RE: [SI-LIST] : HSTL Classes

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From: Jinto N.Jose (jinto.jose@wipro.com)
Date: Fri Jun 01 2001 - 01:28:02 PDT


Vinu,
        Thank you for your inputs . Can you suggest any document by which I can
have a good understanding of HSTL devices ?

Jinto

> -----Original Message-----
> From: vinu@cisco.com [mailto:vinu@cisco.com]
> Sent: Friday, June 01, 2001 3:30 AM
> To: Jinto N.Jose
> Cc: SI List
> Subject: Re: [SI-LIST] : HSTL Classes
>
>
> "Jinto N.Jose" wrote:
>
> > Hello Everybody,
> > I have some more doubts regarding HSTL interfaces.
> >
> > 1. In HSTL standard (EIA/JESD8-6) series terminated load is
> shown only for
> > class 2 . Why can't HSTL class 1,3,&4 drivers can not drive series
> > terminated loads ?
> >
>
> All of them can drive series terminated loads. A Class I driver
> is usually too
> weak to need a series termination. Since Class III and IV drivers are
> asymmetrical, the difference in rise and fall times will affect
> timing when
> operated with series termination.
>
> >
> > 2. For HSTL class 2, the parallel termination option shown is a
> 50 ohm to
> > Vddq/2 both at the driver end and receiver end (Symmetrically double
> > parallel terminated output load). What is the purpose of
> keeping a pull up
> > resistor near the driver ? (same thing is applicable for HSTL
> class 4 where
> > Vtt = Vddq)
>
> HSTL Class II and IV are usually used for bi-directional signals.
> Both ends
> need to be terminated when the signal is bi-directional.
>
> > Will it work fine with a 50 ohm to Vddq or Vddq/2 ?
> >
>
> If an HSTL Class III or IV driver is terminated with 50 ohm to
> Vddq/2 instead
> of Vddq, again timing will be affected due to asymmetric rise and
> fall times.
>
> >
> > 3. For HSTL class 3 the suggested load is 50 ohm to Vddq at the receiver
> > end. Will it work fine if the termination is 50 ohms to Vddq/2
> ? My basic
> > doubt is that why termination is changing with the Class of the driver .
> > Both the schemes of termination 50 ohms to Vddq/2 and 50 ohms
> to Vddq are
> > identical as far as reflections are concerned . Both the
> schemes matches the
> > transmission line impedance (am I right ?). In that case is it
> necessary to
> > flow more than 20mA current ((1.5V -0.4V)/50 ohm) in the case
> of a class3
> > driver ? Or 50 ohm to Vddq termination is shown just to tell that a HSTL
> > class 3 driver can easily sink that much current . Similarly will a HSTL
> > class 4 driver work fine with a single 50 to Vddq/2 ?
> >
>
> HSTL Class III and IV drivers with terminations to Vddq will
> perform better on
> heavily loaded buses than Class I or II.
>
> >
> > 4. How much current an HSTL input usually requires ? Is it in
> micro amps or
> > nano amps ?
>
> I think it would be a few micro Amp.
>
> >
>
> > Thanks
> > Jinto
> >
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