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Re: XGMII: HSTL Classes

Hi Ted,

my own reading of the JEDEC HSTL standard mirrors your own.  I also
agree that the symmetrical drive version is the better implementation.
With respect to the sensitivity of the HSTL line receiver, keep in mind
that the standard calls out different numbers for AC and DC sensitivity.


Ed Grivna
Cypress Semiconductor

> Regarding the choice of IO standard for XGMII, I'd like to offer the
> following perspective from an FPGA vendor.
> Currently, Xilinx supports HSTL Class 1, 3 and 4 but Altera only supports
> HSTL Class 1.  Actel does not support HSTL at the moment but, if at all
> possible, would like to ensure that products in the pipeline can support
> If interoperability is a goal, out  of the shoot, I'd say you're stuck with
> HSTL Class 1 if you want to make the switch to HSTL from SSTL.  
> If that's not enough, here's more:
> CSIX ( a backplane standards org ) has selected HSTL
> Class 1.  Might be relevant to the line card board designer ... minimizing
> the IO standards he has to deal with. 
> The difference among classes is primarily drive strengths which are spec'ed
> to support different termination schemes:
> HSTL Class 1: IOH -8,   IOL 8
> HSTL Class 2: IOH -16, IOL 16
> HSTL Class 3: IOH -8,   IOL 24
> HSTL Class 4: IOH -8,   IOL 48
> HSTL Class 2 is onerous to support for an FPGA vendor because of the
> required size of the pull-up.  Similarly, Class 3 and Class 4 require large
> pull-downs that are expensive to support in a general purpose FPGA.
> VCCO for all HSTL classes is 1.5 V +/- 0.1 V.  There is no 1.8 volt HSTL
> standard that I am aware of (BTW, I'm not an IO guy so I could be lying
> about all of this stuff).
> I also understand that there are other tricky (but not unsolvable) things
> involved with design of the diff amp based around the requirement for a Vref
> (min) of 0.68V.  That doesn't give much headroom in 1.5 volt and lower
> technologies. 
> So, if I was old enough to vote, I would vote for HSTL Class 1 over other
> HSTL standards.  
> Thx,
> Ted Speers
> Sr. Manager, Strategic Marketing
> Actel