NXP Semiconductors QorIQ LS1012A Reference Manual

32.3.35 Age Count Threshold (AGE_CNT_THRESH)32.3.35.1 OffsetRegister OffsetAGE_CNT_THRESH 408h32.3.35.2 FunctionNote that this register uses big-endian byte ordering and is not defined in the EHCIspecification. The age count threshold (AGE_CNT_THRESH) register provides the agingcounter threshold value used to determine the priority state of the USB controller'sinternal system interface. The threshold value is in units of platform clock cycles. Thisregister should be written during system initialization or during normal system operationwhen the system bus interface is idle. It can be read at any time.If the aging counter is less than the AGE_CNT_THRESH value, default (low) priority ischosen. If the aging counter is greater than or equal to the AGE_CNT_THRESH value>and USB_PRI_CTRL[PRI_LVL] = 1, an elevated priority is chosen.The aging counter begins to count from zero when a bus access is requested. Itincrements every bus cycle until the bus transaction completes. At the completion of abus transaction, the counter is synchronously reset to zero. If there are any outstandingbus requests, the aging counter will then begin counting immediately.The AGE_CNT_THRESH is compared against the value of the aging counter duringeach clock cycle of the current transaction. If AGE_CNT_THRESH is equal to zero, anelevated priority is always chosen. If the aging counter is less than theAGE_CNT_THRESH value, default (low) priority is selected. If the aging counter isgreater than or equal to the AGE_CNT_THRESH value andUSB_PRI_CTRL[PRI_LVL] = 1, an elevated priority is chosen.The setting of AGE_CNT_THRESH is highly dependent on both the mix of othercontrollers operating on the system bus as well as the kind of traffic moving through theUSB controller. A recommended approach is first to try leaving the aging mechanismdisabled and see if the USB meets performance requirements. If USB performance doesnot meet application requirements, try the following setting :Chapter 32 Universal Serial Bus Interface 2.0QorIQ LS1012A Reference Manual, Rev. 1, 01/2018NXP Semiconductors 1961