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RE: Single ended XGMII


Depending on vendor a "4 mA" driver can very different.
I have used a 4 mA driver from LSI, 0.25um technology.
This driver has nominally very close to 50 ohm dynamic
characteristics, which is ideal for driving unterminated
point to point signal. This work well for both short and
long (15-20 inches) point to point transmission lines.

However an impedance controlled driver
similar to HSTL, is even better, and much lower noise. 
I would like to stay with this, for improved distance.


-----Original Message-----
From: Devendra Tripathi [mailto:devendra.tripathi@xxxxxxxxx]
Sent: Friday, March 24, 2000 9:25 AM
Subject: Re: Single ended XGMII


I am doubtfull if the rise and fall time will be anywhere close to (say < 
20%) of period
at 4mA drive even for small trace lengths. I guess it has to be 8 or 16 mA. 
I had to use
16 mA at 125 MHz (10 bit interface).


At 08:41 AM 3/24/00 -0800, you wrote:

>Curt and all,
>Could I suggest to reduce the driving capability of the
>output buffers to 4 mA for data and control lines (36 lines
>total) and keep 8 mA only for the recovered clock ? I used it
>the past for Fiber Channel chips and it was very effective.
>Essentially, it reduces the switching noise by half.
>Curt Berg wrote:
> > A few comments on using a single ended XGMII:
> >
> > - Today we have several Quad SERDES chips in production, operating
> >   at 1.25/2.5 Gbit/s with 8B or 10B interfaces. Most
> >   use a 3.3V I/O swing, and I would guess about 40 ohm (nominal)
> >   LVTTL/LVCMOS drivers. This effectively is a 32 bit interface.
> >   I would say, these devices seem to function well for many companies.
> > - Some vendors are event moving to higher integration then
> >   Quads, still using single ended interface. This further indicates
> >   that noise is not a major problem.
> > - Single ended interface on the MAC side is not an issue. There
> >   are many ASIC in production today with 400+ single ended I/Os,
> >   and board with several tens of thousands of single ended nets.
> > - If we for XGMII use a (1.5 V swing) HSTL instead, and
> >   move to a 50-55 ohm impedance controlled driver, we should be
> >   able reduce the effective noise levels by close to a factor
> >   of three.
> >
> > My conclusion is that the industry keeps pushing the
> > capabilities of single ended interfaces, just because
> > differential interfaces double this pincount.
> >
> > If we can reduce the noise by a factor of 3, and not
> > move up much in frequency, from existing proven solutions,
> > I believe we have a workable solution.
> >
> > -Curt Berg-