LVDS inputs on Cyclone II

"Austin Lesea" <austin@xilinx.com> wrote in message 
news:e3dudj$cvm12@xco-news.xilinx.com...
> nospam,
>
> The LVDS input buffer in VII, II Pro, V4, Spartan 3, and 3E is a full CMOS 
> differential comparator, than will 'function' from rail to rail.
>
> Basically, each input ties to both a nmos differential pair, AND a pmos 
> differential pair, so that even when one, or the other is cutoff (out of 
> its range), its complement is still in range, and will operate to sense 
> the difference properly.
>
> Why did we go tothis extra trouble?  Well, these are tiny devices, and it 
> basically costs nothing to do it right the first time, so that you could 
> meet or beat any standard that anyone dreams up without redesigning it.
>
> It is powered from Vccaux, not Vcco!
>
> Since Vccaux is 2.5 volts in Spartan 3, that means that an input voltage 
> has to be in the range of 0 to 2.5 volts just to work at all (common mode 
> range).  We don't specify it use at the rails, because we can't 
> cahracterize all the possible uses of the circuit.  For low speeds, as I 
> said, it will function.  Don't complain if it doesn't switch as fast as it 
> is specified for the LVDS specification (you are not using it there).
>

[snip]

Austin,

Are you saying that, since in V2-V4 the LVDS input buffer is driven from 
VCCAUX, even if a bank's VCCO is not 2.5V you may instantiate an LVDS25 
input? What about an LVDS output in a non-2.5V VCCO bank?

Thanks much,
Bob

"Austin Lesea" <austin@xilinx.com> wrote in message 
news:e3e8sb$cuh15@xco-news.xilinx.com...
>
> As for seeing this sort of data in the spec sheet:  I already said it will 
> not appear, as it is not a supported "use" (there is no standard that 
> requires it, and full support of any swing and common mode voltage is 
> crazy -- there would be a delay table for every Vcm and every swing).
>
> If all you want is a reasonable comparator (which is basically faster than 
> any off the shelf comparator by a factor of hundreds to thousands), it is 
> there.
>
> Austin
>
Hi Austin,
I agree. It's also clear why this behaviour isn't published. I for one am 
impressed with the performance and flexibility of the LVDS inputs, they've 
eased my design problems considerably over the past few years. Good job.
Cheers, Syms.


p.s. Note I didn't even moan about how they're compromised at high 
frequencies by the input capacitance. Oh bugger, I just did. ;-)

Bob,

There are some combinations that may be possible to set the config bits 
to support, but we intentionally choose not to do so (no standard).

So, just because something is possible, does not mean we need to support it.


Safest thing to do here is to power Vcco from 2.5V, Vccaux is 2.5V, and 
you instantiate a LVDS25 input.

If you instantiate a LVDS25 input and the Vcco is 3.3V, all you get is a 
design rule error (3.3V and 2.5V IOs in same bank).  If you can ignore 
the error, and get a bitstream anyway, there you go (success).

Austin


> [snip]
> 
> Austin,
> 
> Are you saying that, since in V2-V4 the LVDS input buffer is driven from 
> VCCAUX, even if a bank's VCCO is not 2.5V you may instantiate an LVDS25 
> input? What about an LVDS output in a non-2.5V VCCO bank?
> 
> Thanks much,
> Bob
> 
> 

nospam wrote:

> So I ask an Altera FAE can I run an LVDS (as a crude voltage comparator)
> input on 3.3v biased at 1.65v.
>
> After an age I get the answer no

Don't worry, the answer "no" is the safest possible answer, so it is
often used as the default reply. :-) Once I asked Cypress whether
I can do something fancy with their GPIF automaton and got the
answer "no". Then I sent the question again with more detailed info
and again received "no". Then I have created a prototype and
the "answer" from it was "oh, yes!". :-)

Rule of thumb: If FAE says "yes", then it "yes". If FAE says "no", then he 
says.

> So I ask the question again and after another age get told no

Don't worry again, I still don't know whether I can temporarily deselect
a Spartan 3 device during reprogramming and grant the bus to another
DMA device... I bet the answer, if any, will be "no". :-)

    Best regards
    Piotr Wyderski

nospam wrote:

> 
> Some time ago I did some experimentation (for a very cost sensitive
> application) with a Spartan 3 part using an LVDS differential input as a
> voltage comparator for a crude delta sigma ADC.
> 
> The I/O bank Vcco was 3.3v the common mode on the LVDS inputs was half the
> 3.3v supply and the differential input voltage was (obviously) limited to
> whatever it took drive the LVDS input one way or the other.
> 
> It worked fine.
> 
> Now the customer says he wants to use Cyclone II.
...

> Can anyone confirm this and has anyone experience of using LVDS inputs as
> voltage comparators on Cyclone II (or Spartan 3/3e for that matter).

I have a bunch of Cyclone I and II designs using LVDS based at 1.65V that
are working just fine at 270MHz input rate. They are fed by either Gennum
9064 or CYV270M0101EQ cable equalizers (I prefer the Cypress, but it has
only just become available in volume).

I remember from my Altera days that there was some reference-ish design that
used the same trick to make an LVDS pair act as a Digta-Selma (or the other
way round) ADC. It had horrible DC properties, but at frequencies over
100Hz it was fairly accurate (16 bit up to 250KHz or so).

Best regards,


Ben