Measuring DisplayPort test pads - Page 1
Forum section: electronics / rf-microwave

author: nitrocaster
author_rank: Newbie
timestamp: July 23, 2016, 01:48:58 am
content: Hi there,Is there a feasible way to measure DisplayPort signal presence on test pads? See the picture attached (pads DRX0P, DRX0N, DRX1P, DRX1N).

author: nitrocaster
author_rank: Newbie
timestamp: July 24, 2016, 11:29:34 am
content: Ok, I know I should use a high-bandwidth oscilloscope with differential probes, but I don't have one. It may sound dumb, but is there any way to make a differential probe myself? I don't even need to see what exactly is happening on the line. Just the fact that it is active (has voltage) or not would be enough. Any thoughts?

author: Rerouter
author_rank: Super Contributor
timestamp: July 24, 2016, 11:55:00 am
content: for the bandwidth, i would say probe them with a normal cro, your not even going to be close to seeing eye diagrams, but you should be able to see a voltage and it toggling, use as short a ground connection as you can, and maybe set to x10 to reduce loading,

author: bktemp
author_rank: Super Contributor
timestamp: July 24, 2016, 03:34:16 pm
content: If your scope has <500MHz bandwidth it is nearly impossible to see anything, maybe an increased level of noise.Probing highspeed signals single ended is fine if you don't care about signal integrity, but you need short connections to GND.Instead of probing the data lines, you can also look at the aux signal, it uses a much lower bitrate. For a short period after power on you should see data on the aux channel.

author: Marco
author_rank: Super Contributor
timestamp: July 24, 2016, 11:33:54 pm
content: Quote from: nitrocaster on July 24, 2016, 11:29:34 amMake a small Schottky RF detector and solder it directly across the test pads, measure the output voltage with a multimeter.Lets say a 1K resistor, in series with a 100 pF coupling capacitor, in series with a RF schottky, in series with a 1 nF capacitor to the other differential signal. Voltage measured across the 1 nF capacitor.PS. oops need another Schottky (or resistor) behind the coupling capacitor, to the other differential signal ... otherwise the coupling cap won't be discharged. Something likethis. Except I'd add the 1K series resistor just to be absolutely certain you don't load the signal much.

author: nitrocaster
author_rank: Newbie
timestamp: July 25, 2016, 01:26:13 pm
content: QuoteI thought so, but according to DisplayPort specs AUX channel is used for initialization procedures (link training, EDID) and remains in idle state (AUX_P = 0V, AUX_N = +3.3V) during the video signal transfer.QuoteAm I getting it right on the schematic attached? It looks a little strange to me that AUX_N has direct connection to ground (just as on the schematic from your link).

author: Marco
author_rank: Super Contributor
timestamp: July 25, 2016, 03:01:34 pm
content: No, like in the link but with a resistor ... or maybe two resistors would be better so capacitance to ground can't screw up the circuit loading either, like in the below picture. There is no direct ground connection at all, measure it with a multimeter.The detector has to be either small or tiny depending on the signalling speed of the display bus.

author: nitrocaster
author_rank: Newbie
timestamp: July 26, 2016, 12:49:29 pm
content: Now it makes sense. I wonder why is -VE grounded on this picture?Or -VE/+VE are probe points for multimeter and RF is a high-speed signal? In that case arrows seem to be reversed, aren't they?

author: Marco
author_rank: Super Contributor
timestamp: July 26, 2016, 12:53:34 pm
content: It's measuring a single ended ground referenced signal in that image, because the multimeter floats it doesn't really matter.Might need something a little lower capacitance than the 1n5711 though, BAT15 maybe? (ESD sensitive BTW.)

author: cirthix
author_rank: Contributor
timestamp: March 13, 2017, 08:23:31 am
content: You want to use a direct DP connection to an eDP display eh?I had to do this once and used a nice little trick. Tossed a sense resistor on the power line going into the panel and when you detect current above a certain threshold, the panel is on and you can turn the backlight on. You'll have to figure out the magic number and use an adc+micro for it, but this will work without hacking the panel itself.If you want to do it from the panel side of things, look for a testpad labeled VGH. It probably won't turn on this power rail until the panel is about to display something. Be warned, it can be in the 15-30v range.