Datasheet

PIC16(L)F1454/5/9
DS41639A-page 324 Preliminary  2012 Microchip Technology Inc.
26.10 USB Operation Overview
This section presents some of the basic USB concepts
and useful information necessary to design a USB
device. Although a lot of information is provided in this
section, refer to the USB 2.0 specification for more
details, as needed.
26.10.1 LAYERED FRAMEWORK
USB device functionality is structured into a layered
framework, graphically shown in Figure 26-11. 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 configurations. 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. 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.
26.10.2 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 26-7 shows an example of a
transaction within a frame.
26.10.3 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 device setup control.
While full-speed devices support all transfer types, low-
speed devices are limited to interrupt and control
transfers only.
FIGURE 26-11: USB LAYERS
To other Configurations (if any)
Device
Configuration
Interface
Interface
Endpoint Endpoint Endpoint
Endpoint Endpoint Endpoint
To other Interfaces (if any)