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1.. SPDX-License-Identifier: GFDL-1.1-no-invariants-or-later2 3.. _touch:4 5*************6Touch Devices7*************8 9Touch devices are accessed through character device special files named10``/dev/v4l-touch0`` to ``/dev/v4l-touch255`` with major number 81 and11dynamically allocated minor numbers 0 to 255.12 13Overview14========15 16Sensors may be Optical, or Projected Capacitive touch (PCT).17 18Processing is required to analyse the raw data and produce input events. In19some systems, this may be performed on the ASIC and the raw data is purely a20side-channel for diagnostics or tuning. In other systems, the ASIC is a simple21analogue front end device which delivers touch data at high rate, and any touch22processing must be done on the host.23 24For capacitive touch sensing, the touchscreen is composed of an array of25horizontal and vertical conductors (alternatively called rows/columns, X/Y26lines, or tx/rx). Mutual Capacitance measured is at the nodes where the27conductors cross. Alternatively, Self Capacitance measures the signal from each28column and row independently.29 30A touch input may be determined by comparing the raw capacitance measurement to31a no-touch reference (or "baseline") measurement:32 33Delta = Raw - Reference34 35The reference measurement takes account of variations in the capacitance across36the touch sensor matrix, for example manufacturing irregularities,37environmental or edge effects.38 39Querying Capabilities40=====================41 42Devices supporting the touch interface set the ``V4L2_CAP_VIDEO_CAPTURE`` flag43and the ``V4L2_CAP_TOUCH`` flag in the ``capabilities`` field of44:c:type:`v4l2_capability` returned by the45:ref:`VIDIOC_QUERYCAP` ioctl.46 47At least one of the read/write or streaming I/O methods must be48supported.49 50The formats supported by touch devices are documented in51:ref:`Touch Formats <tch-formats>`.52 53Data Format Negotiation54=======================55 56A touch device may support any I/O method.57