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5.9 High-Speed, High Bandwidth Endpoints
USB supports individual high-speed interrupt or isochronous endpoints that require data rates up to
192 Mb/s (i.e., 3072 data bytes per microframe). One, two, or three high-speed transactions are allowed in
a single microframe to support high-bandwidth endpoints.
A high-speed interrupt or isochronous endpoint indicates that it requires more than 1024 bytes per
microframe when bits 12..11 of the wMaxPacketSize field of the endpoint descriptor (see Table 5-11) are
non-zero. The lower 11 bits of wMaxPacketSize indicate the size of the data payload for each individual
transaction while bits 12..11 indicate the maximum number of required transactions possible. See
Section 9.6.6 for restrictions on the allowed combinations of values for bits 12..11 and bits 10..0.
Note: This representation means that endpoints requesting two transactions per microframe will specify a
total data payload size in the microframe that is a multiple of two bytes. Also endpoints requesting three
transactions per microframe will specify a total data payload size that is a multiple of three bytes. In any
case, any number of bytes can actually be transferred in a microframe.
The host controller must issue an appropriate number of high-speed transactions per microframe. Errors in
the host or on the bus can result in the host controller issuing fewer transactions than requested for the
endpoint. The first transaction(s) must have a data payload(s) as specified by the lower 11 bits of
wMaxPacketSize if enough data is available, while the last transaction has any remaining data less than or
equal to the maximum size specified. The host controller may issue transactions for the same endpoint one
immediately after the other (as required for the actual data provided) or may issue transactions for other
endpoints in between the transactions for a high bandwidth endpoint.
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