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