Fundamentals of Computer Systems

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1 Fundamentals of Computer Systems Sequential Logic Stephen A. Edwards Columbia University Fall 2012

2 State-Holding Elements

3 Bistable Elements Equivalent circuits; right is more traditional. Two stable states:

4 A Bistable in the Wild This debounces the coin switch. Breakout, Atari 1976.

5 S Latch S S

6 S Latch 0 1 S 1 0 S S Set

7 S Latch 0 1 S 0 0 S S Hold, State 1

8 S Latch 1 0 S 0 1 S S eset

9 S Latch 0 0 S 0 1 S S Hold, State 0

10 S Latch 1 0 S 1 0 S S Huh?

11 S Latch 0 1 S 1 0 S S Set

12 S Latch 0 1 S 0 0 S S Hold, State 1

13 S Latch 1 0 S 1 0 S S Huh?

14 S Latch 0 X S 0 X S S Undefined

15 S Latches in the Wild Generates horizontal and vertical synchronization waveforms from counter bits. Stunt Cycle, Atari 1976.

16 Latch C C inputs outputs C 0 X

17 A Challenge A simple traffic light controller. Want the lights to cycle green-yellow-red. C C Y C G oes this work?

18

19

20

21

22

23

24

25

26

27

28

29

30 Positive-Edge-Triggered Flip-Flop C M Master C Slave C S C C C C M transparent C S opaque

31 Positive-Edge-Triggered Flip-Flop C M Master C Slave C S C C C C M transparent C S opaque

32 Positive-Edge-Triggered Flip-Flop C M Master C Slave C S C C C C M C S transparent opaque opaque transparent

33 Positive-Edge-Triggered Flip-Flop C M Master C Slave C S C C C C M C S transparent opaque opaque transparent

34 Positive-Edge-Triggered Flip-Flop C M Master C Slave C S C C C C M transparent opaque transparent C S opaque transparent opaque

35 Positive-Edge-Triggered Flip-Flop C M Master C Slave C S C C C C M transparent opaque transparent opaque C S opaque transparent opaque transparent

36 The Traffic Light Controller: A second try Let s try this again with flip-flops. Y G Y G

37 The Traffic Light Controller: A second try Let s try this again with flip-flops. Y G Y G

38 The Traffic Light Controller: A second try Let s try this again with flip-flops. Y G Y G

39 The Traffic Light Controller: A second try Let s try this again with flip-flops. Y G Y G

40 The Traffic Light Controller: A second try Let s try this again with flip-flops. Y G Y G

41 The Traffic Light Controller with eset ESET Y G ESET Y G

42 The Traffic Light Controller with eset ESET Y G ESET Y G

43 The Traffic Light Controller with eset ESET Y G ESET Y G

44 The Traffic Light Controller with eset ESET Y G ESET Y G

45 The Traffic Light Controller with eset ESET Y G ESET Y G

46 The Traffic Light Controller with eset ESET Y G ESET Y G

47 Flip-Flop with Enable 0 1 E C C E 0 X X X 1 X X E C What s wrong with this solution?

48 Asynchronous Preset/Clear PE CL PE CL

49 The Traffic Light Controller w/ Async. eset ESET PE CL PE CL Y PE CL G

50 The Synchronous igital Logic Paradigm Gates and flip-flops only INPUTS OUTPUTS Each flip-flop driven by the same clock STATE C L Every cyclic path contains at least one flip-flop CLOCK NEXT STATE

51 Cool Sequential Circuits: Shift egisters A A X X X X 1 0 X X X X X X

52 Universal Shift egister L S 1 S0 S 1 S L S 1 S 0 Operation 0 0 Shift right 0 1 Load 1 0 Hold 1 1 Shift left

53 Cool Sequential Circuits: Counters Cycle through sequences of numbers, e.g.,

54 The 74LS163 Synchronous Binary Counter

55 Timing in Synchronous Circuits C L t c t c : Clock period. E.g., 10 ns for a 100 MHz clock

56 Timing in Synchronous Circuits C L Sufficient Hold Time? t p(min,ff) t p(min,cl) Hold time constraint: how soon after the clock edge can start changing? Min. FF delay + min. logic delay

57 Timing in Synchronous Circuits C L t p(max,ff) Sufficient Setup Time? t p(max,cl) Setup time constraint: when before the clock edge is guaranteed stable? Max. FF delay + max. logic delay

58 Clock Skew: What eally Happens C L 1 2 Sufficient Hold Time? 1 2 t skew t p(min,ff) t p(min,cl) 2 arrives late: clock skew reduces hold time

59 Clock Skew: What eally Happens C L 1 2 Sufficient Setup Time? 1 2 t skew t p(max,ff) t p(max,cl) 1 arrives early: clock skew reduces setup time

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