Example 1: RS Flip-flop

• Basic system:
  • basic digital storage element
    • 
  • behaviour

    • S	R	Q	Qbar	Remark
      1	0	1	0	store is set
      0	1	0	1	store is reset
      0	0	hold	hold	last value is hold ("stored")
      1	1	0	0	forbidden!

  • implementation with simple or and not gates
    • 
  • easily built using standard electronic circuits
• Model in Simulink:
  • direct implementation using OR and NOT gates from library
  • additional input and output elements
  • run →
    • Error: Cannot solve algebraic loop involving 'rsflipflopA/RS flip-flop/Or2' because it consists of blocks that cannot be assigned algebraic variables ...
      Consider breaking the algebraic loop. For example, add a delay or a memory block to the loop ...
• Mathematical problem:
  • S = R = 0 → two possible values for Q/Qbar
• Reality check:
  • initial value for 0/0 is random, but consistent
  • transport of physical signals needs time!
  • different delays in components (transistors) → one output "wins"
  • complex details (inhomogenities in semiconductor crystals)
• Extended Model:
  • include a small delay
    • 
    • using Memory block from library
  • works without additional parameters
  • problem
    • actual delay depends on solver behaviour
    • better solutions available