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Alex makes HDMI Wings for the Papilio Plus!


alex
  • Alex designs an HDMI Wing and a custom PCB that lets him connect his salvaged LVDS laptop monitor to the HDMI Wing on his Papilio Plus. Next we need to get this working on the soon to come Papilio Pro!

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Finally! PCB goodness in them mails. Feels like xmas.

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First off I assembled the super easy Papilio HDMI wing, You can see different views of it in the insets as well as how it fits on the Pap board. It is technically not an 8 bit wing nor a standard 16 bit wing but a kind of modified 16 bit wing as it straddles two 8 bit wing slots due to the requirement for differential pins. The row of 4 pins with the missing pin connects to power and ground, the next row of 8 pins connects to the standard 8 bit wing and finally the last row of 4 pins connects to the adjacent 8 bit wing's pins to get to the differential pair's other pins which are not available on any 8 bit wing alone.

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Next filing the PCB corner to make it fit in the oddly shaped space and line up with the screw holes.

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Finally (for today) assembled the PCB with the tiny 3.3V switch mode power supply regulator, the pot for adjusting the LCD brightness and the HDMI connector where the LCD differential pins will connect to.

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The only testing was to apply power to the board above and check that I get 3.3V which initially I wasn't. This is what happens when it takes a month from designing it until the PCBs arrive. Going back to the schematic I quickly realized I had made a decision to only enable the switch mode supply when the input voltage exceeds 9V (this can be easily changed as it's only a resistor divider) and I was only supplying 8.5V. Quick turn of the knob on the lab PSU to bring the input voltage above that threshold solved the problem :) and I could measure 3.3V on the output.

Next, I have to start cutting into the LCD plastic enclosure to open up spaces for the connectors and the pot which will protrude out the front of the LCD panel. Not really looking forward to that, cutting into plastic and making precisely aligned openings is not my favourite activity.

OK it's nearly 2am and I'm off to bed... satisfied I have accomplished the task I set out to complete :)

After much cutting the plastic case with a box cutter and a metal nibbler and drilling holes I finally had the mechanical side sorted. Finished soldering the wires to the board and hooked it all up to the Papilio then run a quick LVDS test and... nothing. No picture at all. I immediately thought, oh crap, SI issues (signal integrity). How can this be, after all it used to work with just loose wires flying about. Is it the the HDMI wing, is it the trace width, is it the trace spacing, impedance issues? All these thoughts and more run through my head, but as always, one must approach a seemingly unsolvable problem one step at a time.

I checked the power in and out of the switch mode supply, all good, checked the noise on the output, not all that great, about 100mv of a square looking ripple at about 6.6Mhz. Hmmm... whatever.

Checked the trace continuity from the Papilio HDMI wing all the way through the HDMI cable to the panel pins. This is where I uncovered a problem. One of my signal pairs was not coming though. This turned out to be because I had a soldering issue with two of the pins on the HDMI connector on the panel side, they were not making contact with the PCB traces. A quick touch up with the soldering iron fixed that, so after rechecking all signals for continuity I was all cocky thinking I had my issue licked. But alas, on testing, still no picture on the panel. On with the troubleshooting.

Next it was time to check the HDMI wing so off to synthesize Mike's DVID test, but first I must adapt it to the Papilio by creating a suitable ucf file. With ucf done and project flashed on the Papilio I hooked up the HDMI wing to a HDMI monitor and two things became immediately apparent. I had a picture which was great. The quality was not perfect which was not so great. There were a few visible artefacts and I guess that is to be expected since the signal path from FPGA to HDMI connector was not really designed with high speed signals in mind or even matched traces.

At least the HDMI wing was tested to be OK. It was about at this stage when it hit me that while creating the ucf file before, I had a niggling feeling that some signals to the LVDS panel were in a different order. I double checked my LVDS driver project and sure enough that was the case. Initially, a month ago, I had tested the panel with wires soldered directly to a header that plugged into the Papilio expansion header but when I designed the HDMI wing I followed the HDMI pinout. Because of that, some of the differential signals had been swapped around, eg my panel clock now connected to one of the signals... so after fixing up the ucf to match the correct pins, I finally had a picture on my panel.

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With the cover screwed in, it looks like this. I should mention at this point that the pot intended to control the brightness, doesn't. I had honestly thought after checking the datasheet for the CCFL driver that the brightness would be controlled by an analog voltage, but it seems that chip has a few different modes, depending on how three of its pins are wired. Two of the modes are analog brightness control (on different pins) and the third mode is PWM brightness control. It would appear this is the case here. So the pot when turned all the way to one side turns off the backlight, then as it is wound up, when the analog control voltage reaches 1.5V it turns the backlight fully on. Oh well, I can live with that :) If I was bored, I'd attempt some circuit surgery on the CCFL driver board to rewire the pins and change the mode to analog but that's not a huge priority for me right now.

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Final picture of the complete project. I should really remind you again that just because the panel has been fitted with an HDMI connector, it DOES NOT have the smarts to recognize TMDS encoded HDMI signals, it needs to be driven with LVDS.

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