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Adventurer
Adventurer
4,999 Views
Registered: ‎07-30-2013

Why didn't external VREF work?

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I am using a Kintex7 (xc7k160t) and was having trouble with a lpddr2 controller which wouldn't reliably calibrate.  I changed the MIG to use internal VREF and now the calibration reliably works.  

 

What confuses me is that I think the resistor network at the VREF pins are fine.  Each bank has the two VREF pins tied together and share a 100ohm pullup, 100ohm pulldown, and 0.1uF bypass cap.

 

Is there any constraint to explicitly tell the tool to use external VREF?  I assumed that by not using internal, it was pretty evident that it must be external.

 

Dan

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Adventurer
Adventurer
8,818 Views
Registered: ‎07-30-2013

I got to the bottom of it.

The schematic was correct, but BOM somehow wrong, so wrong resistor got populated on the board, causing incorrect voltage reference!  

Correct resistor allows it to calibrate properly.

Thanks for the help @gnarahar@florentw, and @athandr.

 

Dan

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Xilinx Employee
Xilinx Employee
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Registered: ‎07-31-2012
Are you talking about the VRP and VRN pins. Because each byte group has a single VREF pin. The VREFP and VREFN are the internal XADC related pins and note realted to the DDR operation.

The VREF pins should be tied appropriately to the required VREF voltage desired.

Let me know if there is a misinterpretation of your query.
Thanks,
Anirudh

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Moderator
Moderator
4,948 Views
Registered: ‎11-09-2015

Hi @daniel.cogan,

 

Yes, you should not need to constraint the Vref has external (this should be by default if not internal).

Did you make sure you have the correct voltage requirement? (UG471)

 

@athandr,

In fact there are two Vref pins per bank (except configuration bank 0) but there is only one VREF voltage value possible per bank.

Vref.JPG

 

Regards,

 

Florent


Florent
Product Application Engineer - Xilinx Technical Support EMEA
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Moderator
Moderator
4,908 Views
Registered: ‎07-23-2015

@daniel.cogan Can you lets us know whats the VREF voltage when you probe at the resistor divider when using external VREF?

 

 

- Giri
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Adventurer
Adventurer
4,893 Views
Registered: ‎07-30-2013

@florentw

Thank you. I expected I should not need a constraint.  I believe my requirement is correct.  The bank has HSUL12 type, so a 1.2V source.  VREF should be half that.  When I tried internal vref, I set it to 0.6v.

 

I did notice that internal vref is based off of VCCAUX (which appears to be 1.8v on our schematic) whereas the external vref is a voltage divider from a 1.2v source.  Maybe it's a less noisy power rail, although they both come from the same regulator.

 

@gnarahar - Hopefully I can make this measurement today.  If it is 0.6v, I'll be very confused.  

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Adventurer
Adventurer
8,819 Views
Registered: ‎07-30-2013

I got to the bottom of it.

The schematic was correct, but BOM somehow wrong, so wrong resistor got populated on the board, causing incorrect voltage reference!  

Correct resistor allows it to calibrate properly.

Thanks for the help @gnarahar@florentw, and @athandr.

 

Dan

View solution in original post

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