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README for avrlirc

avrlirc implements an IR receiver for use with lircd. It connects via a serial port, and the included host-based daemon relays the IR commands to an lircd daemon locally or elsewhere on the network.


  • Unlike the traditional LIRC "home-brew" serial port IR dongle, or even newer USB-based dongles, there is no requirement here for a special kernel driver. Any serial port capable of at least 38400 baud can be used, including USB-to-serial converters (which can also provide a convenient source of 5V power for the AVR micro).

  • Unlike some other AVR-based solutions (which mostly connect via USB), there is no need for a new type of config -- the same lircd.conf file used for the home-brew receivers will work with this. Nor is there a need for a special kernel driver.

  • A side-effect of using a network connection to access lircd is that the avrlirc device need not be connected to the same machine as that which runs the lircd process that is interpreting the data. In my case, I have IR receivers attached to several workstations, all of which control home automation and home audio tasks which run on a central server. Using avrlirc, the lircd daemons can all run on the the central server (each listening on a differnt port) greatly simplifying configuration changes.

AVR part

The first part is a simple program (avrlirc.c) that runs on an ATTiny2313 AVR microcontroller, or probably many others, without much work -- an onboard hardware-based UART is the main requirement. (The 16-bit timer and the input capture register are used as well.) The code monitors the pulse stream from a standard IR receiver (similar to the Panasonic PNA4602M or Sharp GP1UD261XK0F), and sends the pulse timing information to a host computer via the RS232 serial link. The data format is designed to closely match that of the LIRC ( UDP driver.

Host part(s)

The second part of the project is a host daemon which reads the avrlirc data stream from the serial port and relays it (via a UDP socket connection) to a listening lircd daemon, which would be started using a commandline like:

lircd --driver=udp [ with optional --device=<port-number> ]

There are two versions of the host relay daemon included here:


The simpler one, avrlirc2udp.c, does everything one might need for simple remote control handling: it reliably waits for its configured serial port to be available, converts any received data to UDP, and forwards it to an lircd listening port. It can also forward over TCP, making it possible to pass through an ssh tunnel, for instance. The accompanying "udptcp" script can be used to relay back to UDP on the far end.


The other host daemon, airboard-ir.c, implements all of that plus full support for infrared (IR) keystroke and mouse data from an Airboard keyboard, also known as a Liteon, or an SK-7100, made by Silitek. The keyboard has been rebadged by Motorola and Gateway, perhaps others. It forwards the user input data to the linux kernel, and forwards the IR data to an instance of lircd.

Output from this program goes to three separate destinations:

  • Keyboard and mouse events can be injected into the kernel (using the uinput driver), where they will automatically become available to the rest of the system.

  • The "special" multimedia buttons can be directed to a specified UDP port. (Otherwise they will be "injected" to the kernel, along with the rest of the keys.)

  • Consumer IR codes (from "regular" remote control devices) can be sent to an lircd server, using the LIRC UDP protocol. Visit for more information about lircd.

The code includes documentation for the reverse-engineered protocol from the keyboard, if you wished to write a different sort of driver.

(Rationale: The Airboard has its own IR receiver, which works well enough, except that a) the output is PS/2-only (a "dumb" PS/2-to-USB adapter won't convert it), and worse, b), the mouse is serial-only -- again, a serial to PS/2 adapter won't help. With the disappearance of serial ports, and even of PS/2 ports, this otherwise excellent keyboard is becoming obsolete. Using airboard-ir with an avrlirc receiver has obvious advantages.)

Building the code

The AVR code was written using avr-gcc -- the Makefile is pretty simple. Loading the hex file into the microcontroller, however, is beyond the scope of this README. :-)

The avrlirc2udp and airboard-ir programs assume POSIX termio access for controlling access to the serial port.


See the top of avrlirc.c for a pinout diagram. Other than 5V and ground, the only connections are the IR receiver, one jumper between two pins on the AVR, and the data output line. An RS232 line driver probably isn't necessary, since most consumer-grade serial ports these days will accept TTL-level pseudo-RS232 signalling. But a line driver like the Maxim MAX233 can be used if necessary.

The 38400 baud rate seems to be fast enough to support any remotes I've tried. If it weren't, one might have to resort to adding a crystal (and capacitors) to run the AVR micro faster. At higher speeds, you'll probably need to use the atmel's txd directly, connected via an RS232 line driver (max232 or equiv).

If pin 7 on the ATtiny2313 is grounded, the code will emit a stream of 'U' characters on the serial port, useful for debugging the transmission path.


I've built several of these devices. The first was before I'd figured out that I could get away with fewer parts, so it's the most complex. It includes a 14.7456Mhz crystal, so the AVR can put out 115.2Kbaud, and it includes a proper TTL-to-RS232 level converter (it's a MAX233). Because of this, it also has the capability of receiving serial data from the host. In principle I could extend it to provide other information (I/O port pin status?) on demand, or maybe add IR transmit capabilities, not that I've ever felt the need.

The second that I built is also built around a cannibalized USB-to-serial converter, but with a lower parts count: there's no Maxim chip to convert signal levels. The "case" for the receiver was given to me many years ago, much as you see it here, as a joke. I'm happy to get it out of the cellar and out to where it can be properly displayed.


The code contained here is a follow-on to the avrlirc-0.0.5 project released by Karl Bongers sometime in 2002. This version was initially written in April and May of 2007 by Paul Fox. It bears little relationship to Karl's initial code, but his code was an invaluable jumpstart.

Paul Fox, May 9, 2007
(minor revision, Feb 9, 2009)
(re-hosted at github, April 2013)


AVR-based IR interface for lircd




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