Re: [SI-LIST] : HSTL Classes

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From: Vinu Arumugham (vinu@cisco.com)
Date: Thu May 31 2001 - 14:59:53 PDT


"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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