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Introduction to USB 3.0 By Donovan (Don) Anderson, Vice President, MindShare, Inc. This paper is a brief review of the USB 3.0 implementation, focusing on USB 2.0 backward compatibility and on the major features associated with the Super- Speed (SS) bus. The goal is to provide the reader with a short and concise description of USB 3.0, and enough detail to give a good feel for the technology, protocols, and techniques. Due to the limited scope of this paper, some terminology and concepts are intro- duced but not fully developed. A MindShare Comprehensive USB 3.0 book is in the works that will provide all of the details. In the meantime, please check our website at www.mindshre.com to learn the availabiltity of our USB 3.0 classes and eLearning courses. Motivation for USB 3.0 USB 3.0 enables more demanding applications compared to USB 2.0 by address- ing its limitations: • Bandwidth - 5.0 Gb/sec SuperSpeed (SS) vs. 480 Mb/sec (High Speed) rate Power Conservation - link power states (U0 - U3) and function power man- agement Data Flow Control - poll once versus poll multiple times Error Handling - End-to-end and port-to-port error detection and retries versus only end-to-end retries with USB 2.0. The additional bandwidth provided by USB SS transactions can benefit applica- tions like real-time audio and video streaming that require higher bus band- width at regular intervals. Mass storage applications can also benefit from the SS bandwidth. For example, Table 1 lists approximate download times for the different transmission rates. Visit MindShare Training at www.mindshare.com 1

Introduction to USB 3.0 Table 1: Download Speeds SD Movie - 6GB USB Flash 16GB HD Movie - 25GB USB 1.0 (FS) ~ 2 hours ~ 6 hours ~ 9.25 hours USB 2.0 (HS) ~ 3.25 minutes ~ 9 minutes ~ 14 minutes USB 3.0 (SS) ~ 20 seconds ~ 54 seconds ~ 70 seconds SD= Stanrdard Definition; HD = High Definition (Source: USB-IF) USB 3.0 Topology Figure 1 provides an example of a USB 3.0 topology. A major feature of this topology is its support of all wired USB speeds (LS, FS, HS & SS), and this is accomplished via the two separate buses that are integrated into USB 3.0 cables, connectors and hubs. In the illustration, the SS bus is represented in red and consists of two differential signal pairs, one to transmit packets and one to receive. The standard USB 2.0 bus consists of a single differential pair that oper- ates in a half-duplex model. Notice also that SS devices connect to both the SS and USB 2.0 buses, and provide backward compatibility with older platforms that don’t support SS. 2 Visit MindShare Training at www.mindshare.com

Introduction to USB 3.0 Figure 1: Example USB 3.0 Topology CPU Host Bridge DRAM HS USB Host Controller SS FS Hub SS HS SS HS LS FS SS HS SS LS SuperSpeed Links High-Speed Links Full-Speed Links Low-Speed Links SS HS Hub LS HS FS Visit MindShare Training at www.mindshare.com 3

Introduction to USB 3.0 Figure 2 depicts the cross-section of a USB 3.0 cable and illustrates the SS and USB 2.0 buses along with the VBUS power pin that supplies power at 5 vdc and up to 900mA. The SS bus employs a dual-simplex approach that allows simultaneous trans- mission and reception of packets. There are many cases where an SS device may be both transmitting and receiving data at the same time. For example, during burst transactions a device may be receiving data from the host and returning acknowledgements associated with data already received. Figure 2: USB 3.0 Cable USB 3.0 Composite Cable Jacket Braid GND VBus D+ D- SSTX+ SSTX- SSRX+ SSRX- USB 2.0 UTP UTP VBus SDP GND SDP SSTX, SSRX SDPs W/Drain (2 Sets) GND VBus D+ D- USB 2.0 Tx/Rx Differential pair (UTP) SSRX+ SSRX- SSTX+ SSTX- USB 3.0 SuperSpeed Rx Differential pair (SDP) USB 3.0 SuperSpeed Tx Differential pair (SDP) USB 2.0 Links Versus SuperSpeed Links Unlike the USB 2.0 bus, SS links are constantly transmitting and receiving traffic to maintain synchronization in preparation for delivering the next packet. Each SS link must be trained at startup so the receivers can establish bit lock at the 5.0 4 Visit MindShare Training at www.mindshare.com

Introduction to USB 3.0 Gb/s rate. When an SS link is not transmitting a packet it sends traffic to keep the link operational, known as logical idle packets. Consequently, power con- sumption on SS links would be very high if they didn’t aggressively transition frequently into low power states (more details on link power management fea- tures are covered later). In contrast, USB 2.0 links are in the electrical idle state until it’s time to send a packet (see figure 3). To recognize packets on an SS link, a unique start-of-packet delimiter called an ordered set is required. SS USB uses a variety of ordered sets to identify the type of packet being sent. Figure 3: Link States During Idle Electrical Idle Logical Idle USB 2.0 Packet USB SS Packet Electrical Idle Logical Idle SS Protocol Improvements SS packet protocol is derived from the same Token/Data/Handshake model employed by USB 2.0, often referred to as the end-to-end protocol (See figure 4). Like USB 2.0 all transactions originate at the host, but SS improves the protocol and adds several new features to give better performance, efficiency, and power conservation, such as: Unicast transactions versus broadcast More efficient Token/Data/Handshake Sequence Data Bursting Improved end-to-end data Flow Control (poll once versus poll multiple) Visit MindShare Training at www.mindshare.com 5

Introduction to USB 3.0 Unicast Transactions SS transactions are routed directly from a root port to the target device, so only links in the direct path between the root port and target device see the traffic. That lets other links in the topology to enter or remain in a low power state. Fig- ure 4 illustrates the direct routing used when forwarding packets from the host to a target device. Figure 4: Unicast Transactions CPU Host Bridge DRAM HS USB Host Controller SS SS HS SS SS SS HS SS SS HS SS LS SS HS Hub Token / Data / Handshake Sequences An mentioned earlier, the SS end-to-end protocol is based on the standard USB 2.0 “Token/Data/Handshake” sequence. This section illustrates the differences between the USB 2.0 and SS implementations through an IN and OUT example. 6 Visit MindShare Training at www.mindshare.com

Introduction to USB 3.0 IN Transaction Examples Consider the example IN transaction in figure 5. The left side indicates the sequence of packets required to perform two back-to-back “token/data/hand- shake” transactions, requiring 6 packets be exchanged as follows: 1. Host broadcasts an IN Token packet (1) to initiate the transaction. 2. Device returns the requested DATA packet (2). 3. Host acknowledge receipt of data with an ACK handshake packet (3). 4. The example on the right indicates the packet sequence needed perform two back-to-back SS IN transactions, which requires only 5 packets be exchanged. Steps 1-3 are repeated. SS USB uses an ACK header (packet 1) to initiate an IN transaction. 1. 2. The SS device returns Data (packet 2). 3. The second ACK header (3) both acknowledges receipt of the data and requests a second transaction. 4. The second Data packet (4) is delivered by the device. 5. The final ACK header (5) acknowledges receipt of the data, but does not request additional data. Figure 5: Two Back-to-Back IN Transactions -- USB 2.0 versus SS Host Controller Host Controller 66 44 33 ACK IN Token ACK 11 IN Token DATA 55 55 ACK Header 33 ACK Header DATA 22 11 ACK Header DATA Hdr + Payload DATA Hdr + Payload 44 22 HSHS SSSS Visit MindShare Training at www.mindshare.com 7

Introduction to USB 3.0 OUT Transaction Examples Differences between USB 2.0 and SS OUT transactions are illustrated in Figure 6. The example on the left depicts two back-to-back OUT transactions that require 6 packets: 1. Host broadcasts an OUT Token packet (1) to initiate the transaction. 2. Host sends DATA packet (2) to the Device. 3. Device acknowledges receipt of data with an ACK handshake packet (3). 4. The right side of Figure 6 indicates the packet sequence required to perform two back-to-back SS OUT transactions, but requires only 4 packets be exchanged. 1. SS USB uses a DATA header (packet 1) to initiate an OUT transaction and to deliver data to the device. Steps 1-3 are repeated 2. Device acknowledges receipt of data via an ACK packet (2). 3. The second DATA packet (3) initiates the second transaction and delivers 4. Device acknowledges receipt of data via an ACK packet (4), completing the data to the device. sequence. Figure 6: Two Back-to-Back OUT Transactions -- USB 2.0 versus SS Host Controller Host Controller ACK 66 ACK 33 33 DATA Hdr + Payload 11 DATA Hdr + Payload ACK Header 44 ACK Header 22 55 44 22 11 DATA OUT Token DATA OUT Token HSHS SSSS 8 Visit MindShare Training at www.mindshare.com

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