[转]USB之Part 4 - Protocol

Controlling a Device

Before we go into detail, we need to look at how the host recognises and installs a device when you plug it in. We need to do this in general terms without getting bogged down with the detail.

When you plug a USB device in, the host becomes aware (because of the pullup resistor on one data line), that a device has been plugged in.

The host will now send a request to endpoint 0 of device address 0 to find out its maximum packet size. It can discover this by using the Get Descriptor (Device) command. This request is one which the device must respond to even on address 0.

Typically (i.e. with Windows) the host will now reset the device again. It then sends a Set Address request, with a unique address to the device at address 0. After the request is completed, the device assumes the new address. (And at this point the host is now free to reset other recently plugged-in devices.)

Typically the host will now begin to quiz the device for as many details as it feels it needs. Some requests involved here are:

• Get Device Descriptor
• Get Configuration Descriptor
• Get String Descriptor

At the moment the device is in an addressed but unconfigured state, and is only allowed to respond to standard requests.

Once the host feels it has a clear enough picture of what the device is, it will load a suitable device driver.

The device driver will then select a configuration for the device, by sending a Set Configuration request to the device.

The device is now in the configured state, and can start working as the device it was designed to be. From now on it may respond to device specific requests, in addition to the standard requests which it must continue to support.

We saw in the last chapter that data is transfered in 4 different types of transfer:

• Control Transfers
• Interrupt Transfers
• Bulk Transfers
• Isochronous Transfers

The only transfer type available before the device has been configured is the Control Transfer. The only endpoint available at this time is the bidirectional Endpoint 0.

Configurations, Interfaces, and Endpoints.

The device contains a number of descriptors (as shown to the right) which help to define what the device is capable of. We will examine these descriptors further down the page. For the moment we need to have an idea what the configurations, interfaces and endpoints are and how they fit together.

A device can have more than one configuration, though only one at a time, and to change configuration the whole device would have to stop functioning. Different configurations might be used, for example, to specify different current requirements, as the current required is defined in the configuration descriptor.

However it is not common to have more than one configuration. Windows standard drivers will always select the first configuration so there is not a lot of point.

A device can have one or more interfaces. Each interface can have a number of endpoints and represents a functional unit belonging to a particular class.

Each endpoint is a source or sink of data.

For example a VOIP phone might have one audio class interface with 2 endpoints for transferring audio in each direction, plus a HID interface with a single IN interrupt endpoint, for a built in keypad.

It is also possible to have alternative versions of an interface, and this is more common than multiple configurations. In the VOIP phone example, the audio class interface might offer an alternative with a different audio rate. It is possible to switch an interface to an alternate while the device remains configured.

The SETUP Packet

The Standard requests are all conveyed using control transfers to endpoint 0. Remember that a control transfer starts with a SETUP transaction which conveys 8 bytes. These 8 bytes define the request from the host.

The structure of bmRequestType makes it easy to use it to switch on when your firmware is trying to interpret the setup request. Essentially, when the SETUP arrives, you need to branch to the handler for the particular request, so for example bits 6:5 allow you to distinguish the mandatory standard commands, from any class or vendor commands you may have implemeted for you particular device.

Switching on bit 7 allows you to deal with IN and OUT direction requests in separate areas of the code.

The meaning of the 8
bytes of the SETUP transaction data, which are divided into five
named fields.

GET_DESCRIPTOR

It is probable that this
request (with the descriptor type set to Device) will be
the first that will be received after USB reset. The host needs
to know the max packet length in use by the control endpoint and
this information is available in the 8th byte of the device descriptor.

Typically when the host
is Windows, the device will receive the request with the required
length wLength set to 64. The host will then input 1 packet,
and then reset the device again. Whatever the value of the max packet
length, the host now has the value of the 8th byte and knows what
the packet size is for all future control transfers.

The second reset is probably
to guarantee that the device does not get confused after not being
allowed to complete the transmission of all 18 bytes of the device
descriptor.

Table
of wValues use in Get Descriptor requests to select the required
descriptor.

Device Descriptor

This descriptor will
most likely be the first one fetched by the host. We should point
out some important features.

bLength and bDescriptorType

All descriptors start
with a single byte specifying the descriptor‘s length, and this
is always followed by a single byte defining the descriptor type.

bcdUSB

The only valid version
numbers are 0x0100 (USB1.0), 0x0110 (USB1.1) and 0x0200 (USB2.0).
If you design a new device it should be identified as USB2.0 because
that is the current specification.

bDeviceClass, bDeviceSubClass
and bDeviceProtocol

This triplet of values
is used to describe the class of the device in various ways as defined
in the various class specification documents from the USB-IF.

idVendor, idProduct
and bcdDevice

The combination of idVendor
and idProduct (also known as the VID and PID) must be unique for
the device. This means that the VID you use must be one issued by
the USB-IF and which you have the right to use. You can either buy
a VID from the USB-IF, or you may be able to acquire the right to
use a VID from another manufacturer together with a particular PID
which they have issued to you. If you use a VID/PID combination
which is already in use then you will probably have major problems
with your product in the field.

Device
Descriptor

SET_ADDRESS

After the host has determined
the max packet size for endpoint 0, it is in a position to begin
normal communications with the device. As mentioned above, there
may be a second reset from the host. The host now needs to issue
a SET_ADDRESS request to the device, so that each device on the
bus has a unique address to respond to.

SET_ADDRESS is a simple,
outward direction request in a control transfer with no data stage.
The only useful information carried in the SETUP packet is the required
address.

When implementing this
request in firmware, you should note the following. All other requests
must be actioned before the status stage in completed. But in the
case of SET_ADDRESS, you should not change the device address until
after the status stage. The status stage will not succeed
unless the device is still responding to address 0 while it is taking
place. The device then has 2ms to get ready to respond to the new
address.

Other Information Gathering
Commands

The host is likely to
start using the GET_DESCRIPTOR request mentioned above, to fetch
other information describing the device. A major piece of this information
is the configuration descriptor.

Get Descriptor (Configuration)

The Get Descriptor (Configuration)
warrants special explanation, because the request results in not
just a Configuration Descriptor being returned, but also some or
all of a number of other descriptors:

• Interface Descriptor
• Endpoint Descriptor
• OTG Descriptor
• Class-specific Descriptors
• Vendor-specific Descriptors

A Get Configuration Descriptor
fetches the descriptors for just one configuration depending on
the descriptor index in wValue of the SETUP packet. Most devices
only have one configuration, because built-in Windows drivers always
select the first configuration.

The diagram opposite
shows a typical set of Descriptors which is fetched. It starts with
the configuration descriptor, and the vertical position shows the
correct sequence, with the interfaces being dealt with in turn,
each one followed by its own endpoints.

The position of class
descriptors is defined in the appropriate class specification, and
of course vendors descriptor positions would be up to the vendor
concerned.

An OTG descriptor position
is not defined but typically appears immediately after the configuration
descriptor.

Configuration Descriptor

The configuration descriptor
format is shown to the right.

The wTotalLegth value
is important because it tells the host how many bytes are contained
in this descriptor and all the descriptors which follow.

bNumInterfaces describes
how many interfaces this configuration supports.

Configuration
Descriptor

Interface Descriptor

The interface descriptor
format is shown to the right.

bAlternateSetting needs
some explanation. An interface can have more than one variant, and
these variants can be switched between, while other interfaces are
still in operation.

For the first (and default)
alternative bAlternateSetting is always 0.

To have a second interface
variant, the default interface descriptor would be followed by its
endpoint descriptors, which would then be followed by the alternative
interface descriptor and then its endpoint descriptors.

bInterfaceClass, bInterfaceSubClass
and bInterfaceProtocol

By defining the class,
subclass and protocol in the interface, it is possible to have interfaces
with different classes in the same device. This is referred to as
a composite device.

Interface
Descriptor

Endpoint Descriptor

The endpoint descriptor
format is shown to the right.

Endpoint
Descriptor

Get Descriptor (String)

There are several strings
which a host may request. The strings defined in the device descriptor
are:

• Manufacturer String
• Product String
• Serial Number String

These strings are optional.
If not supported, the corresponding index in the device descriptor
will be 0. Otherwise the host may use the specified index in a Get
Descriptor (String) request to fetch the descriptor.

Get Descriptor (String),
with a descriptor index of 0 in the low byte of wValue, is used
to fetch a special string language descriptor. This contains a series
of 2-byte sized language specifiers. In theory, if the language
of your choice is supported in this list, you can use the index
to this language ID to access the string descriptors in this language
by specifying this in wIndex of the Get Descriptor (String) request.
In practise, with Windows, you will have difficulties if you do
not ensure that the first language specified is English (US).

String
Descriptor Zero

(Specifies
supported string languages)

String
Descriptor

SET_CONFIGURATION

When the host has got
all the information it requires it loads a driver for the device
based on the VID/PID combination in the device descriptor, or on
the standard class defined there or in an interface descriptor.

The driver may also ask
for the same or different information using Get Descriptor requests.

Eventually it will decide
to configure the device using the SET_CONFIGURATION request.
Usually ( when there is one configuration) the Set Configuration
request will have wValue set to 1, which will select the first configuration.

Set Configuration can
also be used, with wValue set to 0, to deconfigure the device.

GET_CONFIGURATION

This request compliments
Set Configuration, and simply allows the host to determine which
configuration it previously set.

SET_FEATURE

CLEAR_FEATURE

This pair of requests
is used to control a small number of on-off features on a device,
an interface or an endpoint.

A device has 5 possible
features, an endpoint has one, and an interface actually has none
at all.

The greyed out features
shown in the table only apply to OTG devices.

ENDPOINT_HALT

Setting this feature
will cause an endpoint to STALL any IN or OUT transactions.

DEVICE_REMOTE_WAKEUP

Setting this feature
allows a device which is then suspended to use resume signalling
to gain the host‘s attention.

Table
of wValues used in Set Feature and Clear Feature requests.

GET_STATUS

This request is used
to fetch status bits from a device, an interface or an endpoint.
In each case the request fetches 16 bits (2 bytes). The tables to
the right show the status bits which are currently implemented.

Note that Remote Wakeup
and Halt status bits can both be controlled by the host using Set.Clear
Feature requests, but the Self-powered bit is only controlled by
the device.

Device
Status Bits

Interface
Status Bits

Endpoint
Status Bits

SET_INTERFACE

GET_INTERFACE

Once a device has been
configured the host may use Set Interface to select an alternative
interface to a particular default interface. It can use the Get
Interface to determine which interface alternative it previous set
for a particular interface.

SYNCH_FRAME

This is used with some
isochronous transfer where the transfer size varies with the frame.
See USB 2.0 specification for more details.

SET_DESCRIPTOR

This Standard request
is optional and not often used. It allows the host to specify a
new set of values for a given descriptor. It is hard to imagine
when this might be of value.

Summary

We have looked at the
set of standard requests which a device must support to become operational.

Coming up...

Next we will examine
the complete enumeration and start of operation of a specific device.