pcDuino and the Adafruit 5" LCD HDMI Display

After my inability to get the 7" LVDS display to work with the pcDuino I thought I would have a go with the 5" Adafruit display that I use with my Raspberry Pi. While not plug and play, it is significantly easier than the LVDS.

The Adafruit 5" 800x480 HDMI Backpack

This backpack features the TFP401 for decoding video, and for the touch version, an AR1100 USB resistive touch screen driver. Connecting the pcDuino to the screen is easy, just plug a HDMI cable into both.

You can power the display from a USB port on your pcDuino/Raspberry Pi but it is probably better to have a separate supply. Particularly for the pcDuino since it only has one USB port which you probably want to use for the keyboard and mouse.

With the default 5" 800x480 display and 50mA backlight current, the current draw is 500mA total. You can reduce that down to 370mA by running the backlight at half-brightness (25mA).

The gotcha with this display is that the TFP401 decoder chip does not contain a video scaler, it will not resize/shrink video! This means that you must feed it a resolution of 800x480 @ 60Hz or you wont see anything!

Forcing the resolution of LightDM

LightDM is the display manager running in Ubuntu. fortunately it is fairly straight forward to set the resolution used by LightDM. To find out the name of your display you can type xrandr into terminal. This indicated that my display was called LCD. Note you can't do this via SSH, you need to be on the pcDuino with the screen attached.

Next you need to find out the parameters for your screen so that we can add a new display mode. Type the following into a terminal screen.

$cvt 800 480

Then copy what comes after modeline. It should look something like: "800x480_60.00"   29.50  800 824 896 992  480 483 493 500 -hsync +vsync. To create and add a new display mode you then type:

$xrandr --newmode "800x480_60.00"   29.50  800 824 896 992  480 483 493 500 -hsync +vsync

xrandr --addmode LCD 800x480_60.00

You can then make sure that the new mode works:

$xrandr --output LCD --mode 800x480_60.00

With a bit of luck LightDM will now fill your LCD screen.

To ensure that this is done every time the pcDuino boots, you can add it to the profile bash script we attempted to use to load the touch driver in the last post.

ubuntu@ubuntu:~$ sudo nano /etc/profile

Add the following lines to the end of the file, save, exit and reboot.

# Set resolution for the Adafruit 5" Screen
xrandr --newmode "800x480_60.00"   29.50  800 824 896 992  480 483 493 500 -hsync +vsync
xrandr --addmode LCD 800x480_60.00
xrandr --output LCD --mode 800x480_60.00