Display Port to VGA conversion.

amarnathsubrama · ‎02-11-2019

Hi,

We have developed a board for converting Display Port standard to VGA standard using PTN3392. We are not able to get Display Port signals and so there is no output in VGA standard.

When I probe on Display Port Main Lane signals all are just high at 0.8V logic and AUXP & AUXN signals are high at 1.2V logic.

And also our board is not receiving power from Display Port cable.

Hereby attaching our schematics for your reference, Can you please help us to overcome the issue?

Thank you.

Amarnath G

+91 9790526746

amarnathsubrama · ‎02-12-2019

Hi Jose,

Thanks for your comments.

Yes first I removed all Pull-Down resistors on TMS,TCK,TDI & TDO and found our board is working fine.

Later on further debugging, Actually Pulling Down the TCK pin alone has created the problem which did not allow to up the display. Except TCK Other JTAG signals (TMS,TDI & TDO) though it is Pulled-Down that doesn't give any problem. This is what I found in further debug.

Now I have another question - Why is Display port cable not carrying +3.3V? I am giving external +3.3V since I am not getting it from Display port cable. Can you please help me on that.

Thank you once again.

Amar

reyes · ‎02-13-2019

Hi Amar,

Glad to know that your board is now working fine.

About your question, I’m not sure what you mean, are you referring to the P3V3-AUX_RTM voltage signal? Is this signal not getting the +3.3V it should?

If my assumption is correct, then this is a big problem for the system because PTN3392 is typically powered by this DP_PWR pin on the DisplayPort connector. This nominal voltage 3.3 V is delivered by the active source whenever the dongle/board is plugged in.

So, if DP_PWR (or P3V3-AUX_RTM in your case) is not powered, then there is no main 3.3V to power the PTN3392, so you would need to check this from your DP source,

If I understand properly your DP_PWR is called “P3V3-AUX_RTM” in your schematic, which is connected to C26 (0.1uF capacitor) and L6 (47 Ohm at 100 MHz inductor), which leads to the main P3V3 voltage rail, correct?

Note: I think C26 should be in the other side of the inductor L6, but it should not cause big problems.

Check Figure 9 on Page 10 of the Application note AN10873: https://www.nxp.com/docs/en/application-note/AN10873.pdf

Did you checked your DP cable to see if DP_PWR pin form your DP cable source have the 3.3V when unplugged form your PTN3392 board?

Have a great day,
Jose

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reyes · ‎02-12-2019

Hi Amarnath,

Seems like the problem comes from the source (DP signal), but not caused by the PTN3392 DP to VGA adapter IC, correct? Did you check the behavior of the DP signal when the PTN3392 is isolated to confirm if the problem is caused by the adapter IC?

Please find below my comments regarding your schematic:

Remove R5 (TCK pull-down resistor), TCK pin can be left floating.

Remove R4 (TMS pull-down resistor), TMS pin can be left floating.

Remove R3 (TRST_N pull-down resistor), TRST_N pin can be left floating.

Remove R2 (TDI pull-down resistor), TDI pin can be left floating.

There seems to be a trace in the schematic between the oscillator pins and GND (in “parallel” to the capacitors C13 and C15), which would cause that OSC_IN and OSC_OUT pins are tied to GND.

You are missing the AC coupling capacitor (0.1uF recommended) in series on the AUX_P and AUX_N pins, check Figure 5 on page 7 of the application note of the following link for details: https://www.nxp.com/docs/en/application-note/AN10873.pdf

Pin 13 and pin 14 of the 20-pin DP connector may either be directly connected to ground or connected to ground through a pull-down device.

Have a great day,
Jose

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