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Newbie bapollonio
Registered: ‎04-10-2019

Power problem with ZC706?

I am mostly posting this in case anyone else runs into the same problem, but also because I don't fully understand the solution.

While troubleshooting a design on my ZC706 (rev2), I became suspicious of my USB connection introducing analog noise and unplugged my hub.  When I plugged it back in the board went almost dead - the fan shut off as did most of the LEDs.  The symptoms very closely resembeld this thread:


Upon further digging I found that VCCINT was at 0.67V (instead of 1.0V) and feared the worst: a short in the FPGA.  But first I wanted to make sure the power controller (U48) wasn't trimming it funny so I took a look at the feedback pin (VADJ) and discovered it was sitting at 0.6V - in other words, it was in regulation.  But this didn't make sense because the maximum trim is a 5.23k resistor which, if shorted to ground, would trim the voltage UP to 1.15V.  Then I looked at U33 and found its VCC pin (VCCF3V3) was sitting at 0.52V and figured I'd found the damaged supply.  Except....VCCF3V3 has no source - at least, not on the schematic.  It's a single pin net!  Figuring I had nothing to lose I tied it to VCC3V3_PS and the board roared back to life.  One rework wire later and everything appears to be working fine.

There does appear to be a via on the board connected to U33 pin 5 but I can't tell where it goes.  Does anyone have any insight into why it would have worked in the first place, or if I might be doing anything bad by tying U33-5 it to VCC3V3_PS?


1 Reply
Xilinx Employee
Xilinx Employee
Registered: ‎06-06-2018

Re: Power problem with ZC706?

Hi @bapollonio ,

U33 is SPDt Switch, which requires Supply voltage of 3.3V. Since VCC3V3 regulator was not supplying the required voltage, you were observing VCCINT trimming. 

Xilinx Doenst recommend customers to do any rework on the board.

Since you have connected U33 pin 5 to VCC3V3_PS. U33 is now supplied with 3.3V but supplying regulator is diffrent i.e VCC3V3_PS. And thats the reason VCCINT trimming didnt happened after rework.

But Still VCC3V3 regulator will be failing in your board.

Please can you share the Power GOOD LEDS Snapshot? And also share the Board front side Snapshot after power up.

Please can you go through this ZC706 Debug Checklist and confirm me till Step 4 you are fine? If you fail in any step please specify.

If VCC3V3 regulator is failing then you need to perform reprogramming by referring this https://www.xilinx.com/support/answers/56811.html.


Hope this helps.



Deepak D N


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