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CENG 241 Digital Design 1 Lecture 8 Amirali Baniasadi

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1 CENG 241 Digital Design 1 Lecture 8 Amirali Baniasadi

2 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 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 4 SR Latches with NAND gates

5 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 6 D Latch Want to get rid of the undesirable SR condition where both S and R are 1. Also called a transparent latch

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

8 8 Graphic symbols for Latches

9 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 10 Edge-Triggered D Flip-Flop The output can change only when clock goes from 1 to Data blocked here while CLK =1

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

12 12 Graphic Symbols

13 13 Other Flip-Flops zEach flip-flop is made of interconnection of gates. zThe edge-triggered D flip-flop is the most efficient flip-flop since it requires the least number of gates. zOther flip-flops are made using the D flip-flop and extra logic. zTwo flip-flops widely used are the JK and T flip-flop.

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

15 15 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’ JK Flip-Flop

16 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 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 18 Characteristic Tables zJK Flip-flop zJ K Q(t+1) z0 0 Q(t) No change z0 1 0 Reset z1 0 1 Set z1 1 Q’(t) Complement

19 19 Characteristic Tables zD Flip-flop zD Q(t+1) z0 0 Reset z1 1 Set zT Flip-flop zT Q(t+1) z0 Q(t) No change z1 Q’(t) Complement

20 20 Summary zLatches, Flip-Flops: SR, D, JK and T


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