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Usb protocol

The document outlines the USB (Universal Serial Bus) design principles, including its architecture, goals, and features that enhance PC peripheral connectivity. It highlights key attributes such as ease of use, low cost, robust error handling, and support for various data transfer types. Additionally, it provides technical details regarding the USB protocol, power management, and the operational characteristics of USB devices and hosts.

Engineering◦Technology◦
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USB Universal Serial Bus
Motivations • Connection of the PC to Telephone – The USB provides a ubiquitous link that can be used across a wide range of PC-to-telephone interconnects. • Ease of use • Hot plug • Port expansion – The lack of a bi-directional, low-cost, low-to-mid speed peripheral bus has held back the creative proliferation of peripherals such as telephone/fax/modem adapters, answering machines, scanners, PDA’s, keyboards, mice, etc.
USB • Fast • Bi-directional • Isochronous • Low-cost • Dynamically attachable serial interface • Consistent with the requirements of the PC platform of today and tomorrow
Goals for USB • The following criteria were applied in defining the architecture for the USB: – Ease-of-use for PC peripheral expansion – Low-cost solution that supports transfer rates up to 12Mb/s – Full support for real-time data for voice, audio, and compressed video – Protocol flexibility for mixed-mode isochronous data transfers and asynchronous messaging – Integration in commodity device technology – Comprehension of various PC configurations and form factors – Provision of a standard interface capable of quick diffusion into product – Enabling of new classes of devices that augment the PC’s capability.
Taxonomy of Application Space
Feature list • Easy to use for end user – Single model for cabling and connectors – Electrical details isolated from end user (e.g., bus terminations) – Self-identifying peripherals, automatic mapping of function to driver, and configuration – Dynamically attachable and re-configurable peripherals • Wide range of workloads and applications – Suitable for device bandwidths ranging from a few kb/s to several Mb/s – Supports isochronous as well as asynchronous transfer types over the same set of wires – Supports concurrent operation of many devices (multiple connections) – Supports up to 127 physical devices – Supports transfer of multiple data and message streams between the host and devices – Allows compound devices (i.e., peripherals composed of many functions) – Lower protocol overhead, resulting in high bus utilization
Feature list (contd…) • Isochronous bandwidth – Guaranteed bandwidth and low latencies appropriate for telephony, audio, etc. – Isochronous workload may use entire bus bandwidth • Flexibility – Supports a wide range of packet sizes, which allows a range of device buffering options – Allows a wide range of device data rates by accommodating packet buffer size and latencies – Flow control for buffer handling is built into the protocol • Robustness – Error handling/fault recovery mechanism is built into the protocol – Dynamic insertion and removal of devices is identified in user-perceived real- time – Supports identification of faulty devices
Feature list (contd…) • Relation with PC industry – Protocol is simple to implement and integrate – Consistent with the PC plug-and-play architecture – Leverages existing operating system interfaces • Low-cost implementation – Low-cost sub-channel at 1.5Mb/s – Optimized for integration in peripheral and host hardware – Suitable for development of low-cost peripherals – Low-cost cables and connectors – Uses commodity technologies • Upgrade path – Architecture upgradeable to support multiple USB Host Controllers in a system
USB System Description • USB interconnect • USB devices • USB host
USB Interconnect • Manner in which devices are connected and communicating with the host – Bus topology • Connection between USB devices and host – Inter-layer Relationship • USB tasks are performed at each layer in system – Data Flow Models – USB Schedule • Access to interconnect is scheduled to support isochronous data transfer • To eliminate arbitration overhead
USB Devices & Host • Hubs – provide additional attachment points to USB • Functions – provide capabilities to the system – e.g. ISDN connection, a digital joysticks • USB Host – Only one host – Interact with USB device through host controller
BUS Topology
Peripherals 1. USB host with host controller 2. 2-port root hub integrated into the host controller 3. 4-port hub integrated into the keyboard (part of the compound device) 4. USB keyboard (part of the compound device) 5. USB keypad (part of the compound device) 6. 4-port hub (part of the 7- port hub) 7. 4-port hub (part of the 7- port hub) 8. USB mouse 9. USB flash drive 10. 4-port hub 11. 4-port hub 12. USB bluetooth adapter
Peripherals Shown Explicitly
Physical Interface pin Name Descryption 1 Vcc +5 Vdc 2 D- Data- 3 D+ Data+ 4 GND Ground
Throughput • Low speed – 1.5 Mbps • Full speed – 12 Mbps • High speed (in USB 2.0) – 480 Mbps • Dynamic mode switching • Clock encoding scheme: NRZI (Non Return to Zero Invert)
Electrical • USB uses differential transmission pair for data – Clock is encoded by NRZI and bit stuffed to ensure adequate transitions in data stream – A SYNC field precedes each packet to allow the receiver to synchronize their bit recovery clock – Differential ‘1’ is transmitted by pulling D+ over 2.8V and D- under 0.3V – Differential ‘0’ is a D- greater than 2.8V and a D+ less than 0.3V – The receiver defines a differential ‘1’ as D+ 200mV greater than D- and a differential ‘0’ as D+ 200mV less than D- • Speed Identification – by pulling either the D+ or D- line high to 3.3 volts
Speed Identification Full Speed Device with pull up resistor connected to D+ Low Speed Device with pull up resistor connected to D-
Power Distribution • Devices: – Bus-powered devices • Low-power Bus Powered – Draw all its power from VBUS and cannot draw any more than one unit load (100 mA) – Designed to work down to a VBUS voltage of 4.40V and up to a maximum voltage of 5.25V • High power bus powered – draw all its power from the bus and cannot draw more than one unit load until it has been configured after which it can then drain 5 unit loads (500mA Max) – When operating at a full unit load, a minimum VBUS of 4.75 V is specified with a maximum of 5.25V • Self-powered devices – draw up to 1 unit load from the bus and derive the rest of it’s power from an external source – Upon failure of external source , it must have provisions in place to draw no more than 1 unit load from the bus • Power management – Host based – Power events: • Suspend • Resume
Bus Protocol • Polled bus • Data transfers initiated only by host controller • Three packets: – Token packet • Type • Direction • Address • End point number – Data packet (Containing the payload) – Handshake packet • ACK • NAK
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
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
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
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
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
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
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
PID Types
Address Fields • Function endpoints are addressed using two fields – The function ADDRess field (ADDR) – The ENDPoint field (ENDP)
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
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
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
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
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
Packet Formats • Token packets • Data packets • Handshake packets
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
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
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
Packets in a split Transaction
Relationship of Interrupt IN Transaction to High- speed Split Transaction
Relationship of Interrupt OUT Transaction to High-speed Split OUT Transaction
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
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
Start-Split (SSPLIT) Transaction Token • ET (Endpoint Type) – Tells the hub which split transaction state machine to use for this full/low speed transaction
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
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
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
Data Packets • Four types of data packets – DATA0, DATA1, DATA2, MDATA
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
Transfer Model • Pipe – Stream pipes • No USB standard format – Message pipes • USB format – Request – Data – Status • End point zero – Default control pipe – Always exists
Data Flow Types • Control Transfers: – Used to configure a device at attach time and can be used for other device-specific purposes, including control of other pipes on the device. • Bulk Data Transfers: – Generated or consumed in relatively large and bursty quantities. • Interrupt Data Transfers: – Used for characters or coordinates with human-perceptible echo or feedback response characteristics. • Isochronous Data Transfers: – Occupy a pre-negotiated amount of USB bandwidth with a pre-negotiated delivery latency. (Also called streaming real time transfers).
USB Devices • All have endpoint zero • Hub • Function • Compound
Typical Configuration
Host Responsibility • Detecting the attachment and removal of USB devices • Managing control flow between the host and USB devices • Managing data flow between the host and USB devices • Collecting status and activity statistics • Providing power to attached USB devices.
Implementer Viewpoints • USB Physical Device: A piece of hardware on the end of a USB cable that performs some useful end user function. • Client Software: Software that executes on the host, corresponding to a USB device. This client software is typically supplied with the operating system or provided along with the USB device. • USB System Software: Software that supports the USB in a particular operating system. The USB System Software is typically supplied with the operating system, independently of particular USB devices or client software. • USB Host Controller (Host Side Bus Interface): The hardware and software that allows USB devices to be attached to a host.
Control Transfer Format • Setup packet • Data packet • Acknowledge packet • USB defined structure • Setup packet: 8 bytes • Maximum data payload size: – Full speed: 8, 16, 32 or 64 bytes – Low speed: 8 bytes
Control Transfer Constraints • Best effort • 10% frame time for control • Use also remaining 90% if no interrupt or isochronous transfer
Isochronous Transfers • Guaranteed access to USB bandwidth with bounded latency • Guaranteed constant data rate through the pipe as long as data is provided to the pipe • In the case of a delivery failure due to error, no retrying of the attempt to deliver the data. • Stream pipe – Uni-directional – Maximum data payload: 1023 bytes – Only in Full speed. – No more 90% of frame for isochronous, interrupt
Bulk Transfers • Large amount of data. • Access to the USB on a bandwidth-available basis. • Retry of transfers, in the case of occasional delivery failure due to errors on the bus. • Guaranteed delivery of data, but no guarantee of bandwidth or latency. • Stream pipe • Only in full speed • Max. Payload: only 8, 16, 32 or 64 bytes
Connectors
Full Speed CMOS Driver
NRZI Encoding
Bit Stuffing
Signaling Levels