Universal Serial Bus Motivation
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- I/O Addresses I/O address conflicts Quite common in PC environment Attachment
- Hardware Host controller / Root Hub Hubs Devices Software
- USB Device Drivers - Host Side - Client Software
- USB Driver - Host Side - Client Driver
- USB Host Controller Driver - Host Side - Client Driver
- USB Host Controller / Root Hub
- USB Devices - On Client Side
Universal Serial BusMotivationEmerged as result of difficulties associated with Cost configuration attachment Peripheral devices to personal computer Creates method of attaching and accessing devices Reduces cost Simplifies attachment and configuration From end user perspective Solves several technical issues Associated with old style peripherals
I/O LegacyPeripherals typically Mapped into CPU’s I/O address space Assigned specific IRQ line Some cases DMA channel Such scheme consumes many I/O resources
InterruptsMost critical issue centered around Allocation of interrupts Exacerbated on ISA bus based devices Do not reliably support sharable interrupts Shortage of interrupts had become major problem
I/O AddressesI/O address conflicts Quite common in PC environment AttachmentNo hot attachment of peripherals Consider booting without a mouse attached Most peripheral devices not usable without rebooting system System software must Detect device Load necessary drivers CostCost of implementing I/O systems based upon legacy technology Fairly high Cables Associated connectors Potential need for expansion cards New SolutionGoalsShould overcome existing shortcomings Provide room for growth and expansion NeedsSingle connector type Ability to attach many peripherals to same connector Method to ease resource conflicts Automatic detection and configuration of peripheral devices Low cost for system and peripheral implementers Enhanced performance capability Support for attaching new peripherals Support for legacy software and hardware Low power implementation - support for green systems
USB ApproachI/O resource limitations no longer exist Each device residing on USB is assigned an address known only to USB subsystem Does not consume any system resources USB supports up to 127 addresses Does pose limitation on number of addresses in single installation Each USB device supports number of ports called endpoints Can be accessed indirectly
OverviewTypical system USB host controller Resides on PCI bus Fetches transaction list that describes USB transactions Scheduled by system For delivery over USB Executes transaction list by performing each transaction in list
Typically originate in USB device driver That wishes to communicate with its associated device
USB device driver - on peripheral device USB host controller - typically in PC
Client requests Into transactions Directed to or from target USB device Primary ComponentsHardwareHost controller / Root Hub Hubs
Devices SoftwareUSB Device drivers USB Driver Controller Driver  USB Device Drivers - Host Side - Client SoftwareThese are the client drivers Located on host side Top box
Second box Use IRP - Interrupt Request Packets Initiate transfer to or from target USB device Example USB keyboard driver Must initiate interrupt transfer Establishing an IRP Supplying memory buffer into which data will be transferred By USB keyboard Note: Client driver has no knowledge of USB serial transfer mechanisms All transactions transpire through lower level software packages Exactly what we want
USB system software USB driver knows Characteristics of USB target device How to communicate with device via USB
Detected by driver when it parses device descriptors Done during device configuration At power up When new device added to system
Amount of bandwidth necessary during transfer of each frame Service only during nth frame
USB driver organizes request into individual transactions Executed during series of 1 ms frames Driver sets up transactions based upon knowledge of USB device requirements Needs of client driver Limitations / capabilities of USB
Shipped with operating system Added as extension via loadable device driver
USB system software Host Controller Driver - HCD Schedules transactions To be broadcast over USB
Building series of transaction lists Each list Consists of pending transactions Targeted for one or more USB devices attached to bus Arise from various USB device drivers Transaction lists executed at 1 ms intervals Transaction or frame list Defines sequence of transactions to be performed During each 1 ms frame
May be performed as series of transactions Scheduled during consecutive 1 ms frames
Initiates transactions via root hub or hubs Each 1 ms frame begins with a start of frame - SOF - transaction Followed by serial transmission of all frames in current list USB Host Controller / Root HubAll communication on USB Originates at host under software control Have basic tree type architecture
USB host controller Initiates transactions over USB Root hub Provides attachment points or ports For USB devices Two USB host controller designs have been developed Open host controller Universal host controller
Implemented in slightly different ways The Host ControllerIn either case host controller responsible for generating transactions Scheduled by host software Host controller driver -HCD Builds linked list of data structures - called transfer descriptors List defines transactions scheduled to be performed During given frame Data structures contain all information host controller needs To generate transactions USB device address Type of transfer Direction of transfer Address of device driver’s memory buffer - remember 472 WritesPerforms writes to target device By reading data to be delivered From memory buffer Buffer supplied by individual USB device driver - top box Perform parallel to serial conversion Create USB transaction Forward to root hub for transmission on bus ReadsPerforms reads from target device By building read transaction Send to root hub Hub transmits read transaction over USB Target device Recognizes That it’s being addressed Data is being requested Device transmits data back to root hub Root hub forwards data to host controller Host controller Performs serial to parallel conversion Transfers data to device driver’s memory buffer Buffer supplied by individual USB device driver - top box Root HubTransactions generated by host controller Forwarded to root hub To be transmitted to USB Consequently every USB transaction Originates at root hub Root Hub Provides connection points For USB devices Performs following key operations Controls power to USB ports Enables and disables ports Recognizes devices attached to ports Sets and reports status events Associated with each port Consists of hub controller and repeater Hub controller responds to accesses made to hub itself Repeater forwards transactions to and from USB and host controller  USB HubsIn addition to root hub USB supports additional hubs Permit extension of USB system Provide means for attaching one or more USB ports Additional peripheral devices
Called compound devices Can also be implemented as stand alone devices
Bus powered hubs limited in amount of power available Hubs contain two major functional elements Hub controller Repeater Hubs must recognize That device has been attached or detached Report event when host software polls hub  Hub ControllerContains USB interface Descriptors needed to identify device as a hub Hub controller gathers Hub and port status information Detect connection and removal of devices Other status info as well Receives commands from host software Control various aspects of hubs operation Powering
Enabling ports Etc.
Hub RepeaterTraffic arriving at hub Must be forwarded Upstream - towards host Downstream - away from host Transmissions originating at host arrive on hub’s root port Forwarded to all enabled ports When target device responds to host initiated transaction Repeater must transmit response upstream USB Devices - On Client SideDevice descriptorsUSB devices contain descriptors Specify devices attributes and characteristics Information specifies to host software Variety of features and capabilities needed to Configure device Locate USB client software driver USB device driver may also use device descriptors To determine additional information Needed to access device in proper fashion
Must be understood by software to Configure and access device correctly
USB DevicesHigh-Speed DevicesHigh speed devices See all transactions broadcast over USB Can be implemented as full feature devices Accept and send serial data at maximum rate of 12 Mbyte / sec Low-Speed DevicesLimited in Throughput - 1.5 Mbyte / sec Feature support See only transactions that follow preamble packet Low speed ports remain disabled during full speed transactions Full speed bus traffic Prevented from being sent over low-speed cables Preamble packets Specify following transaction will be broadcast at low speed Hubs enable low-speed ports After detecting preamble packet Low-speed devices now accept low-speed bus activity
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