Raspberry Pi 2: LIRC with an active-low IR-receiver with raspbian Jessie

On my journey of using the Rasberry Pi 2 for something useful, the moment has come where I want to plug an Infra-Red-receiver to it.

I’m not new to this, for years I used one plugged to my PC’s serial port (RS232). On the software side I used LIRC via the lirc_serial kernel module.

Just typing “lirc rasperry pi 2” on any available search engine gives me hints and How-Tos and motivation telling me that it won’t be difficult. Basically plugging the data-out-line (Vs) to a GPIO (the default being GPIO18), the 3.3V-line to a 3.3-volt-supply and the GND-line to ground should do the trick on the hardware side. An IR-receiver has three “legs”: Vs, 3.3V and GND. Mine has the GND in the middle, Vs left and 3.3V right (from the front view, where the half-bulb is).

I’m using Raspian Jessie and thus a recent kernel which uses DeviceTree for configuring the board’s hardware capabilities (previously this low-level board-layout-configuration was compiled into the kernel). To make the lirc_rpi-module work, the system needs to reserve GPIOs for it at boot-time. On a fresh jessie-installation that means I only need to un-comment one line (a DeviceTreeOverlay-line) of the config.txt-file in /boot:

# Uncomment this to enable the lirc-rpi module

I plug the device using GPIO18 for data-in, I check the voltage, I chang the config.txt, I reboot, I run the mode2-tool and tap on my remote-controls. And nothing appears.

Fortunately, to debug such issues, I have access to superior tools, in this case an oscilloscope and, even more important, a competent colleague with hardware-knowledge. We, or better he, find(s) the problem within 2 minutes. Alone I would have wondered and doubted everything.

IR receiver on RPI, with probe plugged

After plugging the Raspi and its IR-receiver to the oscilloscope (probing the data-line of course) and putting the trigger level low enough we can observe the following sequence appearing when playing around with a remote control:

IR receiver on RPI, pull-down-active

As my colleague takes the first glimpse at this curve he immediately shouts: “it is missing a pull-up-resistor”. What makes him say that is the amplitude-delta of only 1 volt compared to the expected 3.3 volt.

This IR-receiver is an active-low receiver: it forces or drives the line to 0 when it wants to transmit a zero and it does nothing on the line to signal a one. To have the signal bounce back to the 3.3 volt when the device releases a pull-up-resistor is needed. Which is a high-Ohm-resistor (10K for example) coupling the data-line with the 3.3 volt supply line. The opposite is a pull-down-resistor: coupling the data-line with the ground for devices which are active-high and thus driving a 1 to signal a one and nothing to signal a zero.

Neither-nor is present in my setup which is the reason for the 1-values to be stuck at ~1 volt. Depending on the load here and there on the GPIO-PADs of the Raspi we could have also seen 2 volt or even nothing.

While my colleague looks for a high-Ohm resistor I’m sure that the chip-maker has thought of this problem and that there is an internal pull-up or pull-down on each GPIO-pad which can be activated. After quick search I find indeed what I was looking for: setting a GPIO-pull-up (or down) is a parameter to the dtoverlay of lirc-rpi.

# Uncomment this to enable the lirc-rpi module

After applying this change and rebooting, running mode2 gives me the numbers printed out I expect and on the oscilloscope a clean amplitude of 3.3 volts is seen:

IR receiver on RPI, pull-up-active

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