Protocol Layer

1
Protocol Layer

• Bottom-up view of the USB protocol
• Byte/Bit Ordering
• SYNC Field
• Packet Field Formats
• PID Field
• Address Field
• Frame Number Field
• Data Field
• CRC
• Packet Formats
• Token Packets
• Split Transaction Special Token Packets
• Start-of-Frame Packets
• Data Packets
• Handshake Packets

2
Byte/Bit Ordering

• Bits are sent onto the bus
• LSB first, MSB last
• Multiple bytes are interpreted
• Little endian order, i.e., LSB to MSB

3
Endian Order

• Little Endian
• The low-order byte of the number is stored in
  memory at the lowest address, and the high-order
  byte at the highest address (The little end comes
  first)
• Example, a 4 byte Long Int
• Byte3 Byte2 Byte1 Byte0
• Will be arranged in memory as follows
• Base Address0 Byte0
• Base Address1 Byte1
• Base Address2 Byte2
• Base Address3 Byte3
• Big Endian
• Will be arrange in memory as follows
• Base Address0 Byte3
• Base Address1 Byte2
• Base Address2 Byte1
• Base Address3 Byte0

4
SYNC Field

• Synchronization field
• All packets begin with SYNC field
• Align incoming data with the local clock
• Length
• 8 bits for full/low speed
• 32 bits for high speed
• Serves only synchronization mechanism
• The last two bits are a marker of the end of the
  SYNC field

5
Packet Field Formats

• Field formats for
• Token packets
• Data packets
• Handshake packets
• SOP / EOP
• All packets have distinct Start-Of-Packet (SOP)
  and End-Of-Packet (EOP) delimiters

6
PID (Packet IDentifier Field)

• Immediately follows the SYNC field of every USB
  packets
• Indicates the type of packet
• A PID consists of
• 4 bits packet type field
• Packet format, error type
• 4 bits check field
• Complements of the their respective packet
  identifier bits

7
PID (Packet IDentifier Field)

• The host and all functions must perform a
  complete decoding of all received PID fields
• Any PID received with a failed check field or
  which decodes to a non-defined value is assumed
  to be corrupted ? ignored by the packet receiver
• Valid PID for a transaction but does not support,
  the function must not respond
• IN-only endpoint must ignore an OUT token

8
PID Types
9
Address Fields

• Function endpoints are addressed using two fields
• The function ADDRess field (ADDR)
• The ENDPoint field (ENDP)

10
Address Field (ADDR)

• Specifies the function via its address
• Either the source or destination of a data
  packet, depending on the value of the token PID
• 128 addresses are specified as ADDRlt60gt
• Function address zero is reserved as the default
  address

11
Endpoint Field (ENDP)

• More flexible addressing of functions in which
  more than one endpoint is required
• Is defined for IN, SETUP, OUTPUT and PING token
• Device Endpoint
• A uniquely addressable portion of a USB device
  that is the source or sink of information in a
  communication flow between the host and device

12
Frame Number Field

• 11-bit field that is incremented by the host on a
  per-frame basis
• Maximum value of 7FFh and rolls over after
  reaching its maximum value

13
Data Field

• Data packet size varies with the transfer type
• May range from zero to 1,024 bytes
• Data bits within each bytes are shifted out LSB
  first

14
CRC-Cyclic Redundancy Checks

• CRCs are used to protect all non-PID fields in
  token and data packets
• Token and data packet CRCs provide 100 coverage
  for all single and double bit errors

15
Packet Formats

• Token packets
• Data packets
• Handshake packets

16
Token Packets

• PID
• IN, OUT, or SETUP
• ADDR / ENDP
• For OUT, SETUP
• Identify the endpoint that will receive the
  subsequent Data packet
• For IN
• Identify which endpoint transmit a Data packet
• For PING
• Identify which endpoint will respond with a
  handshake packet
• A five bit CRC that covers that address and
  endpoint fields

17
Split Transactions

• A high speed split transaction is used only
  between the host controller and a hub when the
  hub has full/low speed devices attached to it
• Only defined to be used between the host
  controller and a hub
• No other high speed or full/low speed devices
  ever use split transactions

18
Split Transaction Special Token Packets

• SPLIT
• A special token for split transaction
• 4 byte token packets
• Other normal 3 byte token packets
• Provides additional transaction types with its
  specific information
• Is used to support split transactions between the
  host controller communicating with a hub
  operating at high speed with full/low speed
  devices to some of its downstream facing ports
• SSPLIT / CSPLIT
• Start SPLIT transaction
• Complete SPLIT transaction

19
Packets in a split Transaction
20
Relationship of Interrupt IN Transaction to
High-speed Split Transaction
21
Relationship of Interrupt OUT Transaction to
High-speed Split OUT Transaction
22
Start-Split (SSPLIT) Transaction Token

• SC (Start/Complete)
• Zero indicates that this is a start-split
  transaction (SSPLIT)
• Port
• The port number of the target hub for which this
  full/low speed transaction is destined

23
Start-Split (SSPLIT) Transaction Token

• For full-speed isochronous OUT start-splits,
• S and E fields specify how the high speed data
  payload corresponds to data for a full-speed data
  packet as show in this table
• Allow the hub to detect various error cases due
  to lack of receiving start-split transactions

24
Start-Split (SSPLIT) Transaction Token

• ET (Endpoint Type)
• Tells the hub which split transaction state
  machine to use for this full/low speed transaction

25
Complete-Split Transaction Token

• A SPLIT special token packet with the SC field
  set to one
• U reserved/unused and must be reset to zero

26
Start-of-Frame Packets

• Start-of-Frame (SOF) packets are issued by the
  host at a nominal rate of once every 1 ms for
  full-speed bus and 125 µs for a high-speed bus

27
USB Frames and Microframes

• A full speed 1 ms frame time
• A high speed microframe with a 125 µs frame time
• High speed devices see an SOF packet with the
  same frame number each times during each 1 ms
  period

28
Data Packets

• Four types of data packets
• DATA0, DATA1, DATA2, MDATA

29
Handshake Packets

• Used to report the status of a data transaction
• ACK
• The data packet was received without bit stuff or
  CRC errors over the data field and that the data
  PID was received correctly
• NAK
• Unable to accept data from the host (OUT) or has
  no data to transmit to the host (IN)
• STALL
• Unable to transmit or receive data or a control
  pipe request is not supported
• NYET and ERR
• High speed only handshake