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5.6.4 Isochronous Transfer Bus Access Constraints
Isochronous transfers can only be used by full-speed and high-speed devices.
The USB requires that no more than 90% of any frame be allocated for periodic (isochronous and interrupt)
transfers for full-speed endpoints. High-speed endpoints can allocate at most 80% of a microframe for
periodic transfers.
An isochronous endpoint must specify its required bus access period. Full-/high-speed endpoints must
specify a desired period as (2bInterval-1) x F, where bInterval is in the range one to (and including) 16 and F is
125 μs for high-speed and 1ms for full-speed. This allows full-/high-speed isochronous transfers to have
rates slower than one transaction per (micro)frame. However, an isochronous endpoint must be prepared to
handle poll rates faster than the one specified. A host must not issue more than 1 transaction in a
(micro)frame for an isochronous endpoint unless the endpoint is high-speed, high-bandwidth (see below).
An isochronous IN endpoint must return a zero-length packet whenever data is requested at a faster interval
than the specified interval and data is not available.
A high-speed endpoint can move up to 3072 bytes per microframe (or 192 Mb/s). A high-speed
isochronous endpoint that requires more than 1024 bytes per period is called a high-bandwidth endpoint. A
high-bandwidth endpoint uses multiple transactions per microframe. A high-bandwidth endpoint must
specify a period of 1x125 μs (i.e., a bInterval value of 1). See Section 5.9 for more information about the
details of multiple transactions per microframe for high-bandwidth high-speed endpoints.
