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CENG 241 Digital Design 1 Lecture 8

CENG 241 Digital Design 1 Lecture 8. Amirali Baniasadi amirali@ece.uvic.ca. Flip-Flops. Sequential Circuits use flip-flops as storage elements Flip-Flop is a binary storage device that saves one bit of information The outputs can come from flip-flops or combinational logic

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CENG 241 Digital Design 1 Lecture 8

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  1. CENG 241Digital Design 1Lecture 8 Amirali Baniasadi amirali@ece.uvic.ca

  2. Flip-Flops Sequential Circuits use flip-flops as storage elements Flip-Flop is a binary storage device that saves one bit of information The outputs can come from flip-flops or combinational logic Flip-flop inputs come from combinational logic or clock generators

  3. Latches Different flip-flops are different based on the number of inputs and how the inputs affect the binary state. Basic types of flip-flops operate with signal levels and are called latches. Example: SR latch

  4. SR Latches with NAND gates

  5. SR Latches with Control input S and R are allowed to change the flip-flop only when C = 1. If C=0, S and R can’t change output

  6. 1 1 1 D Latch Want to get rid of the undesirable SR condition where both S and R are 1. 0 1 0 1 1 1 0 Also called a transparent latch

  7. 1 1 1 D Latch Want to get rid of the undesirable SR condition where both S and R are 1. 1 0 1 1 0 0 1 Also called a transparent latch

  8. Graphic symbols for Latches

  9. FLIP-FLOPS Many flip-flops are edge triggered: They respond to the input only during transition from 0 to 1 or from 1 to 0.

  10. Edge-Triggered D Flip-Flop The output can change only when clock goes from 1 to 0. Data blocked here while CLK =1 1 1 1 0

  11. Edge-Triggered D Flip-Flop The output can change only when clock goes from 1 to 0. Data passes here while CLK =0 1 1 1 0 1

  12. Graphic Symbols

  13. Other Flip-Flops • Each flip-flop is made of interconnection of gates. • The edge-triggered D flip-flop is the most efficient flip-flop since it requires the least number of gates. • Other flip-flops are made using the D flip-flop and extra logic. • Two flip-flops widely used are the JK andTflip-flop.

  14. JK Flip-Flop • Three flip-flop operations: Set, Reset, Complement output. • JK performs all three

  15. JK Flip-Flop D = JQ’ + K’Q if J=1 , K=0 then D=Q’+Q=1 if J=0 , K=1 then D=0 if j =1 , K=1 then D = Q’

  16. T Flip-Flop T (Toggle) flip-flop is a complementing one. T flip-flop is obtained from a JK when inputs J and K are tied together.

  17. T Flip-Flop If T=0 ( J=K=0) output does not change. If T=1 ( J=K=1) output is complemented. A T flip-flop can also be made of D flip-flop and a XOR. D = T XOR Q = TQ’ + T’Q

  18. Characteristic Tables • JK Flip-flop • J K Q(t+1) • 0 0 Q(t) No change • 0 1 0 Reset • 1 0 1 Set • 1 1 Q’(t) Complement

  19. Characteristic Tables • D Flip-flop • D Q(t+1) • 0 0 Reset • 1 1 Set • T Flip-flop • T Q(t+1) • 0 Q(t) No change • 1 Q’(t) Complement

  20. Summary • Latches, Flip-Flops: SR, D, JK and T

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