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USB Universal Serial Bus
The document summarizes the USB standard and enumeration process. It discusses that USB was developed in the 1990s by major tech companies as a standard cable and protocol for connecting devices. The 12 step enumeration process is then outlined, beginning with a device attaching to a port, the hub detecting it, the host learning about it and assigning an address, retrieving descriptors to identify the device and load appropriate drivers, and ultimately selecting a configuration so communication can begin.
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USB Universal Serial Bus
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- 2. OVERVIEW • Industrial standarddeveloped in 1990s. • Cable + connector + communication protocol • Developed by group of seven companies- Compaq, DEC, IBM, Intel, Microsoft, NEC and Nortel. • Packet based architecture with start frame,transactions,ACK and NAK controls.
- 3. FEATURES OF USB •Ease of Use 1.
- 4. SOME USB KEYTERMS
- 5. FEATURES • 2 CONTROLLERSUSB controller+ Root Hub. • USB controller: Formats the data for the operating system on read and write. • Root Hub: connection point to the host. • Host is always the reference • IN: Host is going to receive the data; OUT: Host is going to send the Data. • 3layers:Lower,Middle,Upper
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- 7. D+>D- =>Enumeration happenat full speed, or vice versa
- 11. EXAMPLE :SET UP TRANSACTION TOKENPACKET DATA PACKET HAND SHAKE PACKET PID+ADDRESS+END POINT NO SUCCESS/FAILURE HOST WANTS TO READ/WRITE
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- 22. ENUMERATION • Before theapplication can communicate with the device host needs to learn about the device and has to assign the device diver • Assigning address to the device-reading descriptors from the device-assigning and loading drivers-selecting configuration(Device power requirement, end point).Then device will be ready for data transfer with any of the endpoint. • Duty of HUB: detect attachment and removal using IN end points. Host polls the root hub to find any device is connected. After boot up also it continues the poll to find any newly attached device.
- 23. ENUMERATION (cont..) • StandardUSB request: Host sends series request to the device HUB .They are followed by control transfers. Endpoint 0. • For a user enumeration process is completely invisible. But get announcement of detection of new devices. • When the enumeration is over OS adds new devices to DEVICE MANAGER list in the control panel. When the user removes device from the bus windows removes the device from the device manager.
- 25. DESCRIPTORS Data structures orformatted block of information which enable the host to learn about the device. The device must store information in the descriptors and should respond for standard USB requests Classification of Descriptors: I. Device II. Configuration III. Interface IV. Endpoint
- 27. bLength:The length ofthe bytes in the descriptor bDescriptortype: 01H for the device descriptor bcdUSB:The USB specification version that the device and its descriptor supports currently.1.00100h;1.10110h Idvendor:deivce manufacturer uses unique id to identify the device by paying money.The host may have an INF file which contains this value to load the drivers. idProduct:The owner of the vendor ID assigns product id to identify the device. bcdDevice:it is the device release No. iManufacturer:Index which points to a string describing the manufacturer IdProduct:Index points a string describing the product. iSerailNo: ’0’ Device with no serial No To identify if the user has more than one identical device on the bus bNumconfigurations:The no of configurations that the device supports. bMax Packet0:The maximum packet size for end point 0 LS:8 bytes;FS:8/16/32/64;HS:64
- 28. Device Class, DeviceSubclass : The devices whose function is defined at device level, this field specifies the device class 02h:Communications. 09h:Hub. E0h:Wireless controller. Device Protocol: It specifies protocol defined by selected class or subclass E.g.: What kind of transfer is going currently
- 33. ENUMERATION STEPS 12 STEPPROCESS
- 34. 1.USER ATTACH DEVICETO PORT • Attachment of USB port to the device or System power up with a device already attached • The port may be on the root hub at a hub which connects the downstream from the host
- 35. 2.HUB DETECTS THEDEVICE • Hub monitors the voltage on the signal lines of each of its ports • If a full speed device is attached D+ is pulled up to V+ • If a full speed device is attached D- is pulled up to V+
- 36. 3.HOST LEARNS ABOUTNEW DEVICE • Each hub uses its interrupt end point to report events at the HUB • Host sends Get_Port_Status request .The returned information tells new device is attached. Get_Port_Status REQ
- 37. 4.DETECTING … HIGH/LOWSPEED? • Checks the voltage on D+ and D- • D+>D- =>full speed device • Hub sends information in response to next Get_Port_Status Get_Port_Status REQ
- 38. 5.HUB RESETS THEDEVICE • When the host learns of new device HOST CONTROLLER send Set_Port_Request which ask the hub to reset the port • Port reset within 10 ms • Reset means D+ and D- ‘0’ • Other USB device may not be affected Set_Port_Status REQ
- 39. 6.HOST CHECKS FULLSPEED DEVICE SUPPORTS HIGH SPEED • If the device supports High speed after reset It will send chirp “K” • A high speed capable hub detects the chirp and responds with a series of alternating chirp “J” &” K”. • Device detects KJKJ..removes full speed pull up, enumeration starts at high speed. • If the Hub is not responding continues at full speed
- 40. 7.HUB ESTABLISHES ASIGNAL PATH B/W DEVICE AND BUS • Verification of reset state by sending Get_Port_Status request • Hub removes the reset Default state • USB registers are at reset state, Device is ready for control transfer with endpoint 0. • Device communicates to host using Default address 00h. • Can draw up to 100mA. Get_Port_Status REQ
- 41. 8.HOST SEND GET_DISCRIPTORTO LEARN PACKET SIZE OF THE DEFAULT PIPE. • Host sends request to device address endpoint 0 • Enumeration of only one device at a time; one device will respond if multiple devices. • 8Th byte of the device descriptor contains maximum packet size supported by endpoint 0. • After receiving one packet ,OS requests 64 bytes. • Host begins status stage transfer
- 42. 9.HOST ASSIGNS ADDRESSTO THE DEVICE • Host assigns a unique address to the device by sending a set_Address request. • The device completes status stage of request using default address- Implementation of new address. • Address is valid until the device is detached, port is reset, system reboots. • Next enumeration new address. Set_Address
- 43. 10.HOST LEARNS ABOUTTHE DEVCIE ABILITY • To read device descriptor Host sends get_Discriptor request • Request for the configuration descriptor
- 44. 11.THE HOST ASSIGNS& LOADS DEVICE DRIVER • Tries to match PC,INF files with vendor ID, Product ID &release No retrieved from device • If no match windows look class, subclass & protocol values from device. If no match user has to browse it externally. • If already enumerated windows look registry file instead of .INF file • Once the driver is obtained it is installed. After installation driver may request the device to resend descriptors or other class descriptors.
- 45. 12.THE HOST DEVICEDRIVER SELECTS A CONFIGURATION • Device driver request configuration by sending set configuration request . • The device reads the request and enables the requested configuration. • The device is in the configured states and interfaces are enabled.