Asynchronous Logic by ewghwehws

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									                      Asynchronous Logic




Some material from “Asynchronous FPGA Risks,” Ken Erickson, 2000 MALPD International Conference
September 26-28, 2000.
Asynchronous Clears
  Synchronous vs. Asynchronous
             Logic
• Asynchronous signals are not synchronized to a
  clock.
• Timing Analysis for Asynchronous Circuits
   – Many tools do not support this
   – Complex, sometimes not tractable
   – Error-prone
• Asynchronous logic may result in smaller, faster,
  or lower power circuits
• Asynchronous logic, well done, is reliable.
                   Is It Or Isn‟t It?

16 MHz high skew clock
                                           1 MHz low-skew clock



             Divide by 16 Ripple                      Synchronous
             Counter                                  Logic


                                   Low-skew buffer
Is It Or Isn‟t It?
Common Asynchronous Design Problems
• Design may be marginal
  – Adequate margin non-verifiable
• Aging and radiation effects
  – Can not test for these
• Failures may occur late in the test program
  – i.e., thermal of thermal/vacuum testing
  – This is always on Friday night
• System may have unexplained glitches
  – Often difficult to troubleshoot
   Some Examples of Problems
• Spacecraft Experienced Inadvertent Reset
  During System Testing
  – Only from 17 to 20 °C
  – FPGAs were redesigned
• Lots and lots of „rookie mistakes.‟
  – No analysis and unknown margin
  – Decoded outputs used as clocks
  – High-skew signals used as clocks
     • Counters
     • Shift Registers
               Case Study
          Potential Race Condition
                 RS Flip-Flop

                              Q



307 KHz                                6-Bit Down Ctr
Clock
                                                    Q5
                                           Enable   Q4
                                                    Q3
                  4-Bit Ctr                         Q2
                          Q3                        Q1
                                                    Q0

                                  19 KHz
                                  Clock

								
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