Microchip Technology DM183037 Data Sheet

DS30575A-page 550

 2012 Microchip Technology Inc.

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.

27.9.1

LAYERED FRAMEWORK

USB device functionality is structured into a layered
framework, graphically illustrated in 

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 configu-
rations. 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 be 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.

27.9.2

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. See 

 for an example of a

transaction within a frame.

27.9.3

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.

27.9.4

POWER

Power is available from the USB. The USB specifica-
tion 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.

Device

Endpoint

Endpoint

Endpoint

Endpoint

Endpoint

To Other Configurations (if any)

To Other Interfaces (if any)

Configuration

Interface

Interface