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PIC18F2450/4450

2006 Microchip Technology Inc.

Advance Information

DS39760A-page 151

The USB specification limits the power taken from the

bus. Each device is ensured 100 mA at approximately 5V

(one-unit load). Additional power may be requested, up

to a maximum of 500 mA. Note that power above a one-

unit load is a request and the host or hub is not obligated

to provide the extra current. Thus, a device capable of

consuming more than a one-unit load must be able to

maintain a low-power configuration of a one-unit load or

less, if necessary.

The USB specification also defines a Suspend mode.

In this situation, current must be limited to 500

μA,

averaged over 1 second. A device must enter a

Suspend state after 3 ms of inactivity (i.e., no SOF

tokens for 3 ms). A device entering Suspend mode

must drop current consumption within 10 ms after

Suspend. Likewise, when signaling a wake-up, the

device must signal a wake-up within 10 ms of drawing

current above the Suspend limit.

14.9.5

ENUMERATION

When the device is initially attached to the bus, the host

enters an enumeration process in an attempt to identify

the device. Essentially, the host interrogates the device,

gathering information such as power consumption, data

rates

and

sizes,

protocol

and

other

descriptive

information; descriptors contain this information. A

typical enumeration process would be as follows:

USB Reset: Reset the device. Thus, the device

is not configured and does not have an address

(address 0).

Get Device Descriptor: The host requests a

small portion of the device descriptor.

USB Reset: Reset the device again.

Set Address: The host assigns an address to the

device.

Get Device Descriptor: The host retrieves the

device descriptor, gathering info such as

manufacturer, type of device, maximum control

packet size.

Get configuration descriptors.

Get any other descriptors.

Set a configuration.

The exact enumeration process depends on the host.

14.9.6

DESCRIPTORS

There are eight different standard descriptor types of

which five are most important for this device.

14.9.6.1

Device Descriptor

The device descriptor provides general information,

such as manufacturer, product number, serial number,

the class of the device and the number of configurations.

There is only one device descriptor.

14.9.6.2

Configuration Descriptor

The configuration descriptor provides information on

the power requirements of the device and how many

different interfaces are supported when in this

configuration. There may be more than one configura-

tion for a device (i.e., low-power and high-power

configurations).

14.9.6.3

Interface Descriptor

The

interface

descriptor

details

the

number of

endpoints used in this interface, as well as the class of

the interface. There may be more than one interface for

a configuration.

14.9.6.4

Endpoint Descriptor

The endpoint descriptor identifies the transfer type

(Section 14.9.3 “Transfers”) and direction, as well as

some other specifics for the endpoint. There may be

many endpoints in a device and endpoints may be

shared in different configurations.

14.9.6.5

String Descriptor

Many of the previous descriptors reference one or

more string descriptors. String descriptors provide

human

readable

information

about

the

layer

(Section 14.9.1

“Layered

Framework”)

they

describe. Often these strings show up in the host to

help the user identify the device. String descriptors are

generally optional to save memory and are encoded in

a unicode format.

14.9.7

BUS SPEED

Each USB device must indicate its bus presence and

speed to the host. This is accomplished through a

1.5 k

Ω resistor which is connected to the bus at the

time of the attachment event.

Depending on the speed of the device, the resistor

either pulls up the D+ or D- line to 3.3V. For a low-

speed device, the pull-up resistor is connected to the

D- line. For a full-speed device, the pull-up resistor is

connected to the D+ line.

14.9.8

CLASS SPECIFICATIONS AND

DRIVERS

USB specifications include class specifications which

operating

system

vendors

optionally

support.

Examples of classes include Audio, Mass Storage,

Communications and Human Interface (HID). In most

cases, a driver is required at the host side to ‘talk’ to the

USB device. In custom applications, a driver may need

to be developed. Fortunately, drivers are available for

most common host systems for the most common

classes of devices. Thus, these drivers can be reused.

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