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- .. SPDX-License-Identifier: GPL-2.0
- Remote Controller devices
- -------------------------
- Remote Controller core
- ~~~~~~~~~~~~~~~~~~~~~~
- The remote controller core implements infrastructure to receive and send
- remote controller keyboard keystrokes and mouse events.
- Every time a key is pressed on a remote controller, a scan code is produced.
- Also, on most hardware, keeping a key pressed for more than a few dozens of
- milliseconds produce a repeat key event. That's somewhat similar to what
- a normal keyboard or mouse is handled internally on Linux\ [#f1]_. So, the
- remote controller core is implemented on the top of the linux input/evdev
- interface.
- .. [#f1]
- The main difference is that, on keyboard events, the keyboard controller
- produces one event for a key press and another one for key release. On
- infrared-based remote controllers, there's no key release event. Instead,
- an extra code is produced to indicate key repeats.
- However, most of the remote controllers use infrared (IR) to transmit signals.
- As there are several protocols used to modulate infrared signals, one
- important part of the core is dedicated to adjust the driver and the core
- system to support the infrared protocol used by the emitter.
- The infrared transmission is done by blinking a infrared emitter using a
- carrier. The carrier can be switched on or off by the IR transmitter
- hardware. When the carrier is switched on, it is called *PULSE*.
- When the carrier is switched off, it is called *SPACE*.
- In other words, a typical IR transmission can be viewed as a sequence of
- *PULSE* and *SPACE* events, each with a given duration.
- The carrier parameters (frequency, duty cycle) and the intervals for
- *PULSE* and *SPACE* events depend on the protocol.
- For example, the NEC protocol uses a carrier of 38kHz, and transmissions
- start with a 9ms *PULSE* and a 4.5ms SPACE. It then transmits 16 bits of
- scan code, being 8 bits for address (usually it is a fixed number for a
- given remote controller), followed by 8 bits of code. A bit "1" is modulated
- with 560µs *PULSE* followed by 1690µs *SPACE* and a bit "0" is modulated
- with 560µs *PULSE* followed by 560µs *SPACE*.
- At receiver, a simple low-pass filter can be used to convert the received
- signal in a sequence of *PULSE/SPACE* events, filtering out the carrier
- frequency. Due to that, the receiver doesn't care about the carrier's
- actual frequency parameters: all it has to do is to measure the amount
- of time it receives *PULSE/SPACE* events.
- So, a simple IR receiver hardware will just provide a sequence of timings
- for those events to the Kernel. The drivers for hardware with such kind of
- receivers are identified by ``RC_DRIVER_IR_RAW``, as defined by
- :c:type:`rc_driver_type`\ [#f2]_. Other hardware come with a
- microcontroller that decode the *PULSE/SPACE* sequence and return scan
- codes to the Kernel. Such kind of receivers are identified
- by ``RC_DRIVER_SCANCODE``.
- .. [#f2]
- The RC core also supports devices that have just IR emitters,
- without any receivers. Right now, all such devices work only in
- raw TX mode. Such kind of hardware is identified as
- ``RC_DRIVER_IR_RAW_TX``.
- When the RC core receives events produced by ``RC_DRIVER_IR_RAW`` IR
- receivers, it needs to decode the IR protocol, in order to obtain the
- corresponding scan code. The protocols supported by the RC core are
- defined at enum :c:type:`rc_proto`.
- When the RC code receives a scan code (either directly, by a driver
- of the type ``RC_DRIVER_SCANCODE``, or via its IR decoders), it needs
- to convert into a Linux input event code. This is done via a mapping
- table.
- The Kernel has support for mapping tables available on most media
- devices. It also supports loading a table in runtime, via some
- sysfs nodes. See the :ref:`RC userspace API <Remote_controllers_Intro>`
- for more details.
- Remote controller data structures and functions
- ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
- .. kernel-doc:: include/media/rc-core.h
- .. kernel-doc:: include/media/rc-map.h
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