[IBIS-Users] sigxp ... buf delays and Prop Delay results

From: Lipsius, JohnX S <johnx.s.lipsius_at_.....> Date: Tue Mar 08 2005 - 16:13:14 PST · This archive was generated by hypermail 2.1.8 : Tue Mar 08 2005 - 16:15:03 PST

ibis-users,

This concerns driver buf delay in Cadence sigxp, so an experienced user
would 
know the answers.  

I understand Rref, Cref, Vref are the ref load ckt elements - for a
simulator 
to calculate buf delay.  And the delay is measured at Vmeas.  

Questions...

1.  In sigxp Results tab from simulation, does the "Propagation Delay"
column
already deduct the driver buf delay calculated?  

2. What is the exact difference (in Preferences) between getting delays 
"from library" and "on the fly" ?  I haven't seen any calculated buf
delays 
in DML models, which would be gotten "from library".  

a.  how are buf delays gotten from each source, library and on-the-fly ?
b.  what exactly is the usage difference after that? 
c.  why would one choose one over the other? 

3. If a driver model has no ref load ckt, why not just edit one into the

ibis (from datasheet)?  I could simulate into the ref load & Vref, but
it seems 
more convenient to edit the ibis.  What is the Cadence feature to view
real world 
ref load waveforms, model integrity V-t ?  Ie: does "falling (or rising)
waveform" 
mean "with ref load" ? 

Regards,
John

-----Original Message-----
From: owner-ibis-users@eda.org [mailto:owner-ibis-users@eda.org] On
Behalf Of Bob Ross
Sent: Thursday, July 22, 2004 5:56 PM
To: RODRIGUEZ LECONA RAMIRO
Cc: ibis-users@eda.org; ibis-info@eda.org
Subject: [IBIS-Users] Re: Question about Vref

Hello Ramiro Lecona:

The Vref value is set to the value that the Rref is connected
(if it exists) for timing test load calibration, per the circuit
specified in the data sheet.

Older devices often used 500
ohms connected to ground.  With higher speed technologies,
50 ohms might be connected to Vcc/2 per intended operation.
For Opne_drain devices, Vref might be some higher voltage
such as Vdd.  For ECL and PECL, the Vref value is often
2 V below the supply voltage.

So the Vref value is set to the value used for timing
test load measurements and does vary according to data
sheet informaiton.

Bob

RODRIGUEZ LECONA RAMIRO wrote:

> 
>   Hi, I am modeling some buffer and I have the question that follows:
>    Some vendors specify Vref subparameter as 0V, other as VDD and
other 
> as VDD/2. What's the difference? Why do they use different values to 
> determine the propagation delay?
>                    Thanks.
> 
> Ramiro Lecona
> 

-- 
Bob Ross
Teraspeed Consulting Group LLC     Teraspeed Labs
121 North River Drive              13610 SW Harness Lane
Narragansett, RI 02882             Beaverton, OR 97008
503-750-6481                       503-430-1065
http://www.teraspeed.com           503-246-8048 Direct
bob@teraspeed.com

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