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B5: Designing for Signal and Power Integrity in FPGA Designs. Mark Alexander 9/10/2002. Agenda. Motivation: Vulnerabilities of high-speed FPGAs Signal Integrity Concerns Transmission Lines Termination I-Grade Parts Power Integrity Concerns Keeping it Clean Conclusion.

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b5 designing for signal and power integrity in fpga designs

B5: Designing for Signal and Power Integrity in FPGA Designs

Mark Alexander


  • Motivation: Vulnerabilities of high-speed FPGAs
  • Signal Integrity Concerns
    • Transmission Lines
    • Termination
    • I-Grade Parts
  • Power Integrity Concerns
    • Keeping it Clean
  • Conclusion
  • Digital device speeds have increased – quickly!
  • Signal transition times have passed a critical point
    • Everything is a Transmission-line
  • Device density has passed a critical point
    • Transient current demands are now wideband
  • New design methods are needed
    • Otherwise, everything fails and we don’t know why
si pi signal and power integrity
SI/PI: Signal and Power Integrity
  • Signal Integrity Engineering
    • Signal waveforms of Data and Clock
    • Signal transition time is the most important factor
      • Dependent on device speed – not clock frequency!
    • Proper PCB Design is the most critical factor
  • Power Integrity Engineering
    • How to build a good PDS (power distribution system)
      • Must accommodate wideband transient currents
    • Proper PCB Design is the most critical factor
signal integrity
Signal Integrity
  • Transition time (rise/fall time)
    • Has gone from 10ns to 1ns in the past ten years
  • For 40cm board
    • 10ns rise time requires no thought – pure digital
    • 1ns rise time has big implications
      • Every PCB trace is a transmission line
      • Any impedance discontinuity can cause reflection
  • Reflections cause:
    • Overshoot
    • Ringing
    • Bad data
    • EMI
  • Need for impedance CONTINUITY
impedance continuity
Impedance Continuity
  • Requires
    • Controlled impedance PCB traces
      • Must be specified by the PCB Designer
      • Reference planes must be present in PCB stackup
    • Knowledge of all impedances in the channel
      • driver Z, trace Z, receiver Z
    • Impedance modifications, in most cases
      • That’s what termination is for!
  • Can be resistors, capacitors or diodes
  • Resistors are easiest to use
    • For details, see Johnson and Graham book (Appendix I)
  • Usually discrete external resistors
  • On-chip termination is now available!
    • Altera and Xilinx both offer this feature
  • Crude on-chip series-termination is available in all PLDs
    • Just select a lower-strength driver – 6mA, 8mA etc.
importance of simulation
Importance of Simulation
  • Simulation is the only way to analyze T-line behavior
  • IBIS and SPICE are the two main methods
    • SPICE is accurate, but difficult and time consuming
    • IBIS is adequate, fast, and it’s easy to get models
  • IBIS runs out of steam at 500 MHz
    • SPICE should be used for multi-gigabit serial designs
  • Allows easy “what-if” analysis
  • Always simulate all process and environment corners
    • Best P, High V, Low T = fast device  signal integrity verification
    • Worst P, Low V, High T = slow device  timing verification
impact of industrial designation
Impact of Industrial Designation
  • High-rel and hostile environment applications often use I-grade parts
  • I-grade has wider temperature range than C-grade parts
    • Commercial grade: 0°C to 85°C junction temperature
    • Industrial grade: -40°C to 100°C junction temperature
  • I-grade parts have faster silicon to accommodate higher temperatures


  • I-grade parts are often used at lower temperatures


  • Edge rates for I-grade parts are even faster than for typical FPGAs!

Simulate all corners!

More signal integrity problems

power integrity

People forget this one!

Power Integrity
  • Power Distribution System (PDS) is central point
  • Purposes of PDS:
    • Deliver power
    • Maintain low noise!
  • Basic parts of PDS:
    • Voltage source (Vreg or Power Supply Circuit)
    • Power planes in PCB stackup
    • Decoupling capacitors
what if there is noise

leads to

What if there is noise?

Excessive noise on the power supplies

Jitter on all signals

Noise on Vcco  timing jitter on I/O signals

Noise on Vccint  timing jitter on core signals

Noise on Vccaux  timing jitter on clock resources

where s the problem
Where’s the problem?
  • Power supply noise DOES NOT come from the Vreg
  • Noise comes from the devices themselves
    • Digital circuits are noisy!
  • Source of all PDS noise:
    • Transient currents flowing through parasitic inductances
  • Where do transient currents come from?
    • Large portions of device enabled/disabled (seconds, days)
    • Clock events – all flops change state at once (nanoseconds)
    • Harmonics of square waves (picoseconds)
  • All these things together make wideband noise
wideband solution
Wideband solution
  • FPGA needs easy access to extra current
  • Bypass capacitors and planes provide this
    • But they have drawbacks
      • They have parasitics of their own
      • Each one only works over a narrow frequency range
    • Solutions to these drawbacks
      • Use many caps in parallel to reduce parasitic effects
      • Use many cap values to cover wide frequency range

470uF, 47uF, 2.2uF, 0.1uF, 0.01uF, 0.001uF

pds design for fpgas
PDS Design for FPGAs
  • One capacitor per Vcc pin
  • Yes, EVERY Vcc pin – Vccint, Vcco, Vccaux, Vref
  • Within the total count for each supply:
    • Allocate some in each frequency range
      • 1% 470 uF
      • 3% 47 uF
      • 6% 2.2 uF
      • 15% 0.1 uF
      • 25% 0.01 uF
      • 50% 0.001 uF
  • Power planes and/or sandwiches are a must
then simulate
Then Simulate
  • Simulation is needed to view network impedance profile
    • Should cover frequencies from 500 kHz to 800 MHz
    • Impedance should be low and flat over this range
then build and measure

Must have 1 GHz or better scope

Must have 1 GHz or better probes

Probe power vias on back of board

Use infinite persistence mode

Then Build and Measure
  • Noise on all supplies should be less than 10% of nominal
  • Measure with a fast oscilloscope
read xapp623
  • Many important details have been left out here
  • System engineers MUST be familiar with
    • System vulnerabilities
    • Common pitfalls
    • Proper design techniques and methods
  • Also consult other PDS references:
    • SI Central
    • Online design communities
    • Get trained!
  • FPGA system design is not just VHDL and Verilog
  • Faster devices mean new challenges
  • Everything is a transmission line
  • Signal Simulation is vital
  • Good PDS Engineering is essential
  • Use lots of capacitors
  • Get trained, stay current, and read the fine print!
appendix i references
  • Howard Johnson and Martin Graham,

High Speed Digital Design: A Handbook of Black Magic,

Prentice Hall, New Jersey, 1993.

  • Mark Alexander,

XAPP623 PDS Design: Using Bypass/Decoupling Capacitors,

Xilinx Appnote, 2002.


appendix ii useful resources
  • Signal Integrity Central – Xilinx site with links to all Xilinx SI literature


  • PCB Checklist – Checklist with all pertinent items and explanations, links


  • SI Reflector – Good discussion list with its share of gurus

Si-list-request@freelists.org, archives athttp://www.freelists.org/archives/si-list

  • Your local EMC Society – Excellent resource to help you stay current


  • SI Training from qualified experts – Fast path to success

http://www.gigatest.com Eric Bogatin

http://www.signalintegrity.com Howard Johnson

http://www.speedingedge.com Lee Ritchey

http://www.ultracad.com Doug Brooks

appendix iii basic decoupling rules
  • Use small capacitor packages
    • Parasitic L is proportional to pkg. size
  • Use largest value in a given package
    • L is dominated by pkg, so maximize C
  • Connect cap lands directly to planes
  • NEVER share cap vias
  • Keep trace between land and via short!!
    • Benefit of small package is lost otherwise