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Using Simulation Techniques to Guarantee Successful Backplane Design

Using Simulation Techniques to Guarantee Successful Backplane Design

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Using Simulation Techniques to Guarantee Successful Backplane Design

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  1. Using Simulation Techniques to Guarantee Successful Backplane Design Shahana Aziz Muniz Engineering, INC MAPLD September 7-9, 2005

  2. Introduction: • Backplane speeds are getting faster due to complex system architectures with high bandwidth • Complex systems now require larger number of loads, higher throughput and dense signal connectivity to meet project requirements. • Backplanes now operate at frequencies of hundreds of MHz, accommodating driver/receiver devices with fast edge rates • Successful backplane design requires careful analysis and simulation MAPLD2005/115

  3. The Challenges of Backplane Design • Backplane design has additional challenges compared to a standalone circuit board deign • Need a design solution that can work across a range of multiple design variables • Plug-in cards may use different possible driver and receiver combinations • System may have a varying number of loads • Finding the optimal solution under a changing load condition is not a trivial problem • Simulation tools provide results for all corner cases quickly and accurately MAPLD2005/115

  4. Simulation Tools Provide a Complete Solution • Tools provide a real world environment • Provide the capability to analyze how loading and parts selection effects issues such as undershoot, overshoot, ringing, crosstalk and timing • Makes it possible to decide about including or excluding termination solutions on the backplane, the choice of backplane trace impedance, backplane dimensions and slot distances and other design parameters • Provides feedback to the plug-in card designers such as driver/receiver component selection, termination schemes for those boards • Provides the environment of determining a complete system solution. • Capability to vary stimulus and models to simulate all possible corner cases, in order to validate circuit performance across all voltage and temperature ranges • Simulations in this paper were completing using Mentor Graphics’ Interconnectix Synthesis (IS) Analyzer and Multiboard, and Sigrity INC’s Speed2000. MAPLD2005/115

  5. Simulation Environment Setup • The Backplane simulation environment is comprised of the following components: • Individual plug-in card simulation models • Backplane simulation model • Interconnection matrix • Device models • Noise rules and simulation stimulus MAPLD2005/115

  6. Simulation Environment Setup Continued • Define System Connectivity • Plug-in cards can be included or excluded from the simulation to create different loading conditions • Specify connector pin matrix. Connection is made by pin number or net name • Import connector model to incorporate pin to pin contact parasitics MAPLD2005/115

  7. Simulation Examples • Simulation was run to investigate the solution for problems such as: • Signal Integrity • Parts Selection • Timing • Crosstalk • Characteristic Impedance • Input Impedance • Transient Voltage Drops MAPLD2005/115

  8. Example 1: Signal Integrity • Simulation was run to study the possible undershoot at sensitive receivers with a maximum of 500mV undershoot tolerance • Using just 10 Ohm stub termination resistor undershoot is seen to be: • ~900mV with 2 loads • ~700mV with 6 loads 2 Load Simulation 6 Load Simulation MAPLD2005/115

  9. Example 2: Parts Selection • To reduce undershoot, an external diode termination scheme was simulated • Two types of diode solutions were compared • Diode with 1V forward voltage drop • Diode with .5V forward Voltage drop • 1V Diode did not make much of a difference • .5V Diode reduced undershoot to value within device tolerance MAPLD2005/115

  10. Example 2: Continued • To reduce undershoot, another approach was to change the stub terminating resistor value at the plug in card • Different values were used, with different driver/receiver combinations to choose the optimal value that allowed 33 MHz operation to continue with undershoot removed • 25 Ohm stub terminators at the target cards were seen to eliminate undershoot • This allowed the problem to be solved without the added component on the backplane MAPLD2005/115

  11. Example 2: Continued • Various loading cases were simulated • Worse case as well as the typical values were recorded in a summary report Worse Case value seen on AD18 Typical value seen on AD23 MAPLD2005/115

  12. Comparison Example • Lab measurement shows close correlation to the simulated results • In populated chassis, measurement cannot be taken at Actel device pin, so for comparisons simulated waveform and lab measurement taken at J1 connector pin • Top right waveform: Lab measurement of signal with 10 Ohm stub resistor only. Top left waveform is the simulated result of same signal – both agree within 100 mV • Simulated: .81V undershoot • Measured: .70V undershoot • Bottom right waveform: Lab measurement of signal with 10 Ohm stub and diode on backplane. Bottom left is the simulated result - again both agree within 100 mV • Simulated: .58V undershoot • Measured: .56V undershoot • Comparison measurement will be made with the 25 Ohm optimal termination in the near future, expected to see strong correlation to simulated result. Simulated Measurements Lab Measurements MAPLD2005/115

  13. Example 3: Timing Analysis • Timing analysis is another area where Multiboard simulation can provide valuable information • Rise and Fall time can be determined at destination devices • Skew between a data bus bits can be measured • System net delay reports provide best and worse case timing analysis to determine system timing budgets and margins Rise/Fall Time Signal Skew MAPLD2005/115

  14. Example 4: Mitigating Crosstalk • Crosstalk reports provide summary information on the victim net crosstalk value and list of aggressors • Detailed reports can be generated to evaluate the contribution of each aggressor signal MAPLD2005/115

  15. Example 5: Controlling Characteristic Impedance • It is possible to measure the characteristic impedance of a signal trace. • Using IS, importing the board construction, material properties, and trace width information the actual value of impedance could be reported for both single ended and differential traces. • The expected versus observed was than compared and changes were made to meet the desired impedance. MAPLD2005/115

  16. Example 6: Predicting Input Impedance • Backplane decoupling was simulated to see if bulk capacitors were sufficient to meet low frequency ( at <50 MHz) impedance requirements • Assuming 10% ripple on each supply, target impedance: • 2.5V * 10/100/1 = .25 Ω (1A current draw) • 3.3 * 10/100/3 = .11 Ω (3A current draw) • 5 * 10/100/1 = .5 Ω (1A current draw) 2.5V Supply Input Impedance 5V Supply Input Impedance 3.3V Supply Input Impedance MAPLD2005/115

  17. Example 7: Simulating Voltage Transients • Simulations were performed to analyze transient noise on the voltage supply due to a current step response. • Voltage requirement is to remain within 10% of nominal, during transient switching event • Worse case current profiles were estimated and simulations were run with each card in the slot drawing worse case power at the same time • Performance noted with and without capacitors on the Backplane 3.3V Supply Transient Behavior 5V Supply Transient Behavior MAPLD2005/115

  18. Conclusion • The examples in this paper were taken from a backplane design completed at NASA Goddard Space Flight Center (GSFC) for the James Webb Space Telescope’s Integrated Science Instrument Module Command and Data Handling Subsystem • As parts get faster, higher throughput is required, and cost and schedule requirements become more competitive – it is becoming imperative to use a tool to aid in the design of a complex high-speed backplane. • Simulating the hardware performance during the layout phase makes it possible to identify and mitigate problems well before fabricating the hardware, greatly eliminating multiple risks from a project’s development cycle MAPLD2005/115

  19. References • http://www.eigroup.org/ibis/ • http://www.actel.com/techdocs/models/ibis.html • http:// www.mentor.com/icx • http://www.sigrity.com/support/techpapers/support_tech_doc.htm MAPLD2005/115