USB Complete fourth- P5:This book focuses on Windows programming for PCs, but other computers and operating systems also have USB support, including Linux and Apple Computer’s Macintosh. Some real-time kernels also support USB. | Chapter 1 Figure 1-1. USB uses a tiered star topology. Each external hub has one upstream-facing port and one or more downstream-facing ports. devices. To increase the available bandwidth for USB devices many PCs have multiple host controllers each controlling an independent bus. Bus Speed Considerations A USB host supports all four speeds. A USB host supports low full and high speed. A USB host supports low and full speeds only. Exceptions include On-The-Go devices and other special-purpose hosts in embedded systems which may support only the speeds needed to access specific peripherals. A USB hub contains both a USB hub and a SuperSpeed hub and handles traffic at any speed. SuperSpeed traffic uses the SuperSpeed hub s circuits and wires and other traffic uses the USB hub s circuits and wires. A SuperSpeed-capable device communicates at SuperSpeed only if the host and all hubs between the host and device are USB Figure 1-2 . Otherwise the device must use a slower speed. For compatibility with USB hosts and hubs 16 USB Basics Figure 1-2. USB hosts and hubs support all four speeds for downstream communications. a SuperSpeed device that doesn t fully function at a lower speed must at least respond to bus resets and standard requests at another speed to inform the host that the device requires SuperSpeed to perform its function. A non-SuperSpeed high-speed-capable device communicates at high speed if the host and all hubs between are USB or higher Figure 1-3 . For compatibility with USB hosts and hubs a high-speed device that doesn t fully function at full speed must at least respond to bus resets and standard requests at full speed to inform the host that the device requires high speed to perform its function. Many high-speed devices function if more slowly at full speed because 17 Chapter 1 adding support for full speed is generally easy and is required to pass USB IF compliance tests. A device that supports full or low .
