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Finite state machines

Finite state machines. Finite State Machines (FSMs). Widely used specification technique Used to specify system behavior in response to events Both output and next state can be determined solely on the basis of understanding the current state and the event that caused the transition

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Finite state machines

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  1. Finite state machines

  2. Finite State Machines(FSMs) • Widely used specification technique • Used to specify system behavior in response to events • Both output and next state can be determined solely on the basis of understanding the current state and the event that caused the transition • H/W engineers have been using FSM for years

  3. Can I interest you in our new long distance service?

  4. Can I interest you in our new long distance service? No thanks.

  5. Can I interest you in our new long distance service?

  6. Can I interest you in our new long distance service? Get Lost!!

  7. Can I interest you in our new long distance service? Get Lost!! Same stimulus event Different response

  8. State Machines • Models how a system responds differently to events over time

  9. State Machines • Models how a system responds differently to events over time button switch

  10. States OFF ON

  11. States Press OFF ON Press

  12. Finite State Machines • State • the memory of previous events • Events • inputs to a system • Actions • output in some form • mechanical, electrical or software event

  13. Finite State Machines • State • the memory of previous events • Events • inputs to a system • Actions • output in some form • mechanical, electrical or software event

  14. Coke Machine • Events • coin insertion • Actions • coke delivery • coins returned • State • the memory of how much money has been deposited

  15. FSM Diagram Event-1 State 1 State 2 Action-k

  16. FSM Event-1 State 1 State 2 Action-k Event-1 Not all state transitions have actions associated with them

  17. ScenarioQuarters Only, 75 cents • Customer • enter quarter • Customer • enter quarter • Customer • enter quarter • Coke Machine • deliver coke

  18. Enumerate Events & Actions Events:E1: Deposit 25 cents Actions: A1: Deliver coke

  19. Define States Start State E1 A B E1 E1/A1 C Events:E1: Deposit 25 cents Actions: A1: Deliver coke States: A: No money B: 25 cents entered C: 50 cents entered

  20. State Transition Table

  21. Coke Machine Scenario 2Coin Return • Customer • enter quarter • Customer • enter quarter • Customer • press coin return (CR) • Coke Machine • return coins

  22. Enumerate Events & Actions Events:E1: Deposit 25 centsE2: Coin Return (CR) Actions: A1: Deliver coke A2: Return coins

  23. Define States Start State E1 A B E1 E1/A1 E2/A2 C Events:E1: Deposit 25 centsE2: Coin Return (CR) Actions: A1: Deliver coke A2: Return coins States: A: No money B: 25 cents entered C: 50 cents entered

  24. Define States E2/A2 Start State E1 A B E1 E1/A1 E2/A2 C Events:E1: Deposit 25 centsE2: Coin Return (CR) Actions: A1: Deliver coke A2: Return coins States: A: No money B: 25 cents entered C: 50 cents entered

  25. Transition Table • State E1 E2 • A B • B C A/coin return • C A/coke A/coin return

  26. Transition Table No BlanksAllowed !! • State E1 E2 • A B • B C A/coin return • C A/coke A/coin return

  27. Transition Table • State E1 E2 • A B A • B C A/coin return • C A/coke A/coin return

  28. Telephone System FSMLocal 4-digit exchange Events d: digit dialed h: hang up Actions c1: connect to caller

  29. Telephone System FSMLocal 4-digit exchange d d d d/c1 A B C D E Events d: digit dialed h: hang up Actions c1: connect to caller StatesA: start B: 1 digit dialed C: 2 digits dialed D: 3 digits dialed E: call in progress

  30. Telephone System FSMLocal 4-digit exchange h d d d d/c1 A B C D E Events d: digit dialed h: hang up Actions c1: connect to caller StatesA: start B: 1 digit dialed C: 2 digits dialed D: 3 digits dialed E: call in progress

  31. Telephone System FSMLocal 4-digit exchange h d d d d/c1 A B C D E Events d: digit dialed h: hang up Actions c1: connect to caller StatesA: start B: 1 digit dialed C: 2 digits dialed D: 3 digits dialed E: call in progress

  32. Problems with FSMs • The state explosion problem • Confusing when too many states • Requirements: • “in all airborne states, when the yellow handle is pulled, the seat will be ejected” • maps event to large collection of states • “when the selection button is pressed, enter the selected mode” • implies a clustering or grouping of states

  33. a A b B c b C c Harel’s State Charts • Allows grouping or clustering of states D a A b B c C c Clustering into new superstate D - reallyan abstraction. To be in D really means to be ineither A or C

  34. 10 C F A 5 5 B E Coke Machine 10/coke 10 5 D 5 5/coke

  35. C F E Coke Machine CR 10/coke ouch! CR 10 10 5 D A 5 5 5 5/coke B CR

  36. Money Entered No Money (A)

  37. Money Entered 5 No Money (A) 10 CR

  38. C Money Entered 5 10 5 B 5 No Money (A) 10 CR

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