Universal Serial Bus Motivation
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- USB Driver IRPs and Frames
- Host Controller Driver and Transactions
- Configuration Descriptor
- Class Specific Descriptors
USB Communications ModelUnlike devices residing on other common bus structures USB devices do not directly consume system resources Not mapped into memory or I/O address space Do not use IRQ lines or DMA channels All transactions originate from host system Only system resources needed USB memory locations used by USB system software Memory and I/O address space and IRQ line used by USB host controller Design eliminates much of difficulty encountered With standard peripheral implementations Communications FlowUSB client initiates transfer When it calls USB system software and requests transfer USB client drivers supply Memory buffer used to store data when Sending or receiving from device Remember 472 model Each transfer between Register or endpoint and Client driver In USB device Occurs via communication pipe Established by USB system software during device configuration
Splits client’s request into individual transactions Consistent with bus bandwidth requirements and USB protocol mechanisms Requests passed to USB Host Controller Driver Transaction to be performed then scheduled Host controller performs transaction Based upon contents of transfer descriptor build by HCD HCD knows all information necessary to perform required transaction
Address of target USB device Type of transfer to be performed Size of data packet Location of client’s memory buffer Host controller fetches transfer descriptors That have been built by HCD Each descriptor defines given transaction that must be performed To satisfy client’s transfer request
From client’s buffer to USB device From USB device to buffer
USB system software notifies client driver Information TransferTransfers initiated by client driver When it issues transfer request to USB driver Ultimately transaction performed via lowlevel packetized transactions
TransfersEach USB function designed with collection Registers or endpoints Used by client driver to access that function Each endpoint Has particular transfer characteristics it supports
Information transferred to speaker Must continue at constant rate Prevents distortion Other endpoints may have different characteristics Require different transfer types Transfer TypesFollowing transfer types supported by USB Isochronous Bulk Interrupt Control High speed transfer Supports all 4 Low speed transfer Supports only last two Client drivers understand nature of transfer Related to each endpoint associated with its particular function Information determined by reading descriptors from device
Calls USB driver to initiate transfer Requested transfer called IO Request Packet - IRP Some transfers require movement of large block of data USB is shared bus Cannot typically perform large block transfer All at one time To accommodate such transfers Transfer split up into smaller segments over longer period of time Segments called transactions Such scheme ensures USB bandwidth can be Allocated amongst other USB devices on bus
Regular intervals called frames Each is 1 ms
Be allocated to it during each 1 ms frame Allocation depends upon Required throughput of device As specified in device descriptor Available bandwidth not required by other devices As device is attached and configured System software parses device descriptors To determine required bandwidth If requirements can be met Device is configured Otherwise Device not configured User notified Host Controller Driver and TransactionsHost controller driver Receives packet requests from USB driver Schedules them to be performed During a series of frames Scheduling order based upon algorithm Defined by host controller driver Algorithm Based upon transfer capabilities other limitations of system Scheduling performed by building series of data structures Called transfer descriptors These define each sequential transaction to be performed Host controller Reads and interprets the descriptors Executes the USB transaction described USB transaction comprises three phases 1. Token phase Define type of transaction 2. Data packet phase 3. Handshake packet phase All USB transfers implemented to guarantee data delivery Except isochronous Provides feedback to sender Whether or not data received without error
Device DescriptorsDevice describes itself to host software Number of descriptors Descriptors include Device DescriptorEach device has single descriptor Contains information about Default communication pipe used to configure device General information about device Also identifies number of possible configurations device supports Configuration DescriptorDevice has configuration descriptor For each configuration device supports Example High power device may support low power mode Has configuration for each power mode
About configuration Defines number of interfaces for device When used in this configuration Interface DescriptorGiven configuration may have Multiple interfaces it supports Example CD Rom
May require 3 device drivers Driver for devices mass storage capability Audio device driver for playing music Video image driver for displaying images Interface descriptors Provide general information about interface Indicates class of device Supported by this interface Number of endpoint descriptors used when communicating with interface
Endpoint DescriptorDevice interface contains multiple endpoints Each defines point of communication Descriptor provides information such as Transfer type supported by endpoint Isochronous Bulk
Interrupt Control
Maximum transfer rate supported String DescriptorsCan be defined for Overall device Given configuration Each interface definition Describe configuration and interfaces In unicode Can be displayed and read by user
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