
CHAPTER 14 USBU (UNIVERSAL SERIAL BUS UNIT)
User’s Manual U14579EJ2V0UM
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Figure 14-7 shows how OpenHCI allocates bandwidth. The HC selects the list to process based on a priority order
algorithm. Control/bulk list processing has priority until the value of the FR area of the HcFmRemaining register from
the beginning of the frame is the same as the value of the PS area of the HcPeriodicStart register.
When the value of the FR area is the same as the value of the PS area, periodic list processing has priority. The
periodic list processing priority will be greater than or equal to the control/bulk list processing priority until the periodic
list processing is completed or the frame time has elapsed. After the periodic list processing is completed, the
control/bulk list processing is restarted.
Figure 14-7. Bandwidth Allocation Method
1.0 ms
SOF
NP
Periodic
NP
Time
Remark
SOF: Start of Frame, NP: Non-Periodic Transfer
During interrupt/isochronous list processing, processing is performed from the InterruptED header pointer that is
processed by the current frame. Since the isochronous list is linked after the interrupt list, the interrupt list always has
a higher priority than the isochronous list. During bulk/control list processing, processing is restarted from the location
where it was previously interrupted in each list. When the end of the list is reached, a value is loaded from the header
pointer and processing is resumed. The control end pointer is compared with the bulk end pointer and an equivalent or
higher access right to the bus is assigned. The ratio of the access rights is set in the CBSR area of the HcControl
register. Figure 14-8 shows an example of a 4:1 control bulk service ratio. While control and bulk transfers have
priority, the HC switches the ED processing of each list according to the value in the CBSR area.
Figure 14-8. 4:1 Control Bulk Service Ratio
Control ED
Bulk ED
The control bulk service ratio is maintained across multiple frames. When the processing of one data packet of a
TD included in a given ED is serviced, the HC processes the next ED.