Freescale Semiconductor MCF52277 Reference Manual page 433

— QH.EPS equals port speed
• Transactions to a device downstream from direct attached FS hub.
— QH.EPS equals downstream device speed
When QH.EPS equals 01 (LS) and PORTSCn[PSPD] equals 00 (FS), a
LS-pre-PID is sent before transmitting LS traffic.
Maximum packet size must equal 64 or less to prevent undefined behavior.
2. siTD (for direct attach FS) – Periodic (ISO endpoint)
• All FS ISO transactions:
— Hub address equals 0
— siTD.EPS equals 00 (full speed)
Maximum packet size must equal to 1023 or less to prevent undefined behavior.
20.5.5.1.5 Operational Model
The operational models are well defined for the behavior of the transaction translator (see USB 2.0
specification) and for the EHCI controller moving packets between system memory and a USB-HS hub.
Because the embedded transaction translator exists within the USB host controller, no physical bus
between EHCI host controller driver and the USB FS/LS bus. These sections briefly discuss the
operational model for how the EHCI and transaction translator operational models combine without the
physical bus between. The following sections assume the reader is familiar with the EHCI and USB 2.0
transaction translator operational models.
Microframe Pipeline
The EHCI operational model uses the concept of H-frames and B-frames to describe the pipeline between
the host (H) and the bus (B). The embedded transaction translator uses the same pipeline algorithms
specified in the USB 2.0 specification for a hub-based transaction translator.
All periodic transfers always begin at B-frame 0 (after SOF) and continue until the stored periodic transfers
are complete. As an example of the microframe pipeline implemented in the embedded transaction
translator, all periodic transfers that are tagged in EHCI to execute in H-frame 0 are ready to execute on
the bus in B-frame 0.
When programming the S-mask and C-masks in the EHCI data structures to schedule periodic transfers
for the embedded transaction translator, the EHCI host controller driver must follow the same rules
specified in EHCI for programming the S-mask and C-mask for downstream hub-based transaction
translators.
After periodic transfers are exhausted, any stored asynchronous transfer is moved. Asynchronous transfers
are opportunistic because they execute when possible and their operation is not tied to H-frame and
B-frame boundaries with the exception that an asynchronous transfer cannot babble through the SOF (start
of B-frame 0.)
Freescale Semiconductor
NOTE
MCF52277 Reference Manual, Rev. 1
Universal Serial Bus Interface – On-The-Go Module
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