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USB System Architecture (USB 2.0).pdf
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USB System Architecture

Figure 17-17: Single or Multiple Transaction Translators

Upstream Port

Transaction Translator

Downstream Ports

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Periodic Split Transactions

Transactions targeting full-speed isochronous endpoints and lowor full-speed interrupt endpoints are expected to be performed during specific 1ms frames, and are therefore termed periodic transactions. These endpoints may be accessed only one time during a single frame. Hence, the execution of these transactions is critical during the frame in which they have been scheduled by host software. On this basis, the transaction translator treats periodic split transactions with higher priority than the control and bulk split transactions.

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