I²C over HDMI

The Raspberry Pi contains several I²C interfaces for accessing peripherals such as sensors, DA-converters or EEPROMs. Two interfaces are accessible via P1 and P5. A third interface is integrated in the HDMI interface. It is disguised as Display Data Channel (DDC), but is nothing than a conventional I²C interface. The integration into the HDMI interface provides the possibility to transmit video, audio and control information via a single cable. Possible applications are reading out the capabilities of your monitor via EDID or controlling brightness and contrast.

In particular, it is interesting if you are planning to build a display for the Raspberry Pi. In this case it is very helpful to have an additional data channel for example to control a backlight circuit. Yet, it is more difficult to use than the other I²C interfaces of the Raspberry Pi and some precautions must be taken:

WARNING

• It might break your monitor irreparably: Via DDC it might be possible to set a monitor in an invalid state with no hope of recovery.
• It might interfere with the GPU: The GPU of the Raspberry Pi claims full control over the DDC interface. Accessing it from the CPU might cause problems.
• In contrast to the other I²C interfaces, it has a 5 V high level. You might want to use level shifting.

Physical Access

Since this interface is accessible via the HDMI connector, you can not just use jumper wires to access it. If the device you want to connect to, has an HDMI connector (e.g. if you want to control a monitor via DDC) you can skip this section. The same applies if it has a DVI connector, because a DVI-HDMI adapter forwards the according lines.

Building an own device is more complicated, because most HDMI connectors have 0.5 mm pitch SMD pins. For testing purposes it is more convenient to build an adapter from a HDMI cable. Get a short, cheap HDMI cable, cut it through and remove jacket and shield.

Now you have to solve a little riddle to match the pins to the correct wires. Apparently you can not rely on the colors. At least for my assignment I could not find any matching color scheme on the Internet. Therefore, I used a multimeter to identify which pin is connected to which wire. Have a look at Wikipedia to get the pin out. It helps to know, that the TMDS channels (Data 1-3, Clock) are seperately shielded. I soldered the wires to a pin header and can now access the HDMI interface via jumper cables. Luckily, there is also a 5 V line available that can be used to drive your circuit. Although, it must not consume much current (below 50 mA).

Kernel Patching

Because of these mentioned drawbacks, the current kernel does not support this I²C interface. You have to apply this patch to the rpi-3.10.y branch:

mkdir linux
cd linux
git init
git fetch git://github.com/raspberrypi/linux.git \
   rpi-3.10.y:refs/remotes/origin/rpi-3.10.y
git checkout rpi-3.10.y
wget https://s3.amazonaws.com/de.koalo.stuff/\
0001-Add-support-for-BSC2.patch
patch -p1 < 0001-Add-support-for-BSC2.patch

Then compile and copy it to the Raspberry Pi as described at elinux.org. You can now access this I²C interface as bus 2. The following should give you an overview about the I²C modules connected via HDMI.

sudo modprobe i2c-dev
sudo i2cdetect 2

Murphy's Touchscreen

It was pretty straightforward to connect the touchscreen of my display to the Raspberry Pi. After plugging in the USB cable, the touchscreen reports itself as eGalax TouchScreen:

pi@raspberrypi ~ $ lsusb
Bus 001 Device 002: ID 0424:9512 Standard Microsystems Corp.
Bus 001 Device 001: ID 1d6b:0002 Linux Foundation 2.0 root hub
Bus 001 Device 003: ID 0424:ec00 Standard Microsystems Corp.
Bus 001 Device 004: ID 0eef:0001 D-WAV Scientific Co., Ltd eGalax TouchScreen

Fortunately, there is a matching driver in the Linux kernel. It only wants to be activated. There are very detailed information about kernel compilation on the web, so I will skip that. In the configuration search for eGalax and activate the option for the corresponding USB driver (TOUCHSCREEN_USB_EGALAX).

After booting the new kernel, there should be a new device in /dev/input/by-id/ named usb-0eef_0001-event-if00. You can test it's functionality with

sudo evtest /dev/input/by-id/usb-0eef_0001-event-if00

Touching the touchscreen should produce some weird output (maybe you have to install evtest first). No further configuration is necessary at this point, everything will be detected automatically. It's time for startx:

I can touch the screen and the cursor moves - not to the position where my finger is, but this is only a matter of calibration. But, before I could start with that, the cursor stopped moving... I tried to reboot the Raspberry Pi, tested it at my PC, but the cursor does no longer react. I gave up and tested it again some time later. Surprise, surprise: It works again, but not for a long time. Furthermore, the evtest does no longer produce any output. Only sometimes it reports

Event: time 1364289593.457921, -------- SYN_REPORT --------

expected 16 bytes, got -1 

This is the same result as unplugging the USB cable. It looks very much as a hardware defect. I also recognized, that the case gets quite hot. Therefore, I opened the case:

There are three cable bundles coming out of the thick cable. It seemed very likely that two of them are for the TFT and one is for the touchscreen. Typically enough, the one for the touchscreen has four wires just like USB and the touchscreen still works when I unplug the two other connectors. We can focus on the little extra board sitting on top of the main board.

At the bottom you can see the USB cable, at the top the connector to the touchscreen. It seems to be a 4-wire resistive touchscreen. As the name implies, you only have to measure the resistance between the wires and calculate the touch position from that. There is a nice application note from Atmel about that topic. I measured the resistance between two of the pins marked in the picture above using a multimeter. The resistance changes when I touch the screen. There is also a small LED that lights up when I touch the screen (while the touchscreen is connected to the computer), but this only happens when the cursor also moves.

I don't think that I can fix the controller itself, but maybe I can connect another controller to the touchscreen some day, but until then I still own Murphy's touchscreen.

8" TFT for the Raspberry Pi

A small device calls for a small screen (or no screen at all). Luckily, I remembered that I bought a 8" TFT some years ago. I never used it for it's intended use case (I don't even remember what that was...), so I found it buried under some other electronic waste.

It is an Innovatek TM-868. It has VGA, audio connectors, an USB connector for touchscreen and a Composite Video connector. VGA is useless for the Raspberry Pi, but the Composite Video connector is of interest here, because there is a corresponding connector on the Raspberry Pi. This works out of the box, but only when you don't connect the HDMI cable at the same time, because the Pi can only drive one display at a time. If you want to leave the HDMI cable connected, you can also set an appropriate setting in the configuration file:

hdmi_ignore_hotplug=1

You can also set the video mode:

sdtv_mode=2 (PAL) or 0 (NTSC)

There are many other configuration options, but in most cases it should work directly as with my setup:

Next step is to connect the touchscreen.