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a) List Priority
The lists built up by the Host Controller Driver are classified as either periodic or non-
periodic. The Interrupt list and the Isochronous list are periodic because the endpoints on
those lists require service at specific times in a deterministic manner. The Control list and
the Bulk list are non-periodic because endpoints on those lists can tolerate latency and
expect service only on a time-available basis.
The Host Controller breaks the USB frame up into three distinct sections with regard to list
service as shown in Figure 18-2. Section 1 is devoted to the non-periodic lists. This is
followed by Section 2, which is a section reserved for the periodic lists in which both the
Interrupt list and the Isochronous list are serviced to completion. Section 3 of the frame is
again devoted to the non-periodic lists.
•
Periodic Lists
The list priority between the periodic lists is fixed with the Interrupt list having priority
over the Isochronous list. When servicing the periodic lists, the Host Controller is
actually servicing a single list, called the Periodic list, which contains both Interrupt
Endpoint Descriptors and Isochronous Endpoint Descriptors. The Host Controller
Driver ensures that all Interrupt Endpoint Descriptors are placed on the list in front of
any Isochronous Endpoint Descriptors.
•
Non-periodic Lists
The priority algorithm between the non-periodic lists is more complicated than that of
the periodic lists. Control endpoints are given equal or more access to the bus in
comparison with Bulk Endpoints. More specifically, N Control Endpoints are given
access to the bus for every 1 Bulk Endpoint. This ratio of N:1 is termed the Control
Bulk Service Ratio. The Host Controller Driver has control over the Control Bulk
Service Ratio via the ControlBulkServiceRatio field of the HcControl Register. The
range of possible Control Bulk Service Ratios is from 1:1 to 4:1. An example of a 4:1
Control/Bulk Service Ratio is shown in Figure 18-3.
CTRL
ED
CTRL
ED
BULK
ED
CTRL
ED
CTRL
ED
Figure 18.3 Example of Control/Bulk Service Ratio of 4:1
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