dev-touch.rst (1926B)
1.. SPDX-License-Identifier: GFDL-1.1-no-invariants-or-later 2 3.. _touch: 4 5************* 6Touch Devices 7************* 8 9Touch devices are accessed through character device special files named 10``/dev/v4l-touch0`` to ``/dev/v4l-touch255`` with major number 81 and 11dynamically allocated minor numbers 0 to 255. 12 13Overview 14======== 15 16Sensors may be Optical, or Projected Capacitive touch (PCT). 17 18Processing is required to analyse the raw data and produce input events. In 19some systems, this may be performed on the ASIC and the raw data is purely a 20side-channel for diagnostics or tuning. In other systems, the ASIC is a simple 21analogue front end device which delivers touch data at high rate, and any touch 22processing must be done on the host. 23 24For capacitive touch sensing, the touchscreen is composed of an array of 25horizontal and vertical conductors (alternatively called rows/columns, X/Y 26lines, or tx/rx). Mutual Capacitance measured is at the nodes where the 27conductors cross. Alternatively, Self Capacitance measures the signal from each 28column and row independently. 29 30A touch input may be determined by comparing the raw capacitance measurement to 31a no-touch reference (or "baseline") measurement: 32 33Delta = Raw - Reference 34 35The reference measurement takes account of variations in the capacitance across 36the touch sensor matrix, for example manufacturing irregularities, 37environmental or edge effects. 38 39Querying Capabilities 40===================== 41 42Devices supporting the touch interface set the ``V4L2_CAP_VIDEO_CAPTURE`` flag 43and the ``V4L2_CAP_TOUCH`` flag in the ``capabilities`` field of 44:c:type:`v4l2_capability` returned by the 45:ref:`VIDIOC_QUERYCAP` ioctl. 46 47At least one of the read/write or streaming I/O methods must be 48supported. 49 50The formats supported by touch devices are documented in 51:ref:`Touch Formats <tch-formats>`. 52 53Data Format Negotiation 54======================= 55 56A touch device may support any I/O method.