Spartan 3 LVDS

Hi, I'm using the Spartan 3 XC3S400-TQ144.  Does this device have internal 
100-ohm termination for LVDS input pairs, or must I use an external 100-ohm 
resistor?  When I designed my board, I assumed internal termination would be 
enabled automatically by instantiation of the IBUFGDS; but looking at my 
signal levels, although my board is working, I would say there is no 
termination.  The only statement I can find in the datasheet is note 5 
hidden away below table 37, which I think I can be forgiven for overlooking. 
I only have two input pairs, and it's a prototype board, so I can just 
solder 0201 resistors between the pins; but maybe someone here knows better 
...

TIA

On Sun, 22 Mar 2009 13:50:36 +0000, Andrew Holme wrote:

> Hi, I'm using the Spartan 3 XC3S400-TQ144.  Does this device have internal 
> 100-ohm termination for LVDS input pairs, or must I use an external 100-ohm 
> resistor?  When I designed my board, I assumed internal termination would be 
> enabled automatically by instantiation of the IBUFGDS; but looking at my 
> signal levels, although my board is working, I would say there is no 
> termination.  The only statement I can find in the datasheet is note 5 
> hidden away below table 37, which I think I can be forgiven for overlooking. 
> I only have two input pairs, and it's a prototype board, so I can just 
> solder 0201 resistors between the pins; but maybe someone here knows better 
> ...
> 
> TIA

It does have internal termination but termination depends on the
reference resistors connected to the VRP and VRN pins on the bank that
needs termination, Are these installed?

Peter Wallace

"Peter Wallace" <pcw@www.karpy.com> wrote in message 
news:pan.2009.03.22.14.44.00.712281@www.karpy.com...
> On Sun, 22 Mar 2009 13:50:36 +0000, Andrew Holme wrote:
>
>> Hi, I'm using the Spartan 3 XC3S400-TQ144.  Does this device have 
>> internal
>> 100-ohm termination for LVDS input pairs, or must I use an external 
>> 100-ohm
>> resistor?  When I designed my board, I assumed internal termination would 
>> be
>> enabled automatically by instantiation of the IBUFGDS; but looking at my
>> signal levels, although my board is working, I would say there is no
>> termination.  The only statement I can find in the datasheet is note 5
>> hidden away below table 37, which I think I can be forgiven for 
>> overlooking.
>> I only have two input pairs, and it's a prototype board, so I can just
>> solder 0201 resistors between the pins; but maybe someone here knows 
>> better
>> ...
>>
>> TIA
>
> It does have internal termination but termination depends on the
> reference resistors connected to the VRP and VRN pins on the bank that
> needs termination, Are these installed?
>
> Peter Wallace

That would be DCI, correct?  I haven't fitted those reference resistors, and 
one of my pairs is in bank 5.

Looks like popping an 0201 between the pins is still my best bet.

"Andrew Holme" <ah@nospam.co.uk> wrote in message 
news:Kmrxl.223082$Dz4.54592@newsfe20.ams2...
> Hi, I'm using the Spartan 3 XC3S400-TQ144.  Does this device have internal 
> 100-ohm termination for LVDS input pairs, or must I use an external 
> 100-ohm resistor?  When I designed my board, I assumed internal 
> termination would be enabled automatically by instantiation of the 
> IBUFGDS; but looking at my signal levels, although my board is working, I 
> would say there is no termination.  The only statement I can find in the 
> datasheet is note 5 hidden away below table 37, which I think I can be 
> forgiven for overlooking. I only have two input pairs, and it's a 
> prototype board, so I can just solder 0201 resistors between the pins; but 
> maybe someone here knows better ...
>
> TIA
>

Spartan 3 family does not have the true "differential termination" 
(DIFF_TERM) capability, but Spartan 3A does. In Spartan 3A, you still have 
to have the "DIFF_TERM = yes" contraint in the constaint file in order to 
enable this differential termination.

Spartan 3 family does have pseudo differential termination by using 
digitally-controlled impedance (DCI), but you have to have the Rref 
resistors (on the Vrp and Vrn pins) stuffed in order to use this feature. 
However, I would not use this feature without also using the "DCI FREEZE" 
option because of the amplitude jitter that DCI causes by its constant 
adjustment of this impedance. DCI can also generate significant amounts of 
heating if you use a bunch of it.

0201's are fun to work with. Enjoy.

Bob
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