From the day i received my beagleboard rev. B7 i was missing an easy way to use the digital video interface with a small LCD screen. Many people seem to have the same problem and often end up with big and expensive adaptors. Further indication that this is a common problem comes from the fact that the latest incarnation of the beagleboard includes a way to directly interface to the OMAPs parallel video interface. Unfortunatately this only provides 1.8v signals and still needs a converter to interface to the 3.3v signals most raw LCD use.
This page describes a cheap and simple yet flexible HDMI to parallel 3.3v interface. This allows to connect most LCDs frames to the beagleboard without any further interface required. I am using this myself with some 7inch 800x480 displays which i am using to run Angstrom Linux as well as the maemo on beagleboard project. I have used these lately for some work on Meego.
The hardware is rather simple and doesn't consist of much more than the TFP101A panel link receiver from TI. This chip directly outputs 3.3V signals. The dvi2par routes these signals though 22R resistor arrays to reduce reflections. The resulting signals can be fed directly into a LCD screen as depicted below.
The main components is the TFP101A. I got mine as a free sample from TI. All remaining parts have been bought in germany from Reichelt and Conrad. The part numbers are:
|Reichelt Part No.
|Conrad Part No.
|ST 24C01 MN
|Resistor array 4*22R SMD
|SMD 0805 4,70K
|SMD 0805 100
|SMD 0805 47,0K
|Diode 1N 4001
|SMD 1N 4001
|SL 1X36G 2,54
The dvi2par is meant to directly interface to the LCD board. It can be setup to either be run from >5V using the included voltage regulator. Alternally it can be run directly from the 3.3V likely already present to supply the dispay itself as seen in the image below.
Also visible in this picture is the fact that only the upper 6 of the 8 color bits available on the dvi2par are being used. It's important to make sure that the upper bits are being used if the display supports less than 24 bit color depth. Also visible in the image is the fact that a tiny wire goes from pin 8 of the TFP101A to some other parts on the board is mounted on. Pin 8 carries the SCDT signal which indicates that a valid signal has been detected in the input. In this example the signal is routed to the backlight inverter and makes sure that the backlight is only switched on if a valid HDMI signal is being received.
The dvi2par also includes an optional i2c eeprom which is meant to allow to use the dvi2par on hosts other than the beagleboard which support to use a displays ddc/edid configuration. However, this part has only been verified to be operational and to be usable on a standard linux desktop PC. But since this part is not necessary on a beagleboard, it isn't tested to all detail.
In order to use it, you need to build an EDID description for your particular display using e.g. the Phoenix EDID designer 1.3 (A windows software which runs fine in wine). The output of this editor needs then to be programmed into the dvi2par ddc eeprom. The jumper JP1 has to be set to enable writing the eeprom. The i2c port of the hdmi connector then needs to be connected to some linux controlled i2c hardware like e.g. my i2c-tiny-usb. Below is an image of my i2c-tiny-usb based adptor used to write the ddc eeprom on the dvi2par.
The archive contains some scripts (prom.sh) and a tool (eeprom.c) to write the ddc data to the eeprom using the depicted i2c-tiny-usb setup.
I have two spare converted 7inch EEEPC screens i am willing to trade for interesting parts like e.g. a beagleboard xm or similar goods. These screen come with a little stand, and backlight inverter. The displays are ready-to-run and only require a single 12V 1A power supply. I can provide a beaglebaord linux kernel configured to match these displays. Send me an email if you are interested.