Chris_C

vga controller gottcha...

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So I get my head round clock tiles and instancing components and other such obvious stuff! when I hit this nice little gottcha...

 

I had a VGA output, but only 2/3 the screen width, and it wouldn't sync horizontally nor vertically, even worse it seemed to be just a little out....

 

Well I tinkered and faffed, and even twiddled but to no avail... :(

 

then it dawned on me, I was using a tiny portable tv with vga input...

 

and let me tell you they are NOT vga monitors, 2-3 resolutions and although I didn't check I bet only one sync per resolution, when I finally got to connecting it to my (actual) spare vga monitor low and behold it was working all along...

 

The moral of the story if you are designing a circuit to drive *anything* make sure the bit of kit can actually do what you're trying to do with it!!!

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Flat panel monitors are "iffy" in any situation where the sync signals may not be quite spot on officially supported standards. More than one I've tried won't display the output from the FPGA arcade games at all. A CRT multisync monitor is still the most versatile.

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I have an old Dell P190S LCD monitor that seems pretty good at locking onto all my generated signals. It has a VGA input and a DVI input for which I use a passive DVI to HDMI cable so I can also connect it to the Pipistrello. I wonder if compiling a list of known good and bad monitor models would be beneficial?

 

Oh this reminds me, I noticed something unexpected recently, when I have the Pipistrello connected to that monitor via the HDMI cable, I usually pull the USB cable to reset the board, but when the HDMI cable is plugged in the Pipistrello board sucks just enough power from it to keep the FPGA design from being cleared out. In fact with the USB disconnected and just the HDMI in, I still get a valid picture on the monitor!

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Yeah, the HDMI signals are terminated on the monitor side to 3.3V via a 50 ohm resistor.  So with 8 signals terminated like that there is enough current flowing to keep the core voltage high enough for it to work.

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Looking at the schematic for the Pipistrello, it might also be pin 18 of the HDMI powering the board via R26 zero ohm resistor. I haven't checked if R26 is populated on the board. There's a (supposed) 50mA limit on pin 18 as well.

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