The Usb communications architecture is designed to enable computing devices such as desktop Pcs and laptops to interconnect with many other peripheral devices. A Usb uses a two wire serial communications link running at one of three dissimilar speeds: 1.5 megabits per second (low speed), 12 Mbps (full speed) and 480 Mbps (high speed). Usb devices are categorized into discrete device classes such as display, communication, audio, mass warehouse and human interface.
The Human Interface device (Hid) Class defines coarse behaviors and functions for touch screen interfaces. The Hid class includes peripherals such as keyboards, mice, trackballs, joysticks, touch pads and touch screens that enable end users to operate and interact with the computing system. Hid also includes provisions for output directed to the end user. The Hid specification requires one operate endpoint and one interrupt endpoint with the host.
The operate pipe typically handles enumeration and device configuration while the interrupt pipe is used for data transfer. Descriptors define a Usb device to the host regarding the type of device, the device manufacturer, Usb1.x or 2.0 or 3.0 support, device configuration, endpoints quantities and types, etc. Touch screen device descriptors tell the host which endpoints and transfer types are supported. In addition to handling approved Usb requests, Hid devices must riposte to all approved Hid requests. Data must be transferred within defined structures called reports, which can be transferred over either the operate pipe or interrupt pipe.
Hid devices can transfer information stored in reports with the host through the operate pipes. This transfer is typically reserved for configuration information or device identification. The Usb specification defines the transfer sequence for the operate pipe, which has a higher level of error checking than other transfer methods. The host application programming interface (Api) calls used to send data through the operate pipe differ from those used for the interrupt pipe. While enumeration, the device requests how often it wants the host to ask for the data or send the data. After enumeration is complete, the host schedules data transfers on a periodic basis. If the enumerated device is a touch screen, the touch screen Usb bridge or the touch controller (e.g., a controller that supports Usb natively) assembles the coordinates of the finger(s), as well as other associated information such as finger width, signal strengths, finger speed and gestures, and shop that data in a buffer. The data is transmitted to the host at the interval specified previously or upon unavoidable events such as a finger's touch.
Usb Microcontroller Communications Architecture for Human Interface device (Hid)
