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Universal Serial Bus System Architecture (1)

Universal Serial Bus System Architecture (1). overview. The big picture – overview. USB Bus Topology. USB Host There is only one host in any USB system. The USB interface to the host computer system is referred to as the Host Controller.

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Universal Serial Bus System Architecture (1)

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  1. Universal Serial Bus System Architecture (1) overview

  2. The big picture – overview

  3. USB Bus Topology

  4. USB Host • There is only one host in any USB system. • The USB interface to the host computer system is referred to as the Host Controller. • A root hub is integrated within the host system to provide one or more attachment points. • USB Devices • USB devices are one of the following: • Hubs, which provide additional attachment points to the USB • Functions, which provide capabilities to the system, such as an ISDN connection, a digital joystick, or speakers.

  5. USB electrical

  6. USB bus protocol • The USB is a polled bus. • The Host Controller initiates all data transfers. • All bus transactions involve the transmission of up to three packets. • Token packet • Each transaction begins when the Host Controller, on a scheduled basis, sends a USB token packet describing the type and direction of transaction, the USB device address, and endpoint number. • Data packet • The USB device that is addressed selects itself by decoding the appropriate address fields. In a given transaction, data is transferred either from the host to a device or from a device to the host. The direction of data transfer is specified in the token packet. The source of the transaction then sends a data packet or indicates it has no data to transfer. • Handshake packet • The destination, in general, responds with a handshake packet indicating whether the transfer was successful.

  7. USB pipe • The USB data transfer model between a source or destination on the host and an endpoint on a device is referred to as a pipe. • stream pipe • Message pipe • Pipes have associations of data bandwidth, transfer service type, and endpoint characteristics like directionality and buffer sizes. Most pipes come into existence when a USB device is configured. • One message pipe, the Default Control Pipe, always exists once a device is powered, in order to provide access to the device’s configuration, status, and control information. • The transaction schedule allows flow control for some stream pipes. At the hardware level, this prevents buffers from underrun or overrun situations by using a NAK handshake to throttle the data rate. When NAKed, a transaction is retried when bus time is available.

  8. Plug and play • 當pc一接上電源時, 所有利用usb所連接的裝置與集線器都預設為位址0 • 接著pc就向usb查詢, 若發現第一個device其位址為0, 則assign 一個新位址給他, 然後再向下尋找第二個位址為0的裝置或集線器 • 最多可連接127個devices • 在配給新位址的同時, pc亦為每個device或hub載入其驅動程式 • 若有一新的裝置被接上時, 其預設為位址0, pc就會確認並載入其相對應的驅動程式, 並分配一個尚未使用的位址給它 • 一旦某個device突然被拔取後, pc可經由D+或D-的電壓變化測到裝置被移除掉, 就將其位址收回並列入可使用的位址名單中. • 每個device (function)有4-bit endpoint address (為register或memory位址) , 可當輸入或輸出 port • 所以在一個單獨的小管線(pipe)內最多可再分割成16組的微管線(micro-pipe), 也就是可對16個輸出/入的端點(共32個端點endpoint)定址 • 位址0 的in, out是給開機時之初始控制傳輸用, 真正可用的endpoint number為15, 即30個endpoint

  9. Data flow types • The USB architecture comprehends four basic types of data transfers: • 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 (ex: printer or scanner) • 用於傳輸大量資料，這種資料是非週期性或是無傳輸速度限制，例如：USB印表機，較不即時的資料傳輸並不會導致列印工作資料的流失或損毀. • Interrupt Data Transfers (polling, ex: mouse or keyboard) • 用於傳統個人電腦中被認為是靠中斷驅動的裝置。因為USB不支援硬體中斷，那些必須靠中斷驅動的USB裝置必須被週期性的POLLING，以得知是否有資料需要傳送。例如：在傳統個人電腦系統中，每當鍵盤上有一個鍵被壓下，鍵盤會發出一個硬體中斷以通知處理器去執行一段軟體斷程序，以便服務鍵盤。如果是USB鍵盤，它將週期性的被詣問以便決定有是否有資料要傳送。詢問的週期當然是很重要(它的頻率必須足夠以免資料移失，但也不能太高以免佔用太多匯流排頻寬。 • Isochronous Data Transfers • 需要固定之傳送速率應用， 使用這種傳應用必須確保發送端與接收端的速率是相互吻合的。例如：USB麥克風與喇叭即是用此種傳輸。

  10. USB Implementation Areas

  11. USB bus interface layer • provides physical/signaling/packet connectivity between the host and a device. • USB Device Layer • is the view the USB System Software has forperforming generic USB operations with a device. • Function Layer • provides additional capabilities to the host via an appropriate matched client software layer.

  12. USB Host • The host’s logical composition is • USB Host Controller • Aggregate USB System Software (USB Driver, Host Controller Driver, and host software) • Client.

  13. USB Devices • A USB physical device’s logical composition is • USB bus interface • USB logical device • Function.

  14. Physical Bus Topology • USB attachment points are provided by a special class of USB device known as a hub. • The additional attachment points provided by a hub are called ports. • A host includes an embedded hub called the root hub. • USB devices that provide additional functionality to the host are known as functions. • When multiple functions are combined with a hub in a single package, they are referred to as a compound device.

  15. Logical Bus Topology

  16. USB Communication Flow

  17. USB Communication Model • 與一般其它匯流排上的裝置不同, USB並不宜接消耗系統資源 • USB裝置並不被對映到記憶體或I/O位址空間, 也不會佔用IRQ或DMA channel • 所用transaction皆由host啟動

  18. Transfer Management

  19. Transfer Management • Client Software: • Consumes/generates function-specific data to/from a function endpoint via calls and callbacks requesting IRPs with the USBD interface. • A software client normally requests data transfers via I/O Request Packets (IRPs) to a pipe and then either waits or is notified when they are completed. • USB Driver (USBD): • Converts data in client IRPs to/from device endpoint via calls/callbacks with the appropriate HCD. A single client IRP may involve one or more transfers. • Host Controller Driver (HCD): • Converts IRPs to/from transactions (as required by a Host Controller implementation) and organizes them for manipulation by the Host Controller. • Interactions between the HCD and its hardware is implementation-dependent and is outside the scope of the USB Specification. • Host Controller: • Takes transactions and generates bus activity via packets to move function-specific data across the bus for each transaction.

  20. Transaction Tracking • 當一個client driver希望對某個endpoint of device做傳輸(transfer)時, 它利用IO Request Packet (IRP)去呼叫USB Driver, 以啟動transfer • 一個transfer可能需要傳輸大量資料,因為USB是shared bus, 所以將傳輸分成多個transaction來完成 • 一個transaction乃是由多個packet組成(看是那種傳輸, 其transaction的packet組成方式不太一樣 • USB的傳輸基礎是以1ms為一個frame來排程

  21. Arrangement of IPRs to Transactions/Frames

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