Re: [SI-LIST] : HSTL Termination

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From: Vinu Arumugham (vinu@cisco.com)
Date: Thu May 31 2001 - 14:09:35 PDT


Manoj,

With a 38 ohm driver driving a 55 ohm (50 ohm +10%) unterminated line, you
can have 300mV to 500mV of overshoot depending on the max. Vddq variation
allowed between the transmitter and the receiver. With lower driver
impedances, some receivers may begin to clamp and that could be a problem.

Thanks,
Vinu

manoj-kumar.sharma@st.com wrote:

> Subject: HSTL Termination
> From: manoj-kumar.sharma@st.com
> To: vinu@cisco.com
> CC: majordomo@silab.eng.sun.com
>
> Hi Vinu,
>
> As you write for Class 1 HSTL driver the output impedence should be
> greater than 38 ohms.
> What I interprate from this is, to design the driver for voh & Vol (Ioh
> and Iol) the minimum load is 38 ohms.
> Voh =1.1V, Vol = 0.4 V and Ioh & Iol >= 8mA.
>
> For this specs, say Voh =1.1V, Ioh =8mA, we can have the value of
> R=1.1/8m = 137.5ohms,
>
> Can you oblige me by putting some light on how you calculate the minimum
> load (ie. how you decide max. current).
>
> Regards
> Manoj
>
> ------------------------------
>
> Date: Wed, 30 May 2001 10:25:50 -0700
> From: Vinu Arumugham <vinu@cisco.com>
> Subject: Re: [SI-LIST] : HSTL Termination
>
> "Jinto N.Jose" wrote:
>
> > Hello Everybody!
> > One of the suggested termination for HSTL class-I is
> >
> > ___ 0.75V
> > |
> > /
> > \ 50 ohms.
> > /
> > |\ \ |\
> > | \_____________|_| \
> > | / | /
> > |/ |/
> >
> > How is this usually realized ? Do any body use a +0.75 power
> supply for
> > this ?
>
> Yes, this termination with a 0.75V power supply that is capable of
> sourcing and
> sinking current, is sometimes used.
>
> >
> > What about the topology shown bellow - is it a valid termination for
> HSTL- I
> > signals
> >
> > ___ 1.5V
> > |
> > /
> > \ 100 ohms.
> > /
> > |\ \ |\
> > | \_____________|_| \
> > | / | | /
> > |/ / |/
> > \
> > / 100 ohms
> > \
> > |
> > ---
> > ///
> >
>
> This is a valid HSTL I termination.
>
> >
> > In all HSTL specifications I have seen , I have found Ioh and
> Iol min
> > values and not the max values. What is the significance of this minimum
> > output current specification ?
>
> This spec. guarantees that the driver will source or sink at least
> Ioh/Iol while
> meeting Voh/Vol levels.
>
> There is no max. Ioh/Iol spec. According to the spec., an HSTL Class II
> driver is
> also HSTL Class I compliant. If you plan to use HSTL Class I drivers to
> drive
> unterminated lines, you may have to design the buffer to not only meet
> the HSTL
> spec. but also limit the output impedance to be greater than about 38
> ohm, to
> avoid overshoot/undershoot.
>
> >
> >

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