Subject: Re: [IBIS-Users]Difference in rise-fall waveform due to c_comp!!
From: Alok Rungta (a0875973@india.ti.com)
Date: Wed Mar 03 2004 - 00:43:54 PST
Arpad,
I am not sure if I understood things correctly .. First I would further
clarify the experiment I did. I simulated a simple circuit with a
buffer(ibis model) and a load. I tried two loads one resistive and other
only cap. For each load condition I did 2 simulations with different
ibis model files wherein in the 2nd simulation I just changed the
c_comp(made it 10x the actual) value to see the effect of this
parameter on the simulation. What I observe is:
For Resistive load:(which is same as the load I used for generating the
V-t curve in IBIS model):
1>When there is a transition from low to high the waveform in both the
cases(with actual ibis model of the buffer and with the one which has
c_comp modified to 10x) is exactly same.
2>But when there is a fall transition at output the waveforms are
different for two cases.
For Cap load:
3> Waveform is different for the rise case in both the cases.
4> Waveform is different for the fall cases in both the cases.
From what I understand observation 2 can be explained because of
current flowing in the buffer(outside in) and thus c-comp coming in
picture.observation 1 => no role of c_comp in low to high transition
because its inside out.
Observation 4 can also be attributed to an outside in effect where
c_comp comes into effect. But observation 3 is what I am not very clear
about .. One thing I deduce is that probably the v-t curves in the ibis
model are generated for resistive loads so when it comes to resistive
loads the simulator just uses IV curve and V-t curves in the model for
interpolation and generates the curve(accuracy depending on difference
between the loads you simuated for and load being used in the
simuation). When using the cap. load it cannot possibly use this V-t
curve in the same way because the load is of entirely different nature.
So it has to look at the c-comp. How is it using the IV information in
this case is not very clear to me ?
Can you please throw some more light on this ...
Thanks in Anticipation
Alok Rungta
Muranyi, Arpad wrote:
>Alok,
>
>What you see is the correct behavior.
>
>The meaning of C_comp is die capacitance which includes
>everything that is on the die, pad, metal, transistors,
>etc... When you obtain waveforms (Vt curves), this
>capacitance is in place, so the waveform includes its
>effects. For this reason, IBIS simulators do not use
>this capacitance to load the waveform that is in the
>IBIS model, i.e. no matter what value you provide for
>C_comp, the simulation waveforms will remain the same
>(within reasonable limits) with the same resistive
>load on the output. On the other hand, any external
>capacitance connected to the output of the IBIS model
>will change its simulation waveforms, because that
>will act as a load to the buffer.
>
>On the other hand, the value of C_comp will have an effect
>the reflections which come to the buffer from the outside.
>
>In summary, C_comp is invisible from the inside out, but
>visible from the outside in.
>
>I hope this helps,
>
>Arpad Muranyi
>Intel Corporation
>=========================================================
>
>-----Original Message-----
>From: owner-ibis-users@eda.org [mailto:owner-ibis-users@eda.org]On
>Behalf Of Alok Rungta
>Sent: Tuesday, March 02, 2004 8:57 AM
>To: ibis-users@eda.org
>Subject: [IBIS-Users]Difference in rise-fall waveform due to c_comp!!
>
>
>Hello IBIS users,
>
>I am trying to understand how the simulation tool is using the c_comp
>parameter in the ibis model. For the same ibis model i just change the
>c_comp parameter and resimulated to see if there are any differences in
>the observed output voltage waveform. When using a cap as a load I see
>differences in both the rise and the fall transition .. but when using
>a pure resistive load I find that both the models on simulation give the
>same rise waveform but different fall waveforms. Is this the expected
>behavior ?? Isnt c_comp supposed to be used only in case of reflection ??
>
>Thanks in Anticipation
>Alok Rungta
>
>
>|------------------------------------------------------------------
>|For help or to subscribe/unsubscribe, email majordomo@eda.org
>|with just the appropriate command message(s) in the body:
>|
>| help
>| subscribe ibis <optional e-mail address, if different>
>| subscribe ibis-users <optional e-mail address, if different>
>| unsubscribe ibis <optional e-mail address, if different>
>| unsubscribe ibis-users <optional e-mail address, if different>
>|
>|or email a written request to ibis-request@eda.org.
>|
>|IBIS reflector archives exist under:
>|
>| http://www.eda.org/pub/ibis/email_archive/ Recent
>| http://www.eda.org/pub/ibis/users_archive/ Recent
>| http://www.eda.org/pub/ibis/email/ E-mail since 1993
>|------------------------------------------------------------------
>|For help or to subscribe/unsubscribe, email majordomo@eda.org
>|with just the appropriate command message(s) in the body:
>|
>| help
>| subscribe ibis <optional e-mail address, if different>
>| subscribe ibis-users <optional e-mail address, if different>
>| unsubscribe ibis <optional e-mail address, if different>
>| unsubscribe ibis-users <optional e-mail address, if different>
>|
>|or email a written request to ibis-request@eda.org.
>|
>|IBIS reflector archives exist under:
>|
>| http://www.eda.org/pub/ibis/email_archive/ Recent
>| http://www.eda.org/pub/ibis/users_archive/ Recent
>| http://www.eda.org/pub/ibis/email/ E-mail since 1993
>
>
>
>
|------------------------------------------------------------------
|For help or to subscribe/unsubscribe, email majordomo@eda.org
|with just the appropriate command message(s) in the body:
|
| help
| subscribe ibis <optional e-mail address, if different>
| subscribe ibis-users <optional e-mail address, if different>
| unsubscribe ibis <optional e-mail address, if different>
| unsubscribe ibis-users <optional e-mail address, if different>
|
|or email a written request to ibis-request@eda.org.
|
|IBIS reflector archives exist under:
|
| http://www.eda.org/pub/ibis/email_archive/ Recent
| http://www.eda.org/pub/ibis/users_archive/ Recent
| http://www.eda.org/pub/ibis/email/ E-mail since 1993
This archive was generated by hypermail 2b28 : Wed Mar 03 2004 - 01:14:35 PST