Posts Tagged ‘interface’
Human Interface Device
Posted by Parkzone Corsair in USB on December 3rd, 2009
The name human interface device suggests that HIDs interact directly with people, and many HIDs do just that. A mouse detects when someone moves it or presses a key. A host may send data that translates to an effect that a user senses on a joystick. Besides keyboards, mice, and joysticks, devices with HID interfaces include remote controls; telephone keypads; game controls such as data gloves and steering wheels; barcode readers; and UPS units.
Devices with physical control panels can use a HID interface to send control-panel input to the host. Devices with virtual control panels on the host can use a HID interface to send control-panel data to the device.
A virtual control panel can be cheaper to implement than traditional physical controls on a device.A HID doesn’t have to have a human interface. The device just needs to be able to function within the limits of the HID class specification.
Of USB’s Four Transfer Types
Posted by Parkzone Corsair in USB on November 19th, 2009
Because all devices share the bus, a device has no guarantee that a particular rate or maximum latency will be available on attachment. If the bus is too busy to allow a requested transfer rate or maximum latency, the host refuses to complete the configuration process that enables the host to schedule transfers. The device’s driver can then request a configuration or interface that requires less bandwidth. To take full advantage of reserved bandwidth, the device driver and application software and device firmware must eliminate retries as much as possible.
The device should have data ready to send when the host requests it and should be ready to accept data when the host sends it.Of USB’s four transfer types, the fastest on an otherwise idle bus are bulk transfers,with theoretical maximums of around 1.2 MB/s at full speed, 53 MB/s at high speed, and 400 MB/s at SuperSpeed. Isochronous transfers can request the most bandwidth (1.023 MB/s at full speed, 24.576 MB/s at high speed, and 393 MB/s at SuperSpeed). Low speed doesn’t support bulk or isochronous transfers, and the maximum guaranteed bandwidth for a single low-speed transfer is 800 bytes per second.
