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CSE 140 Lecture 8 Sequential Networks

This lecture covers the various types of memory devices used in Sequential Networks, including latches, flip-flops, SR latches, and D latches. It also discusses implementation techniques and characteristic expressions.

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CSE 140 Lecture 8 Sequential Networks

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  1. CSE 140 Lecture 8Sequential Networks Professor CK Cheng CSE Dept. UC San Diego

  2. Part II. Sequential Networks (Ch. 3) Memory / Time steps si xi yi yi=fi(St,X) sit+1=gi(St,X) Clock Memory: Flip flops Specification: Finite State Machines Implementation: Excitation Tables

  3. Memory Devices • Memory Storage • Latches • Flip-Flops • SR, D, T, JK • State Tables • Characteristic Expressions

  4. Memory Storage: Capacitive Loads • Fundamental building block of other state elements • Two outputs: Q, Q • No inputs

  5. Capacitive Loads • Consider the two possible cases: • Q = 0: then Q’ = 1 and Q = 0 (consistent) • Q = 1: then Q’ = 0 and Q = 1 (consistent) • Bistable circuit stores 1 bit of state in the state variable, Q (or Q’ ) • But there are no inputs to control the state

  6. SR (Set/Reset) Latch • SR Latch • Consider the four possible cases: • S = 1, R = 0 • S = 0, R = 1 • S = 0, R = 0 • S = 1, R = 1

  7. SR Latch Analysis • S = 1, R = 0: then Q = 1 and Q = 0 • S = 0, R = 1: then Q = 0 and Q = 1

  8. SR Latch Analysis • S = 1, R = 0: then Q = 1 and Q = 0 • S = 0, R = 1: then Q = 0 and Q = 1

  9. SR Latch Analysis • S = 0, R = 0: then Q = Qprev • S = 1, R = 1: then Q = 0 and Q = 0

  10. y S y = (S+Q)’ Q Q = (R+y)’ R

  11. Flip-flop Components SR F-F (Set-Reset) y S R Q Inputs: S, R State: (Q, y)

  12. Id Q y S R Q* y Q** y** Q*** y*** 0 0 0 0 0 1 1 0 0 1 1 1 0 0 0 1 0 1 0 1 0 1 2 0 0 1 0 1 0 1 0 1 0 3 0 0 1 1 0 0 0 0 0 0 4 0 1 0 0 0 1 0 1 0 1 5 0 1 0 1 0 1 0 1 0 1 6 0 1 1 0 0 0 1 0 1 0 7 0 1 1 1 0 0 0 0 0 0 8 1 0 0 0 1 0 1 0 1 0 9 1 0 0 1 0 0 0 1 0 1 10 1 0 1 0 1 0 1 0 1 0 11 1 0 1 1 0 0 0 0 0 0 12 1 1 0 0 0 0 1 1 0 0 13 1 1 0 1 0 0 0 1 0 1 14 1 1 1 0 0 0 1 0 1 0 15 1 1 1 1 0 0 0 0 0 0 State Q y SR Transition State Diagram 00 01 10 00 10 10 01 10 01 11 10 01 11 10 SR 00 11 01 00 11 00 11

  13. CASES: SR=01, (Q,y) = (0,1) SR=10, (Q,y) = (1,0) SR=11, (Q,y) = (0,0) SR = 00 => if (Q,y) = (0,0) or (1,1), the output keeps changing Solutions: 1) SR = (0,0), or 2) SR = (1,1). State table SR inputs 00 01 10 11 PS 0 0 0 1 - 1 1 0 1 - Characteristic Expression Q(t+1) = S(t)+R’(t)Q(t) Q(t) Q(t+1) NS (next state)

  14. SR Latch Analysis • S = 0, R = 0: then Q = Qprev and Q = Qprev (memory!) • S = 1, R = 1: then Q = 0 and Q = 0 (invalid state:Q ≠ NOT Q)

  15. SR Latch Symbol • SR stands for Set/Reset Latch • Stores one bit of state (Q) • Control what value is being stored with S, R inputs • Set: Make the output 1 (S = 1, R = 0, Q = 1) • Reset: Make the output 0 (S = 0, R = 1, Q = 0) • Must do something to avoid • invalid state (when S = R = 1)

  16. D Latch • Two inputs: CLK, D • CLK: controls when the output changes • D (the data input): controls what the output changes to • Function • When CLK = 1, D passes through to Q (the latch is transparent) • When CLK = 0, Q holds its previous value (the latch is opaque) • Avoids invalid case when Q ≠ NOT Q

  17. D Latch Internal Circuit

  18. D Latch Internal Circuit

  19. D Flip-Flop • Two inputs: CLK, D • Function • The flip-flop “samples” D on the rising edge of CLK • When CLK rises from 0 to 1, D passes through to Q • Otherwise, Q holds its previous value • Q changes only on the rising edge of CLK • A flip-flop is called an edge-triggered device because it is activated on the clock edge

  20. D Flip-Flop Internal Circuit • Two back-to-back latches (L1 and L2) controlled by complementary clocks • When CLK = 0 • L1 is transparent, L2 is opaque • D passes through to N1 • When CLK = 1 • L2 is transparent, L1 is opaque • N1 passes through to Q • Thus, on the edge of the clock (when CLK rises from 0 1) • D passes through to Q

  21. D Flip-Flop vs. D Latch

  22. D Flip-Flop vs. D Latch

  23. Latch and Flip-flops (two latches) Latch can be considered as a door CLK = 0, door is shut CLK = 1, door is unlocked A flip-flop is a two door entrance CLK = 1 CLK = 0 CLK = 1

  24. D Flip-Flop (Delay) Id D Q(t) Q(t+1) 0 0 0 0 1 0 1 0 2 1 0 1 3 1 1 1 Q D CLK Q’ State table Characteristic Expression Q(t+1) = D(t) D PS 0 1 0 0 1 1 0 1 NS= Q(t+1)

  25. JK F-F State table Q J JK 00 01 10 11 PS CLK 0 0 0 1 ? 1 1 0 1 ? Q’ K Q(t+1)

  26. JK F-F State table Q J JK 00 01 10 11 PS CLK 0 0 0 1 1 1 1 0 1 0 Q’ K Q(t+1) Characteristic Expression Q(t+1) = Q(t)K’(t)+Q’(t)J(t)

  27. T Flip-Flop (Toggle) State table Q T T PS 0 1 CLK 0 0 1 1 1 0 Q’ Q(t+1) Characteristic Expression Q(t+1) = Q’(t)T(t) + Q(t)T’(t)

  28. Using a JK F-F to implement a D and T F-F Q Q’ D J K CLK D flip flop Q Q’ T J K CLK T flip flop

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