PIC16F1459-ESSQTP datasheet(324/418 Pages) MICROCHIP

324 / 418 page

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

Endpoint

To other Interfaces (if any)

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