An Introduction to IBIS Models and Signal Integrity Simulation by wio18411

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									    An Introduction to
        IBIS Models
            and
Signal Integrity Simulation


    Shahana Tanveer
   Northrop Grumman
     April 13, 2004
What is SI?




              Peak voltage: ~3V




GND
       1.7V
              Maximum
              undershoot: 1.7V
              MFG Abmax is -.5V




                                  2
Common Signal Integrity Problems

Common Signal Integrity (SI) problems include:
 Undershoot/overshoot
 Ringing
 Crosstalk
 Power/ground plane noise, ground bounce


It is important to lay out the PWB to eliminate possible SI problems.

Things to consider:
    Termination Schemes
    Layer Stack up
    Trace width/spacing/length/impedance
    Bypassing




                                                                        3
Modification: Harness Came in Too Long




                       Board Dimensions 3" x 5"   4
Solving SI problems - Simulation

 SI simulation is becoming more common with
  the availability of quality tools that fit
  seamlessly with the PWB layout flow.
 SI simulation becomes more important with
  faster operational speeds and higher density
  requirements.

A line is electrically long when:
Propagation delay, tp > Rise Time, tr/4,
Where tp = Length, L/ velocity, v

Assuming v = 1.4 x 108 m/s
When tr = 2.5 ns, L = 8.75 cm
But when tr = 1 ns, L = 1.05 cm

So as edge rates are getting faster and faster with
the newer technologies, it is becoming more
important to consider transmission line effects,
termination and possible signal integrity problems
in the PWB design.
                                                      5
Simulation Models: SPICE

SPICE….
 SPICE simulations model a circuit at transistor level, thus SPICE models
  contain detailed information about the circuit and process parameters which is
  regarded as proprietary and IC vendors are reluctant to provide..
 Not all SPICE simulators are fully compatible.
 Although SPICE simulation accuracy is typically very good, a significant
  limitation with this type of modeling is simulation speed.
 SPICE has various simulator options that control accuracy, convergence and
  the algorithm type, and any options that are not consistent might give rise to
  poor correlation in simulation results across different simulators.



           IBIS, is an alternative to SPICE simulation.




                                                                                   6
Simulation Models: IBIS
  IBIS: Input/Output Buffer Information Specification from the
                  Electronics Industry Alliance
  IBIS behavioral data is taken from actual devices.
  IBIS models tend to simulate much faster than SPICE models.
  IBIS Modeling provides a simple table-based buffer model for
     semiconductor devices.
    IBIS models can be used to characterize I/V output curves, rising/falling
     transition waveforms, and package parasitic information of the device.
    IBIS models are intended to provide nonproprietary information about
     I/O buffers and are more easily available from different IC vendors
    Non-convergence is eliminated in IBIS simulation.
    Virtually all EDA vendors presently support IBIS models, and ease of
     use of these IBIS simulators is generally very good.
    IBIS models for most devices are freely available over the Internet
     making it easy to simulate several different manufacturers’ devices on
     the same board.


                                                                                 7
Elements of an IBIS Model



        [2]




                    [3]
      [4]


                            [5]
            [1]



                                  8
        Elements of an IBIS Model
Element 1: Pull-down
 Describes the I/V characteristics during pull-down.
 Data for minimum and maximum current for given
   voltages.
 Data is taken for -Vcc to 2Vcc as that allows a
   behavioral model for signal reflections caused by
   improper termination and overshoot and undershoot
   situations when the protection diodes are forward
   biased.
Element 2: Pull-up                                         Element [1]
 Describes the pull-up state of the buffer when the
   output drives high.
 Data is entered using the formula Vtable =
   Vcc – Voutput
 The minimum and maximum values are determined
   by the minimum and maximum operating
   temperatures, supply voltages and process variations.
 Combining the highest current values with the fastest
   ramp time and minimum package characteristics, a
   fast model can be derived. A slow model can be          Element [2]
   derived by combining the lowest current with the
   slowest ramp time and maximum package                                 9
   characteristics.
       Elements of an IBIS Model
Element 3: GND and Power Clamps
 Describes the ground and power clamp diodes.
 The GND clamp curve is derived from the
   ground relative data gathered while the buffer
   is in the high-impedance state and illustrates
   the region where the ground clamp diode is
   active. The range is from -Vcc to Vcc.
 The power clamp curve is derived from the
   Vcc relative data gathered while the buffer is
   in a high impedance state and shows the
   region where the power clamp diode is active.
   This measurement ranges from Vcc to 2Vcc.




                                                    Element [3]



                                                                  10
      Elements of an IBIS Model
Element 4: Ramp
 Describes the ramp time for the pull-up and pull-
   down devices. Ensures proper AC operation of the
   model.
 The min and max columns represent the minimum
   and maximum slew rates for the buffers.                 Element [4]
 The values represent the intrinsic values of the
   transistors with all package parasitics and external
   loads removed.

Element 5: Package
 Adds the component and package parasitics.
 C_comp is the capacitance of the die itself,
   excluding the package capacitance.
 Package characteristic resistance, inductance and
   capacitance are added by R_pkg, L_pkg, and
   C_pkg, respectively.                                   Element [5]



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Putting it all together – the IBIS File

 A standard IBIS model file consists of three sections:
  Header Info–this section contains basic information about the
    IBIS file and what data it provides.
  Component, Package, and Pin Info –this section contains all
    information regarding the targeted device package, pin lists, pin
    operating conditions, and pin-to-buffer mapping.
  V-I Behavioral Model–this section contains all data to recreate I-
    V curves as well as V-t transition waveforms, which describe the
    switching properties of the particular buffer.




                                                                        12
Putting it all together



          Header




                           Component




                          Model I-V Data




                                           13
  Caution!
                Not all models can be trusted out of the box!
     |***********************************************************************
     | RTSX-S IBIS Model (RT54SX32S and RT54SX72S)
     |***********************************************************************
     [IBIS ver]   3.2
     [File name]   rtsxs.ibs
     [File Rev]   1.1
     [Date]      March 1, 2004
     [Disclaimer] All V/I data was verified for accuracy against bench measurements. The measurements were done
                 on typical production parts. 3.3V PCI model has not been verified against silicon measurements.
                 Please check Actel IBIS page for updates at http://www.actel.com/


     |******************************************************************************
     | IBIS file 6325q83f.ibs created by Jason Lew
     |******************************************************************************
        [IBIS ver] 2.1
        [File name] 6325q83f.ibs
        [File Rev] 2.x
        [Date]      April 9, 2003
        [Source]     From Lab mesurement at Quicklogic.
        [Disclaimer] This information is for modeling purposes only, and is not guaranteed.



                           Be aware of what you are using!
Make certain that models come from trusted sources and are verified
                                                                                                                   14
      Setting up the Simulation
                                     Import CAD data into Simulation tool (translation
                                      is seamless if using the simulator tool supported
                                      Layout tool). This incorporates trace routing, via,
                                      power plane information into simulation.
                                     Set up simulation environment with different
                                      parameters (as shown below).



                                                           Link models
                                                           to devices



Generate Stimulus


  Define Board Construction




              Specify Noise Rules
                                                                                      15
Viewing Results

                  Various Result options:

                  Simulation Waveforms



                   Violation Reports




                                            16
   Simulation Examples

Example 1: Actel AX driving Xilinx input       Simulation helped choose termination
 1V overshoot and undershoot                 value, topology
                                               Use simulation to test different IO
                                              buffers for optimum buffer selection
                                              that meets both timing and SI
   AX High Slew Driver
                                              requirements.
   Violates Device Abs Max

                  AX Slow Slew Driver          AX Fast Slew Driver with 45 Ohm
                  Cannot meet device timing       Termination: Cleans Signal




                                                                                      17
 Simulation Examples

 Question: How would RTSX-S                 Use RTSX-S Buffer model with same
 behave in this environment?               routing
                                            Simulate both High Slew and Slow Slew
                                           RTSX-S
SX-S High or Slow slew: Same
                                           SX-S with 45 Ohm Termination:
rising edge, slower only on falling edge
                                                     Cleans Signal




                                                                                 18
Simulation Examples

Example 2: RTSX-S on multi driver signal (other end Xilinx Virtex)




Overshoot and undershoot violates Abmax
Use Simulation to try different Termination
schemes:
     - Series termination at RTSX-S
     - RC termination at Xilinx




                                               Unterminated RTSX-S driver




                                                                            19
Simulation Examples

With 45 Ohm at RTSX-S – overshoot and undershoots removed




  45 Ohm at RTSX-S, RTSX-S driving   45 Ohm at RTSX-S, Xilinx driving (12S
                                     driver)

                                                                             20
Simulation Examples
RC termination at Xilinx:
                                                 12S driver
 Reduces overshoot and undershoot
 But reveals problems with the
Xilinx driver




  RC termination, RTSX driving       RC termination, Xilinx driving


                                                    16F Driver




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Advanced Simulation

 Include true power/ground characteristics instead of assuming ideal
  planes
 Simulate the effects of Simultaneously Switching Outputs, identify
  possible false logic switching
 Optimize decoupling strategies
 Determine worst case power/ground voltage fluctuations




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References

 High Speed Digital Design by Howard W. Johnson and Martin
    Graham.
   http://www.eigroup.org/ibis/
   www.actel.com
   www.mentor.com/icx
   www.sigrity.com




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