[转]USB之Part 4

原地址http://www.usbmadesimple.co.uk/ums_4.htm

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 now signals a USB Reset to the device, in order that it should start in a known state at the end of the reset. In this state the device responds to the default address 0. Until the device has been reset the host prevents data from being sent downstream from the port. It will only reset one device at a time, so there is no danger of two devices responding to address 0.

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 can now see that there is a set of requests which a device must respond to, and need to look at the detailed means by which the requests are conveyed.

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.

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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.

Offset	Field	Size	Value	Description
0	bmRequestType	1	Bitmap	D7 Data direction 0 - Host-to-device 1 - Device-to-host D6:5 Type 0 = Standard 1 = Class 2 = Vendor 3 = Reserved D4:0 Recipient 0 = Device 1 = Interface 2 = Endpoint 3 = Other 4-31 = Reserved
1	bRequest	1	Value	Specific Request
2	wValue	2	Value	Use varies according to request
4	wIndex	2	Index or Offset	Use varies according to request
6	wLength	2	Count	Number of bytes to transfer if there is a data stage

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

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Here is a table which contains all the standard requests which a host can send. The first 5 columns are the SETUP transaction fields in order, and the last column describes any accompanying data stage data which will have the length wLength.

bmRequestType	bRequest	wValue	wIndex	wLength	Data
00000000b 00000001b 00000010b	CLEAR_FEATURE (1)	Feature Selector	Zero Interface Endpoint	Zero	None
10000000b	GET_CONFIGURATION (8)	Zero	Zero	One	Configuration Value
10000000b	GET_DESCRIPTOR (6)	Descriptor Type (H) and Descriptor Index (L)	Zero or Language ID	Descriptor Length	Descriptor
10000001b	GET_INTERFACE (10)	Zero	Interface	One	Alternate Interface
10000000b 10000001b 10000010b	GET_STATUS (0)	Zero	Zero Interface Endpoint	Two	Device, Interface or Endpoint Status
00000000b	SET_ADDRESS (5)	Device Address	Zero	Zero	None
00000000b	SET_CONFIGURATION (9)	Configuration Value	Zero	Zero	None
00000000b	SET_DESCRIPTOR (7)	Descriptor Type (H) and Descriptor Index (L)	Zero or Language ID	Descriptor Length	Descriptor
00000000b 00000001b 00000010b	SET_FEATURE (3)	Feature Selector	Zero Interface Endpoint	Zero	None
00000001b	SET_INTERFACE (11)	Alternate Setting	Interface	Zero	None
10000010b	SYNCH_FRAME (12)	Zero	Endpoint	Two	Frame Number

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.

Descriptor Types	Value	Comments
Device	1
Configuration	2	Request for this also returns OTG, interface and endpoint descriptors
String	3	Qualified by an index to specify which string is required
Interface	4	Not directly accessible
Endpoint	5	Not directly accessible
Device Qualifier	6	Only for high speed capable devices
Other Speed Configuration	7	Only for high speed capable devices
Interface Power	8	Obsolete
On-The-Go (OTG)	9	Not directly accessible

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.

Offset	Field	Size	Value	Description
0	bLength	1	Number	Size of this descriptor in bytes
1	bDescriptorType	1	Constant	DEVICE descriptor type (= 1)
2	bcdUSB	2	BCD	USB Spec release number
4	bDeviceClass	1	Class	Class code assigned by USB-IF 00h means each interface defines its own class FFh means vendor-defined class Any other value must be a class code
5	bDeviceSubClass	1	SubClass	SubClass Code assigned by USB-IF
6	bDeviceProtocol	1	Protocol	Protocol Code assigned by USB-IF
7	bMaxPacketSize0	1	Number	Max packet size for endpoint 0. Must be 8, 16, 32 or 64
8	idVendor	2	ID	Vendor ID - must be obtained from USB-IF
10	idProduct	2	ID	Product ID - assigned by the manufacturer
12	bcdDevice	2	BCD	Device release number in binary coded decimal
14	iManufacturer	1	Index	Index of string descriptor describing manufacturer - set to 0 if no string
15	iProduct	1	Index	Index of string descriptor describing product - set to 0 if no string
16	iSerialNumber	1	Index	Index of string descriptor describing device serial number - set to 0 if no string
17	bNumConfigurations	1	Number	Number of possible configurations

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.

When are requests valid?

The device can be in one of three states which determine whether a particular request is valid at the time.

The states are:

Default

After reset but before receiving Set Address.

In the Default state, the only valid requests are Get Descriptor, and Set Address.

Addressed

After the device has been assigned an address via Set Address.

Now the device must recognise the following additional requests:

• Set Configuration
• Get Configuration
• Set Feature
• Clear Feature
• Get Status
• Set Descriptor (optional)

Configured

After the host has sent Set Configuration with a non-zero value, to select a configuration. The device is now operational.

In the Configured state, only Set Address is not a valid request. Three further requests are restricted to Configured state only:

• Get Interface
• Set Interface
• Synch Frame

Note that this was only a brief overview. The specification gives more detailed information, which you should read when implementing a USB device.

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.

The actual descriptor which is fetched by a GET_DESCRIPTOR request is determined by the high byte of the wValue word in the SETUP data.

So the request we call here 'Get Descriptor (Configuration)' is simply a Get Descriptor request with the high byte of wValue set to 2.

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.

Offset	Field	Size	Value	Description
0	bLength	1	Number	Size of this descriptor in bytes
1	bDescriptorType	1	Constant	CONFIGURATION descriptor type (= 2)
2	wTotalLength	2	Number	Total number of bytes in this descriptor and all the following descriptors.
4	bNumInterfaces	1	Number	Number of interfaces supported by this configuration
5	bConfigurationValue	1	Number	Value used by Set Configuration to select this configuration
6	iConfiguration	1	Index	Index of string descriptor describing configuration - set to 0 if no string
7	bmAttributes	1	Bitmap	D7: Must be set to 1 D6: Self-powered D5: Remote Wakeup D4...D0: Set to 0
8	bMaxPower	1	mA	Maximum current drawn by device in this configuration. In units of 2mA. So 50 means 100 mA.

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.

Offset	Field	Size	Value	Description
0	bLength	1	Number	Size of this descriptor in bytes
1	bDescriptorType	1	Constant	INTERFACE descriptor type (= 4)
2	bInterfaceNumber	1	Number	Number identifying this interface. Zero-based value.
3	bAlternateSetting	1	Number	Value used to select this alternate setting for this interface.
4	bNumEndpoints	1	Number	Number of endpoints used by this interface. Doesn't include control endpoint 0.
5	bInterfaceClass	1	Class	Class code assigned by USB-IF 00h is a reserved value FFh means vendor-defined class Any other value must be a class code
6	bInterfaceSubClass	1	SubClass	SubClass Code assigned by USB-IF
7	bInterfaceProtocol	1	Protocol	Protocol Code assigned by USB-IF
8	iInterface	1	Index	Index of string descriptor describing interface - set to 0 if no string

Interface Descriptor

Endpoint Descriptor

The endpoint descriptor format is shown to the right.

Offset	Field	Size	Value	Description
0	bLength	1	Number	Size of this descriptor in bytes
1	bDescriptorType	1	Constant	ENDPOINT descriptor type (= 5)
2	bEndpointAddress	1	Endpoint	The address of this endpoint within the device. D7: Direction 0 = OUT, 1 = IN D6-D4: Set to 0 D3-D0: Endpoint number
3	bmAttributes	1	Bitmap	D1:0 Transfer Type 00 = Control 01 = Isochronous 10 = Bulk 11 = Interrupt The following only apply to isochronous endpoints. Else set to 0. D3:2 Synchronisation Type 00 = No Synchronisation 01 = Asynchronous 10 = Adaptive 11 = Synchronous D5:4 Usage Type 00 = Data endpoint 01 = Feedback endpoint 10 = Implicit feedback Data endpoint 11 = Reserved D7:6 Reserved Set to 0
4	wMaxPacketSize	2	Number	Maximum packet size this endpoint can send or receive when this configuration is selected
6	bInterval	1	Number	Interval for polling endpoint for data transfers. Expressed in frames (ms) for low/full speed or microframes (125us) for high speed.

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).

Offset	Field	Size	Value	Description
0	bLength	1	Number	Size of this descriptor in bytes
1	bDescriptorType	1	Constant	STRING descriptor type (= 3)
2	wLANGID[0]	2	Number	LANGID Code 0
...	...	...	...	...
2 + x*2	wLANGID[x]	2	Number	LANGID Code x

String Descriptor Zero
(Specifies supported string languages)

 

Offset	Field	Size	Value	Description
0	bLength	1	Number	Size of this descriptor in bytes
1	bDescriptorType	1	Constant	STRING descriptor type (= 3)
2	bString	2	Number	UNICODE encoded string

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.

A Configured Device

Once a device has been configured, it is allowed to respond to other transfer types than Control transfers.

As we have seen, the other transfer types are

• Interrupt Transfers
• Bulk Transfers
• Isochronous Transfers

As a result of the information in the descriptors, the host will now know what particular transfers on which particular endpoints the device is prepared to support. There may now also be new class or vendor-specific requests which may now be supported on the control endpoint in addition to the standard requests.

It is all these additional transfers which perform the functionality that the device was designed for.

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.

Feature Selector	Recipient	Value
ENDPOINT_HALT	Endpoint	0
DEVICE_REMOTE_WAKEUP	Device	1
TEST_MODE	Device	2
B_HNP_ENABLE	Device	3
A_HNP_SUPPORT	Device	4
A_ALT_HNP_SUPPORT	Device	5

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.

Status Bit	Purpose	Comment
D0	Self Powered	Set to 1 by the device when it is self-powered
D1	Remote Wakeup	Set to 1 if the device has been enabled to signal remote wakeup.
D2 - D15	reserved	Must be set to 0

Device Status Bits

Status Bit	Purpose	Comment
D0 - D15	reserved	Must be set to 0

Interface Status Bits

Status Bit	Purpose	Comment
D0	Halt	Set to 1 when endpoint is halted
D1 - D15	reserved	Must be set to 0

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.

Forward

Copyright © 2006-2008 MQP Electronics Ltd