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PIC18F2450/4450

DS39760A-page 150

Advance Information

2006 Microchip Technology Inc.

14.9

Overview of USB

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.

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.

For

each

configuration,

there

may

multiple

interfaces. Each interface could support a particular

mode of that configuration.

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.

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 a

transaction within a frame.

14.9.3

TRANSFERS

There are four transfer types defined in the USB

specification.

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.

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.

14.9.4

POWER

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.

FIGURE 14-13:

USB LAYERS

Device

Configuration

Interface

Endpoint

Interface

Endpoint

To other Configurations (if any)

To other Interfaces (if any)

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