5.1. Introduction¶
LIRC stands for Linux Infrared Remote Control. The LIRC device interface is a bi-directional interface for transporting raw IR and decoded scancodes data between userspace and kernelspace. Fundamentally, it is just a chardev (/dev/lircX, for X = 0, 1, 2, ...), with a number of standard struct file_operations defined on it. With respect to transporting raw IR and decoded scancodes to and fro, the essential fops are read, write and ioctl.
Example dmesg output upon a driver registering w/LIRC:
$ dmesg |grep lirc_dev
rc rc0: lirc_dev: driver mceusb registered at minor = 0, raw IR receiver, raw IR transmitter
What you should see for a chardev:
$ ls -l /dev/lirc*
crw-rw---- 1 root root 248, 0 Jul 2 22:20 /dev/lirc0
5.2. LIRC modes¶
LIRC supports some modes of receiving and sending IR codes, as shown on the following table.
LIRC_MODE_SCANCODE
This mode is for both sending and receiving IR.
For transmitting (aka sending), create a
struct lirc_scancode
with the desired scancode set in thescancode
member,rc_proto
set the IR protocol, and all other members set to 0. Write this struct to the lirc device.For receiving, you read
struct lirc_scancode
from the lirc device, withscancode
set to the received scancode and the IR protocolrc_proto
. If the scancode maps to a valid key code, this is set in thekeycode
field, else it is set toKEY_RESERVED
.The
flags
can haveLIRC_SCANCODE_FLAG_TOGGLE
set if the toggle bit is set in protocols that support it (e.g. rc-5 and rc-6), orLIRC_SCANCODE_FLAG_REPEAT
for when a repeat is received for protocols that support it (e.g. nec).In the Sanyo and NEC protocol, if you hold a button on remote, rather than repeating the entire scancode, the remote sends a shorter message with no scancode, which just means button is held, a “repeat”. When this is received, the
LIRC_SCANCODE_FLAG_REPEAT
is set and the scancode and keycode is repeated.With nec, there is no way to distinguish “button hold” from “repeatedly pressing the same button”. The rc-5 and rc-6 protocols have a toggle bit. When a button is released and pressed again, the toggle bit is inverted. If the toggle bit is set, the
LIRC_SCANCODE_FLAG_TOGGLE
is set.The
timestamp
field is filled with the time nanoseconds (inCLOCK_MONOTONIC
) when the scancode was decoded.
LIRC_MODE_MODE2
The driver returns a sequence of pulse and space codes to userspace, as a series of u32 values.
This mode is used only for IR receive.
The upper 8 bits determine the packet type, and the lower 24 bits the payload. Use
LIRC_VALUE()
macro to get the payload, and the macroLIRC_MODE2()
will give you the type, which is one of:
LIRC_MODE2_PULSE
Signifies the presence of IR in microseconds.
LIRC_MODE2_SPACE
Signifies absence of IR in microseconds.
LIRC_MODE2_FREQUENCY
If measurement of the carrier frequency was enabled with ioctl LIRC_SET_MEASURE_CARRIER_MODE then this packet gives you the carrier frequency in Hertz.
LIRC_MODE2_TIMEOUT
If timeout reports are enabled with ioctl LIRC_SET_REC_TIMEOUT_REPORTS, when the timeout set with ioctl LIRC_GET_REC_TIMEOUT and LIRC_SET_REC_TIMEOUT expires due to no IR being detected, this packet will be sent, with the number of microseconds with no IR.
LIRC_MODE_PULSE
In pulse mode, a sequence of pulse/space integer values are written to the lirc device using LIRC write().
The values are alternating pulse and space lengths, in microseconds. The first and last entry must be a pulse, so there must be an odd number of entries.
This mode is used only for IR send.
5.3. Remote Controller protocol¶
An enum rc_proto
in the LIRC Header File lists all the
supported IR protocols:
-
enum
rc_proto
¶ the Remote Controller protocol
Constants
RC_PROTO_UNKNOWN
- Protocol not known
RC_PROTO_OTHER
- Protocol known but proprietary
RC_PROTO_RC5
- Philips RC5 protocol
RC_PROTO_RC5X_20
- Philips RC5x 20 bit protocol
RC_PROTO_RC5_SZ
- StreamZap variant of RC5
RC_PROTO_JVC
- JVC protocol
RC_PROTO_SONY12
- Sony 12 bit protocol
RC_PROTO_SONY15
- Sony 15 bit protocol
RC_PROTO_SONY20
- Sony 20 bit protocol
RC_PROTO_NEC
- NEC protocol
RC_PROTO_NECX
- Extended NEC protocol
RC_PROTO_NEC32
- NEC 32 bit protocol
RC_PROTO_SANYO
- Sanyo protocol
RC_PROTO_MCIR2_KBD
- RC6-ish MCE keyboard
RC_PROTO_MCIR2_MSE
- RC6-ish MCE mouse
RC_PROTO_RC6_0
- Philips RC6-0-16 protocol
RC_PROTO_RC6_6A_20
- Philips RC6-6A-20 protocol
RC_PROTO_RC6_6A_24
- Philips RC6-6A-24 protocol
RC_PROTO_RC6_6A_32
- Philips RC6-6A-32 protocol
RC_PROTO_RC6_MCE
- MCE (Philips RC6-6A-32 subtype) protocol
RC_PROTO_SHARP
- Sharp protocol
RC_PROTO_XMP
- XMP protocol
RC_PROTO_CEC
- CEC protocol
RC_PROTO_IMON
- iMon Pad protocol