USB System Architecture

The Feedback/Feed Forwarding Solution

In the previous example, the sink may be able to make the isochronous to synchronous conversion depending on its synchronization type. The sink device could potentially reconstruct the source sample clock by averaging the number of samples within the frame or microframe. In other cases, the synchronization type may make it necessary to exchange information between the source and sink in order to establish a synchronous connection.

Feedback and feed forwarding techniques are used as a way for information to be passed between the source and sink, allowing them to establish a synchronous connection. Whether feedback or feed forwarding is used depends on the synchronization type defined by the endpoint.

Synchronization Types

The specification defines three synchronization types for devices that need to establish a synchronous connection with another device. These are:

•Asynchronous — Endpoints that send or receive data at a rate that is locked to an external clock or a free running internal clock. The device cannot synchronize the transfer rate to the USB clock (based on the 1ms Start of Frame).

•Synchronous — Endpoints whose isochronous endpoints can be locked to the SOF clock.

•Adaptive — These devices have a specific range at which they can source or receive data. The actual rate used is based on the desired data rate.

Table 6-3 on page 129 lists the synchronization types for both source and sink and specifies whether the corresponding endpoint uses feedback or feed forwarding to establish a synchronous connection.

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