and must be implemented by the application designer. To assist in this task a unified control API is presented in Section AVB API. The control thread can pass XC channels connected to other components into the avb_init() function and subsequently use the control API calls to configure the other components.

For an example of how an application can use this API, see the example code walkthrough presented in Section An XMOS AVB application (tutorial).

Legacy Mode

AVB protocols require an AVB compatible switch to function correctly. For testing/demonstration purposes the software platform provides a “legacy” mode which alters the protocols to be non standard but function through non-AVB switches. In this mode the destination addresses of the protocols change to become legacy traffic and some of the PTP protocol behavior changes. In this case:

  • The protocols are non-longer AVB standard so do not work with any other AVB hardware. They will work with other endpoints set to this mode.
  • There is no longer any quality of service guarantee so audio may be disrupted. Furthermore the traffic from the endpoint may disrupt other non-AVB ethernet devices on the network.

Remote Configuration

The AVB standard protocols control the time synchronization, streaming and routing of audio data. However, it is likely that a higher level configuration protocol will be required to configure an AVB endpoint.

Such a protocol needs two parts: discovery and control. The discovery part must determine higher level information about other endpoints on the network. For example:

  • Discover which other Talkers/Listeners are on the network.
  • Discover which streams are available and meta-information about them (sample rate, description, global clock synchronization participation etc.).

The XMOS TCP/IP implementation canbe used with the AVB solution to provide services such as MDNS.

The control part controls the device. For example it controls:

  • The streams the Talker outputs/Listener inputs.
  • Audio input/output (including sample rate, gain, dsp).
  • Other non-audio aspects.

A control stack will link into the control thread to provide a bridge to the local control API. To aid implementation of this, a the XMOS TCP/IP stack can be used. A demonstration control API is provided for the XR-AVB-LC-BRD, for more details see the AtteroTech/XMOS XR-AVB-LC-BRD Quickstart Guide.

TCP/IP Stack

The AVB software solution includes a port of the uIP protocol stack which is a small memory footprint stack that can be used for UDP/TCP communication to aid implementation of an upper layer configuration protocol. For details on this stack see the XTCP Component Guide.


The Zeroconf stack (sometimes known as “Bonjour”) provides:

  • Multicast DNS - Allows endpoints to have a local name related to their IP address.
  • DNS Service Discovery - Allows endpoints to advertize a particular service (such as a configuration/control service over TCP/IP) for other entities on the network to discover.

The stack is configured via the API described in Section MDNS/Bonjour API.