From: ARiazi ([email protected])
Date: Sat Feb 10 2001 - 15:50:19 PST
Suchitha V Wrote:
> First of all, Min, Typ, Max represent the SLOW, TYPICAL and FAST BUFFER
> CORNERS respectively.
> 2% implies that Vref can have a tolerance of +/- 2% on either side of the
> typical value.
> For the purpose of simulation, the SLOW, TYPICAL, and FAST buffer models
> included in the IBIS models
> which are derived from the SPICE simulations/measurements.
> SLOW corner is a buffer in its weak performance i.e, slow speed, weak
> strength, weak voltage.
> In this case the variation of Vref= 2/3Vtt -2% is considered for deriving
> the VI data for the IBIS models.
> FAST corner is a buffer in its fullest performance i.e, fast speed, strong
> driver strength, strong voltage.
> In this case the variation of Vref=2/3Vtt +2% is considered for deriving
> VI data for the IBIS models.
> TYPICAL corner corner is a buffer with typical performance and therefore
> typical values are to be
As you stated: "SLOW corner is a buffer in its weak performance i.e, slow
speed, weak driver strength, weak voltage".
Additionally, temperature and package impedance also play a role so that:
Slow Corner Model: slowest (Min) ramp (dV/dt), Min VI curves, Min voltage,
Max temperature, and Max package impedance.
Fast Corner Model: fastest ramp, Max VI, Max voltage, Min temperature, and
Min package impedance.
Typical Corner Model: All parameters at nominal values.
In many cases, just a single set (i.e. Min, Typ, Max) of models
(representing each bus agent) proves sufficient for all signal quality and
timing margin evaluations. However, situations can arise which demand
multiple sets of models (of SAME component) for a comprehensive
investigations of overshoot/undershoot, crosstalk, synchronous timing skew,
etc. Furthermore, models often evolve through changes and revisions.
Therefore, care must be taken to obtain the most complete and recent models
for each simulation.
The receiver input reference voltage ( Vref ) combined with a specified
tolerance dictate the input threshold region; i.e. :
VIL(max) = Vref - 50 mV
VIH(min) = Vref + 50 mV
The Input Low Voltage ( VIL ) and Input High Voltage ( VIH ) thresholds in
turn influence the lower and upper noise margins of the bus.
Furthermore, Vref in conjunction with a test load are used (by the processor
manufacturer) to specify the signals' valid timing parameters.
Consequently, for XTK simulations, the same test load should be utilized
for TIME_TO_VM calibration of Quad driver models with the
measurement voltage set at Vref.
The termination voltage Vtt (equivalent to the processor core supply
voltage) also varies over a range of values. The smallest value of Vtt
be used in SS (i.e Slow buffer, Slow environment) and the largest Vtt value
in FF (i.e Fast buffer, Fast PCB) simulations.
Vtt, Vref, noise margins, and the models are among critical considerations
when designing or simulating a high speed bus. Other essential bus elements
include topology, termination plans, stackup, decoupling techniques,
bus width, bandwidth, trace impedance, velocity, etc. The final paragraph
presents comments and examples regarding impedance and propagation delay.
The formulas for computing bus impedance include numerous parameters
such as trace width, thickness, height over reference plane, substrate
dielectric constant, etc. Therefore, to efficiently verify for instance
50 +/- 10% Ohms for GTL or 60 +/- 10% Ohms for PCI-X require
use of a field solver program. The signal propagation delay
Tpd (inverse of velocity) involves only the substrate dielectric constant
and simple equations:
Tpd = 1.017* SQRT(0.475Er + 0.67) nS/ft [For microstrip] and
Tpd = 1.017* SQRT(Er) nS/ft [For stripline traces]
For example, assuming FR-4 substrate and Er= 4.2, yields:
Tpd = 1.66 nS/ft [for microstrip] and
Tpd = 2.084 nS/ft [for stripline signals]
When analyzing a high speed bus, it is frequently desirable to ascertain a
range of Tpd values for the outer and inner traces. This can be easily
achieved by first assigning appropriate values to Er for the best and the
worst case corners and then applying the above Tpd equations.
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