tailieunhanh - USB Complete fourth- P12

USB Complete fourth- P12: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 3 More about Time-critical Transfers Just because an endpoint is capable of a rate of data transfer doesn t mean that a particular device and host will be able to achieve the rate. Several things can limit an application s ability to send or receive data at the rate that a device requests. The limiting factors include bus bandwidth the capabilities of the device the capabilities of the device driver and application software and latencies in the host s hardware and software. Bus Bandwidth When a device requests more interrupt or isochronous bandwidth than is available the host refuses to configure the device. A high-speed interrupt endpoint can request up to three 1024-byte data packets in each microframe using as much as 40 of the bus bandwidth. To help ensure that devices can enumerate without problems the interrupt endpoints in default interfaces must specify a maximum packet size of 64 bytes or less. The device driver is then free to try to increase the endpoint s reserved bandwidth by requesting alternate interface settings or configurations. Many drivers don t support requesting alternate interface settings or configurations however. Isochronous endpoints might also request more bandwidth than is available. In particular full-speed endpoints on a bus and SuperSpeed endpoints can request over half of the bus bandwidth. To help ensure that devices will enumerate without problems default interfaces in USB and USB devices must request no isochronous bandwidth. In other words a default interface can transfer no isochronous data at all and typically includes no isochronous endpoints. After enumeration the device driver can request isochronous bandwidth by requesting an alternate interface setting or a configuration with one or more isochronous endpoints. Note that even full-speed endpoints must meet this requirement to comply with USB . A specific host might configure a device with non-compliant default interfaces but future editions of the .