Information

Universal Serial Bus Interface
MPC8308 PowerQUICC II Pro Processor Reference Manual, Rev. 1
Freescale Semiconductor 13-41
If the aging counter is less than the AGE_CNT_THRESH value, priority state zero is chosen. If the aging
counter is greater than or equal to the AGE_CNT_THRESH value, priority state one is chosen.
The aging counter begins to count from zero when a bus access is requested. It increments every bus cycle
until the bus transaction completes. At the completion of a bus transaction, the counter is synchronously
reset to zero. If there are any outstanding bus requests, the aging counter will then begin counting
immediately.
The AGE_CNT_THRESH is compared against the value of the aging counter during each clock cycle of
the current transaction. If AGE_CNT_THRESH is equal to zero, priority state one is always chosen. If the
aging counter is less than the AGE_CNT_THRESH value, priority state zero is selected. If the aging
counter is greater than or equal to the AGE_CNT_THRESH value, priority state one is selected.
The two priority states of the aging counter function each have corresponding register bits which are
programmed by the CPU. Thus, when the aging counter function is at priority state zero,
PRI_CTRL[30–31] are selected and used to drive bus priority levels. When the aging counter function is
at priority state one, PRI_CTRL[28–29] are selected and used to drive the priority.
Figure 13-31 shows the age count threshold register.
Table 13-34 describes the age count threshold register fields.
The setting of AGE_CNT_THRESH is highly dependent on both the mix of other controllers operating on
the system bus as well as the kind of traffic moving through the USB controller. A recommended approach
is first to try leaving the aging mechanism disabled and see if the USB meets performance requirements.
If USB performance does not meet application requirements, try the following settings:
• Set PRI_CTRL[pri_lvl0] to 0.
• Set tPRI_CTRL[pri_lvl1] to 3.
• Set AGE_CNT_THRESH to 40.
This combination works for a wide variety of applications. If this combination still does not meet
application requirements, try lowering AGE_CNT_THRESH by 5. On the contrary, the setting 40 may be
too conservative for some applications. If USB performance is acceptable at 40, try raising the value in
Offset 0x408 Access: Read/Write
0 17 18 31
R
— Threshold
W
Reset All zeros
Figure 13-31. Age Count Threshold (AGE_CNT_THRESH)
Table 13-34. AGE_CNT_THRESH Register Field Descriptions
Bits Name Description
0–17 — Reserved, should be cleared
18–31 Threshold Aging counter threshold value.