PIC18F2450/4450
14.9.3
TRANSFERS
14.9 Overview of USB
There are four transfer types defined in the USB
specification.
This section presents some of the basic USB concepts
and useful information necessary to design a USB
device. Although much information is provided in this
section, there is a plethora of information provided
within the USB specifications and class specifications.
Thus, the reader is encouraged to refer to the USB
specifications for more information (www.usb.org). If
you are very familiar with the details of USB, then this
section serves as a basic, high-level refresher of USB.
• Isochronous: This type provides a transfer
method for large amounts of data (up to
1023 bytes) with timely delivery ensured;
however, the data integrity is not ensured. This is
good for streaming applications where small data
loss is not critical, such as audio.
• Bulk: This type of transfer method allows for large
amounts of data to be transferred with ensured
data integrity; however, the delivery timeliness is
not ensured.
14.9.1
LAYERED FRAMEWORK
USB device functionality is structured into a layered
framework graphically shown in Figure 14-13. Each
level is associated with a functional level within the
device. The highest layer, other than the device, is the
configuration. A device may have multiple configura-
tions. For example, a particular device may have
multiple power requirements based on Self-Power Only
or Bus Power Only modes.
• Interrupt: This type of transfer provides for
ensured timely delivery for small blocks of data;
plus data integrity is ensured.
• Control: This type provides for device setup
control.
While full-speed devices support all transfer types,
low-speed devices are limited to interrupt and control
transfers only.
For each configuration, there may be multiple
interfaces. Each interface could support a particular
mode of that configuration.
14.9.4
POWER
Below the interface is the endpoint(s). Data is directly
moved at this level. There can be as many as
16 bidirectional endpoints. Endpoint 0 is always a
control endpoint and by default, when the device is on
the bus, Endpoint 0 must be available to configure the
device.
Power is available from the Universal Serial Bus. The
USB specification defines the bus power requirements.
Devices may either be self-powered or bus powered.
Self-powered devices draw power from an external
source, while bus powered devices use power supplied
from the bus.
14.9.2
FRAMES
Information communicated on the bus is grouped into
1 ms time slots, referred to as frames. Each frame can
contain many transactions to various devices and
endpoints. Figure 14-9 shows an example of
transaction within a frame.
a
FIGURE 14-13:
USB LAYERS
Device
To other Configurations (if any)
Configuration
To other Interfaces (if any)
Interface
Endpoint
Interface
Endpoint
Endpoint
Endpoint
Endpoint
DS39760A-page 150
Advance Information
© 2006 Microchip Technology Inc.
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