Re: [SI-LIST] : LVCML vs. SLVS

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From: D. C. Sessions (si-list@lumbercartel.com)
Date: Fri May 04 2001 - 08:25:53 PDT


On Friday 04 May 2001 02:01, Franz Smith wrote:
> Hi,
> is there anyone interested to compare this 2 interfaces?
>
> I have to connect through a backplane 2 ASICs @2.5Gbps,
> and I'm looking for a SerDes block and high speed interfaces.
> Both can drive a coax cable or a backplane (stripeline)
> carrying data up to 2.5Gbps (3.125Gbps with 8bit/10bit coding?).
> The power consumption is reasonable, probably 20mW for the driver (?)
> using Vcc=1.8V, but SLVS could work with a lower Vcc.
> I think to use a CMOS 0.18um technology, but why not 0.12um?
> Do you see any benefit of using external 50ohm resitors?
>
> SLVS
> ----
> Vcc >= 800mV, because Voh=400mV (+)
> standard Jedec JC-16, under discussion (=)
>
> LV-CML
> ------
> Vcc >= 1.8V (?), because Voh=1.5V (-)
> standard (?)
>
> As you see there are a lot of "?", but I haven't found
> much documentation on these to interfaces.

I'm the author of SLVS, and chair of JEDEC's JC-16 committee
which is considering it as a standard. So I'm not exactly unbiased ...

That said, I'm not sure that I'd advise using either of these for a cabled
connection. Both have quite narrow common-mode ranges and depend
on high-quality ground integrity between the driver and the receiver.
Which is Not Guaranteed in a cabled environment. For all of its faults,
LVDS really is pretty well-suited to a cabled environment.

Aside from that issue, I would (remember, I'm biased) choose SLVS for
two main reasons:

1) It has a longer future due to being (relatively) independent of the power
supply. Giving the rapidly dropping ceiling with CMOS, this is important.
2) Since SLVS is ground-referenced instead of supply-referenced, there
is no net positive bias on the connection. Which is a (minor) advantage
both in backplane and cabled connections. We're looking into hot-plug
issues for SLVS and although we don't want to promise anything it looks
good so far.

Copies of the SLVS spec _used_ to be available from my personal website,
but my (former) ISP bailed and the new one hasn't gotten the domain site up
yet. If someone wants to volunteer a site, I'll happily provide a copy for
publication; in the meantime I can mail copies on request. (I don't think
Ray would appreciate a binary bomb on the list!)

-- 
| The race is not always to the swift, nor the battle to the strong. |
| Because the slow, feeble old codgers like me cheat.                |
+--------------- D. C. Sessions <dcs@lumbercartel.com> --------------+

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