HDMI and paired track length importance - Page 1
Forum section: electronics / projects

author: johnnyfp
author_rank: Frequent Contributor
timestamp: August 23, 2013, 03:28:01 am
content: Hi,I'm putting together a derivative copy of a 4.3" HDMI Touchscreen shownhere. As much as I've tried I can't get anything to show up on the screen.The prototype that I've made has straight connection between the HDMI port and the HDMI receiver, and has no considerations that the pair lengths are matched, IE R1+ is 22mm where are R1- is 23mm long.I haven't found any shorts or broken links yet, but would like to rule out if track length could be the issue.According to the Datasheet of the TFP401 the status output should be high if the DE is actively switching, but this is not active.However the LCD display controller indicates that it's receiving signals, but nothing is displayed.Is track length matching important across the pair or across the entire input signal, if it is, how critical is it?Cheers

author: JuKu
author_rank: Frequent Contributor
timestamp: August 23, 2013, 07:29:14 am
content: Track matching is critical. Even more so is to have stray capacitances under strict control. And then, almost always the issues are failed HDCP or data handshake.

author: johnnyfp
author_rank: Frequent Contributor
timestamp: August 23, 2013, 07:45:55 am
content: Does that include limiting or not having via's?

author: JuKu
author_rank: Frequent Contributor
timestamp: August 23, 2013, 07:51:38 am
content: I've never seen a via that wouldn't be several picofarads at 2-3GHz. I guess that is possible, but if you have such a beaty, you would know it.

author: johnnyfp
author_rank: Frequent Contributor
timestamp: August 23, 2013, 08:42:22 am
content: Mmh doesn't bode well. According tothis design guidemy adaptation is very broken.Back to the drawing board, me thinks.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: August 23, 2013, 09:19:50 am
content: Bear in mind that the specs take into account that there could be however many metres of cable, so there are some margins, and a short cable will to some extent compensate for non-optimalness of the PCB.Have you looked at what's coming out of the TFP401A on a scope? It should be fairly obvious if there's meaningful RGB & sync. present.the clock rate for a 4.3" display isn't that high (as digital video goes) so I wouldn't expect even a pretty poor layout to stop it working. I'd be carefully checking the TFP401 configuration (software & pin -selectable options) and the display connections.

author: cyr
author_rank: Frequent Contributor
timestamp: August 23, 2013, 02:37:45 pm
content: What signal are you actually feeding it? Are you sure that it is an appropriate resolution and clock rate for the display?At low resolutions, with short cables I would be amazed if your board was bad enough to completely destroy the signal...

author: ejeffrey
author_rank: Super Contributor
timestamp: August 23, 2013, 03:31:43 pm
content: 1 mm track length difference won't matter by itself. That adds less than 10 picoseconds skew. Even at the highest bit rate allowed by HDMI (1.65 Gb/s) that is less than 2% of the bit time. At lower resolutions it is even less. What can/will matter is impedance mismatches if the signal is not run as differential pair.There is plenty of other stuff that can go wrong, both on the signal integrity side and the protocol.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: August 23, 2013, 03:58:27 pm
content: Quote from: cyr on August 23, 2013, 02:37:45 pmSome LCDs are pretty fussy - you should definitely be looking closely at what's coming out of the TFP401

author: johnnyfp
author_rank: Frequent Contributor
timestamp: August 23, 2013, 11:28:11 pm
content: So on the scope I can see data on the HDMI input, but VSYNC and HSYNC are a constant 1 (3.3v) where as Output dataclock is giving a 170ns pulse at about 1.5mhz.So there is certainly data comming into the chip. But it's not generating any HSYNC and VSYNC.Could my EDID be incorrect especially for the VSync and Hsync polarity? I would just expected it to be inverted if it was + or - but still output something.Also I'm a bit concerned that I can actually see data on the HDMI input. I've only got a 25Mhz Oscilloscope, and would expect the HDMI signal to be in the Ghz, or multiple 100th of MHZ anyway.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: August 24, 2013, 09:06:44 am
content: Quote from: johnnyfp on August 23, 2013, 11:28:11 pmWhat about the data-out?QuoteI'd start off by eliminating the EDID stuff from the equation, e.g. copy the EDID from a monitor, or splice a cable so the ID is physically connected to a real monitor, however the (presumably) unusual resolution may be causing issues.Bear in mind the DVI to parallel process is just fairly dumb deserialisation, and ISTR each lane decodes to a fixed group of the output lines, so even if the EDID is bad and/or the frequencies are wierd, you should still see stuff coming out. Using EDID from a real monitor would at least confirm that your device is outputting data - it could just be that it sees a resolution it's not happy with and shuts of the sync

author: johnnyfp
author_rank: Frequent Contributor
timestamp: August 24, 2013, 10:33:35 am
content: Good idea. Thanks I'll try that.I can see data on the rgb lines but the DE line is not pulsing hence the reason there is no vsync or hsync. According to the datasheet the syncs are latched on a low DE.I'll also try and put another unit together, I'll see if that one is working. Maybe a duff tp401.The irritating thing is that this circuit is a clone of an existing circuit, created by the original designer, just with the sockets changed and an eeprom added, so should work. He's got two units working.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: August 24, 2013, 03:20:17 pm
content: Quote from: johnnyfp on August 24, 2013, 10:33:35 amLook at the datasheet to see which outputs come from the same lane as the sync and see if they are also down - see if it could be one lane down.

author: johnnyfp
author_rank: Frequent Contributor
timestamp: August 25, 2013, 05:21:12 am
content: F$*k, it is indeed the EDID.I put a copy of an EDID from a LG HD TV. And low and behold, I have the DE pulsing away and vsync and hsync working.Now I have to figure out what in the EDID is causing the fault.Thanks Mike, brilliant idea about replacing the EDID, one step closer at ruling the world!!!! err I mean, getting my 4.3" screen connected to my BeagleBoneBlack.

author: johnnyfp
author_rank: Frequent Contributor
timestamp: August 28, 2013, 09:04:30 am
content: I've got it working!Check it outThanks again Mike for your help.