[SI-LIST] Re: Simulation problems with SSTL2 IBIS model (TI CDCV8 50)

Dirk,

Based on looking at the model, I would say that the tools you mentioned may be 
troubled by the large time delays at the start of the [Rising 
Waveform]/[Falling Waveform] V-T table data.  The [Falling Waveform] times 
start at 10 ns, the [Rising Waveform] at 110 ns.  There clearly is no need for 
the long lead-in times.  In fact, if a tool runs this model at 100 Mhz, and 
dutifully tries to follow the timing in the V-T table data exactly as given, it 
may never switch!  But, as Timothy Coyle mentioned, most tools do allow you to 
truncate the lead-in times.

Though it might seem wise to always strip constant-voltage lead time from the 
start of the tables, the main issue is the timing problem Andrew Ingraham 
mentioned.  Until recently, the IBIS spec did not require a common timing 
reference between all of a model's V-T tables; it only required all rising 
tables to be time-correlated and all falling tables to be time-correlated.  
There was no requirement that a rising and a falling edge table share a common 
timing reference.  But this has always been a practical necessity for modeling 
the duty cycle, especially in differential buffers, and IBIS has formalized the 
requirement in version 4.0 (see BIRD 68.1 at 
http://www.vhdl.org/pub/ibis/birds/).  

In your model, the 10 ns and 110 ns "setup" times are clearly arbitrary, and 
the actual skew between non-inverting and inverting outputs is unknown.  So 
although the output signals probably should cross mid way between low and high 
state levels, the exact cross-over voltage here is ill-defined within the model 
and depends on whether and how each tool chooses to adjust the V-T tables.  
Model makers should spot these problems ahead of time by testing their models 
in an IBIS development package, such as Mentor's IBIS Development System.  (not 
sure where it's located on the Web; the Visual IBIS Editor is at 
http://www.innoveda.com/products/datasheets_HTML/ibis.asp; call if you need 
more info.)  Another resource is the IBIS Model Review Committee, info at 
http://www.eigroup.org/ibis/support.htm.

Best regards,

John Angulo
Hyperlynx Products
Mentor Graphics Corp.
john_angulo@xxxxxxxxxx
425-869-2320
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