1.. SPDX-License-Identifier: GFDL-1.1-no-invariants-or-later 2.. c:namespace:: V4L 3 4.. _touch: 5 6************* 7Touch Devices 8************* 9 10Touch devices are accessed through character device special files named 11``/dev/v4l-touch0`` to ``/dev/v4l-touch255`` with major number 81 and 12dynamically allocated minor numbers 0 to 255. 13 14Overview 15======== 16 17Sensors may be Optical, or Projected Capacitive touch (PCT). 18 19Processing is required to analyse the raw data and produce input events. In 20some systems, this may be performed on the ASIC and the raw data is purely a 21side-channel for diagnostics or tuning. In other systems, the ASIC is a simple 22analogue front end device which delivers touch data at high rate, and any touch 23processing must be done on the host. 24 25For capacitive touch sensing, the touchscreen is composed of an array of 26horizontal and vertical conductors (alternatively called rows/columns, X/Y 27lines, or tx/rx). Mutual Capacitance measured is at the nodes where the 28conductors cross. Alternatively, Self Capacitance measures the signal from each 29column and row independently. 30 31A touch input may be determined by comparing the raw capacitance measurement to 32a no-touch reference (or "baseline") measurement: 33 34Delta = Raw - Reference 35 36The reference measurement takes account of variations in the capacitance across 37the touch sensor matrix, for example manufacturing irregularities, 38environmental or edge effects. 39 40Querying Capabilities 41===================== 42 43Devices supporting the touch interface set the ``V4L2_CAP_VIDEO_CAPTURE`` flag 44and the ``V4L2_CAP_TOUCH`` flag in the ``capabilities`` field of 45:c:type:`v4l2_capability` returned by the 46:ref:`VIDIOC_QUERYCAP` ioctl. 47 48At least one of the read/write or streaming I/O methods must be 49supported. 50 51The formats supported by touch devices are documented in 52:ref:`Touch Formats <tch-formats>`. 53 54Data Format Negotiation 55======================= 56 57A touch device may support any I/O method. 58