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Universal Serial Bus System Architecture (2)

Universal Serial Bus System Architecture (2). Physical layer. Data Encoding/Decoding The USB employs NRZI data encoding when transmitting packets. In NRZI encoding, a “1” is represented by no change in level and a “0” is represented by a change in level. Bit Stuffing

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Universal Serial Bus System Architecture (2)

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  1. Universal Serial Bus System Architecture (2)

  2. Physical layer • Data Encoding/Decoding • The USB employs NRZI data encoding when transmitting packets. • In NRZI encoding, a “1” is represented by no change in level and a “0” is represented by a change in level. • Bit Stuffing • In order to ensure adequate signal transitions, bit stuffing is employed by the transmitting device when sending a packet on USB • A zero is inserted after every six consecutiveones in the data stream before the data is NRZI encoded, to force a transition in the NRZI data stream. • gives the receiver logic a data transition at least once every seven bit times to guarantee the data and clock lock.

  3. The Example of NRZI and Bit Stuffing

  4. Protocol Layer- Token Packet format 8-bit 8-bit 7-bit 4-bit 5-bit • Bit Ordering • Bits are sent out onto the bus least-significant bit (LSB) first • SYNC Field • All packets begin with a synchronization (SYNC) field, which is a coded sequence that generates a maximum edge transition density. • The SYNC field pattern is 00000001 • Packet Identifier Field (PID) • identify the packet is the token, data, and handshake packets • PID[7:4]=PID[3:0]#

  5. Address(7-bit) • 用來定址最多127個週邊裝置, 但位址0為default address. • endpoint(4-bit) • 可看成微管線, 為每個device內的register (或窗口), 最多一個device有32個endpoint (16* (IN/OUT)), endpoint 00為default communication point. • crc (cycle redundancy checks, 5-bit) • covers the ADDR and ENDP fields of IN, SETUP, and • OUT tokens or the time stamp field of an SOF token. The generator polynomial is: • G(X) = X5 + X2 + 1

  6. Data Packet format • PID為data0或data1 • 最初的資料封包都以DATA0為開始, 其後才是DATA1, 然後依此方式交替切換, 此動作稱為DATA TOGGLE 可確保整個傳輸過程中, 主機能與DEVICE維持同步 • 根據不同的傳輸型態, 其data field的長度限制將不同 • 控制型傳輸的DATA FIELD固定為8-BYTE • 中斷型傳輸在全速度傳輸時data為64 bytes，慢速傳輸僅為8 bytes，當資料小於設定值時，就表示最後一次的傳輸了 • 巨量型傳輸之data最大限制為8、16、32 or 64 bytes • 即時傳輸之data可為0~1023 bytes • CRC is a 16-bit polynomial applied over the data field of a data packet. • The generating polynomial is: G(X) = X16 + X15 + X2 + 1 8-bit 8-bit 0 ~1023 bytes 16-bit

  7. Handshake Packet 與SOF Packet Handshake packet • 接收端對傳送端送過來的資料回應 • Handshake packet有下列三種 • Ack packet ：device己成功接收資料 • Nak packet：device尚未就緒 • Stall packet：發生接收錯誤 • SOF packet是用於即時型傳輸 • 讓device可利用SOF packet來辨識frame的起點 • 即在1ms的frame開始時, 即時型傳輸會利用SOF啟動傳輸並達到同步傳輸的作用. 8-bit 8-bit Start of Frame packet 8-bit 8-bit 11-bit 5-bit

  8. Transaction Format • Packet transaction format varies depending on the endpoint type. • There are four endpoint types: • bulk,control, interrupt, and isochronous. • Bulk Transactions • guarantee error-free delivery of data between thehost and a function • By means of error detection and retry. • use a three-phase transaction • token, data, and handshake packets

  9. Bulk Transfer

  10. Control Transfers • have two or three transaction stages: • Setup and Status. • May optionally contain a Data stage between the Setup and Status stages. • SETUP stage • SETUP transaction is used to transmit information to the control endpoint of a function.

  11. Control Transfers • Data stage • consists of one or more IN or OUT transactions • follows the same protocol rules as bulk transfers (consisting three packet, token, data, handshake). • All the transactions in the Data stage must be in the same direction • all INs or all OUTs • The amount of data to be sent during the data phase and its direction are specified during the Setup stage. • If the amount of data exceeds the prenegotiated data packet size, the data is sent in multiple transactions (INs or OUTs) that carry the maximum packet size. • Any remaining data is sent as a residual in the last transaction.

  12. Status stage • is the last operation of control transfer in the sequence. • A status stage is delineated by a change in direction of data flow from the previous stage and always uses a DATA1 PID.

  13. The Status stage reports to the host the outcome of the previous Setup and Data stages of the transfer. • Three possible results may be returned: • The command sequence completed successfully. • The command sequence failed to complete. • The function is still busy completing command.

  14. Control Transfer – three stages

  15. Example of Control Transfer (1) • device剛連上USB界面時, 其default位址為0, 主機利用default位址向其取得device descriptor(get_descriptor), 才知此device是做何用途, 將以何種傳輸方式來傳送資料, 其device driver為何, 等等. 因此, pc端必需先執行一setup token packet, 做為get_descriptor的transaction的開始.

  16. Example of Control Transfer (2) • 在setup token packet (為控制型傳輸)後面所接的data packet為 控制型傳輸的DATA FIELD固定為8-BYTE

  17. Example of Control Transfer (3) • handshake packet

  18. Interrupt Transactions • may consist of IN or OUT transfers. • the data toggle protocol must be followed.

  19. Interrupt Transfer

  20. Isochronous Transactions • have a token and data phase, but no handshake phase, • The host issues either an IN or an OUT token followed by the data phase in which the endpoint (for INs) or the host (for OUTs) transmits data.

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