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Protocol Layer

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.

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Protocol Layer

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  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 Address+0 Byte0 • Base Address+1 Byte1 • Base Address+2 Byte2 • Base Address+3 Byte3 • Big Endian • Will be arrange in memory as follows • Base Address+0 Byte3 • Base Address+1 Byte2 • Base Address+2 Byte1 • Base Address+3 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 ADDR<6:0> • 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

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