USB

1. VARUN K
varunkpa@gmail.com

2. OVERVIEW
• Industrial standard developed 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 KEY TERMS

5. FEATURES
• 2 CONTROLLERS USB 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

6. USB LOWER
LAYER

7. D+>D- =>Enumeration happen at
full speed, or vice versa

11. EXAMPLE :SET UP
TRANSACTION
TOKEN PACKET
DATA PACKET
HAND SHAKE
PACKET
PID+ADDRESS+END POINT NO
SUCCESS/FAILURE
HOST WANTS TO
READ/WRITE

21. USB
MIDDLEWARE

22. ENUMERATION
• Before the application 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..)
• Standard USB 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 or formatted 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 of the bytes in the descriptor
bDescriptortype: 01H for the device descriptor
bcdUSB:The USB specification version that the device and its descriptor
supports currently.1.00100h;1.10110h
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, Device Subclass
: 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 STEP PROCESS

34. 1.USER ATTACH DEVICE TO 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 THE DEVICE
• 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 ABOUT NEW 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/LOW SPEED?
• 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 THE DEVICE
• 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 FULL SPEED 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 A SIGNAL 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_DISCRIPTOR TO
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 ADDRESS TO 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 ABOUT THE 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 DEVICE DRIVER 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.