tailieunhanh - USB Complete fourth- P25

USB Complete fourth- P25: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 8 more about how to implement low-level communications one source of information is the source code and other documentation from the Linux USB Project. Host Controller Types To access low- and full-speed devices a USB or USB host can use a controller that conforms to the Open Host Controller Interface OHCI standard or the Universal Host Controller Interface UHCI standard. To access high-speed devices a USB host uses a host controller that conforms to the Enhanced Host Controller Interface EHCI standard. USB hosts use a single Extensible Host Controller Interface xHCI controller for all speeds. The USB-IF s website has links to the specifications. For information about which host-controller types are in a PC in Windows Device Manager look under Universal Serial Bus controllers. To view a driver s name right-click a host controller s entry and select Properties Driver Driver Details. One of the drivers listed should have ohci uhci or ehci in the name. Chapter 9 has more about the Device Manager. OHCI and UHCI Differences OHCI and UHCI controllers both provide a way for the low- and full-speed USB hardware to communicate with higher-level drivers but each takes a different approach. UHCI places more of the communications burden on software and thus can use simpler cheaper hardware. OHCI places more of the burden on the hardware and allows simpler software control. UHCI was developed by Intel and OHCI was developed by Compaq Microsoft and National Semiconductor. Motherboards tend to have UHCI controllers and expansion cards tend to have OHCI controllers. The differences between host controllers should be transparent to driver developers and application programmers. Both controller types comply fully with the USB specification. Their performance can differ however. Developers shouldn t assume a device works fine based on tests with one host-controller type. An OHCI controller can schedule more than one stage of a control transfer in a single .