Controlling insanity by parsing IR codes with Linux::IRPulses

Sending information with pulsing invisible light can be surprisingly complex. Disentangle the problem with LIRC and Linux::IRPulses.

Infrared remotes are one of those things where every manufacturer thinks they have the One True Way™ of doing it. You would think that there’s only one or two straightforward ways to pulse a little IR light. Clearly, we’re all wrong, because the home entertainment industry invents new ones all the time. That’s not even counting the other sectors and hobbyist projects that come up with entirely different methods.

The Linux Infrared Remote Control (LIRC) project has produced mappings for a lot of remotes out there. That doesn’t help with some of the more fringe devices. Also, some of the components at the top of the stack are geared towards executing a program after detecting a valid series of pulses.

If we would rather take the pulses and handle them within our own program, then we need to ignore the top layers of LIRC and parse the pulse data directly. That is what Linux::IRPulses does.

We first need the hardware to detect the pulses. On a regular computer, there are many modules available which can be plugged in to a USB port. On a single board computer like the Raspberry Pi, we have General Purpose Input/Output (GPIO) pins, which can read the timing of the pulses.

Setting up the Raspberry Pi

Skip this section if you’re using a regular IR device. If you want to set up a module on the Raspberry Pi’s GPIO pins, then read on.

First, you need a module of the right frequency for the IR data you’re trying to receive. If you’re using a remote from an old TV, then searching around for “ IR protocol” should get you the right answer. 38 KHz is a common frequency, but this is just the first thing that manufacturers all did differently.

The TSOP38138 is an IR remote receiver that runs at 38KHz. It’s part of a family of devices that run at different frequencies, any of which are likely adequate.

IR receivers for picking up remote data have three pins: power, ground, and data. Connect power to a +3.3V pin on the Raspberry Pi, ground to ground, and data to GPIO 23. See the Raspberry Pi GPIO documentation for the location of the pins.

Now we need to configure LIRC. Start with a simple installation of the package with apt-get install lirc. Next, we need to load the kernel module, tell LIRC where to find the pin, and also configure some Raspberry Pi boot options.

In /etc/modules-load.d/modules.conf, put:

lirc_dev
lirc_rpi gpio_in_pin=23 gpio_out_pin=22

That will make GPIO 23 as your input pin. LIRC can also be setup to send IR data, so we set GPIO 22 for that as long as we’re here. Next, modify /etc/lirc/hardware.conf:

# /etc/lirc/hardware.conf
#
# Arguments which will be used when launching lircd
LIRCD_ARGS="--uinput"

#Don't start lircmd even if there seems to be a good config file
#START_LIRCMD=false

#Don't start irexec, even if a good config file seems to exist.
#START_IREXEC=false

#Try to load appropriate kernel modules
LOAD_MODULES=true

# Run "lircd --driver=help" for a list of supported drivers.
DRIVER="default"
# usually /dev/lirc0 is the correct setting for systems using udev
DEVICE="/dev/lirc0"
MODULES="lirc_rpi"

# Default configuration files for your hardware if any
LIRCD_CONF=""
LIRCMD_CONF=""

The particularly important things to note here are DEVICE for the device path, and MODULES for the Raspberry Pi GPIO driver.

Finally, edit /boot/config.txt and add this somewhere in the file:

dtoverlay=lirc-rpi,gpio_in_pin=23,gpio_out_pin=22

And then reboot. Once you’re back up, you can test it by plugging in your IR module to the right pins and pointing a remote at it. Using mode2 -d /dev/lirc0, you should see the pulse and space data being sent.

Decoding the undecodable

Sony runs their remotes at 40KHz. It starts by sending a header of a 2400μs pulse and 600μs space. After the header, a 1 bit is sent by a 1200μs pulse, and a zero with a 600μs. Between those ones and zeros are 600μs spaces. Codes could be 12, 15, or 20 bits long depending on the remote. This is about as straightforward as things get.

NEC uses a 38KHz carrier frequency. There’s a 9000μs header followed by a 4500μs space. A 1 bit is sent by a 562.5μs pulse. A 0 bit is sent by a 562.5μs pulse. Wait, what? No, that’s not a typo. NEC differentiates ones and zeros by the length of the space that comes after the pulse: 1687.5μs for 1, and 562.5μs for 0.

EasyRaceLapTimer (an Open Source quadcopter race timer system) is on a 38KHz frequency. It sends a 300μs pulse followed by a 300μs space. It then alternates sending pulses and spaces, with a 1 bit being 600μs, and a 0 bit being 300μs.

All the timing numbers above are big fat lies. The noisy, analog nature of the world means the actual values coming from the IR receiver will be different from the specified values, perhaps by as much as 15%. It’s safe to assume that reverse engineered specifications are only guessing at the actual values that the manufacturer intended.

All that is to say that we have a complicated job on our hands, and the above only covers a few of the examples out there.

Linux::IRPulses

The goal of this module is to simplify the process reading these pulses and spaces while tolerating the numbers being off.

At present, the module works by parsing the output of LIRC’s mode2 program. This may change to reading directly from /dev/lirc0 in the future. For now, we’ll start by opening a pipe to mode2:

open( my $in, '-|', 'mode2 -d /dev/lirc0' ) or die "Can't exec mode2: $!\n";

We now need to define the protocol to Linux::IRPluses’s constructor. To help with this, adding a use Linux::IRPulses will export the subroutines pulse(), space(), and pulse_or_space(). These are used to specify what you expect to come in for pulses or spaces.

For instance, we know that NEC sends a 9000μs pulse and 4500μs space for its header. We tell the constructor this with:

my $ir = Linux::IRPulses->new({
    header => [ pulse 9000, space 4500 ],
    ...
});

The parser goes through each entry in the array, checking off that the given pulse or space data is what we expect. Once it reaches the end of the header array, it marks the header as good and then looks for valid data for ones and zeros. We specify those in much the same way. We’ll add in the other constructor parameters here, as well:

my $ir = Linux::IRPulses->new({
    header => [ pulse 9000, space 4500 ],
    zero => [ pulse 563, space 563 ],
    one => [ pulse 563, space 1688 ],
    bit_count => 32,
    callback => sub {
        my ($args) = @_;
        my $code = $args->{code};
        say "Received code $code";
    },
});

The parser will continue looking for ones and zeros until it’s collected enough for the given bit_count. Once the right number has been met, it calls the subref specified in callback with a hashref. The hashref contains keys for code (the IR code that was detected) and pulse_obj (the Linux::IRPulses object). All the length numbers are checked with a tolerance of 20%.

We don’t promise you will keep your sanity after working with IR data, but hopefully Linux::IRPulses can help you go mad with dignity.

(Original photo CC-BY 2.0 by Stefanus Ming at https://flic.kr/p/7djHYP)

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Timm Murray

Timm Murray is a Perl programmer and hardware specialist. He is the creator of UAV::Pilot the de-facto standard Perl drone library. He regularly blogs about Perl and hardware hacking.

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