Universal Serial Bus InterfaceMCF5253 Reference Manual, Rev. 124-152 Freescale SemiconductorCAUTIONLimiting the number of tracking pipes in the EMBedded—TT to four (4) willimpose the restriction that no more than 4 periodic transactions(INTERRUPT/ISOCHRONOUS) can be scheduled through theembedded-tt per frame. The number 16 was chosen in the USB specificationbecause it is sufficient to ensure that the high-speed to full-speed periodicpipeline can remain full. Keeping the pipeline full puts no constraint on thenumber of periodic transactions that can be scheduled in a frame and theonly limit becomes the flight time of the packets on the bus.— Complete-split transaction searching.NOTEThere is no data schedule mechanism for these transactions other than themicroframe pipeline. The embedded TT assumes the number of packetsscheduled in a frame does not exceed the frame duration (1 msec) or elseundefined behavior may result.24.12.1.5.5 Multiple Transaction TranslatorsThe maximum number of embedded Transaction Translators that is currently supported is one as indicatedby the N_TT field in the HCSPARAMS register. See Section 24.6.2.3, “Host Controller StructuralParameters (HCSPARAMS),” for more information.24.12.2 Device OperationThe co-existence of a device operational controller within the USB OTG module has little effect on EHCIcompatibility for host operation. However, given that the controller is initialized in neither host nor devicemode, the USBMODE register must be programmed for host operation before the EHCI host controllerdriver can begin EHCI host operations.24.12.3 Non-Zero Fields the Register FileSome of the reserved fields and reserved addresses in the capability registers and operational registers haveuse in device mode, the following must be adhered to:• Write operations to all EHCI reserved fields (some of which are device fields in the USB OTGmodule) in the operation registers should always be written to zero. This is an EHCI requirementof the device controller driver that must be adhered to.• Read operations by the module must properly mask EHCI reserved fields (some of which aredevice fields in the USB OTG module registers).24.12.4 SOF InterruptThe SOF interrupt is a free running 125 μsec interrupt for host mode. EHCI does not specify this interrupt,but it has been added for convenience and as a potential software time base. Note that the free runninginterrupt is shared with the device-mode start-of-frame interrupt. See Section 24.6.3.2, “USB Status
