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

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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

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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

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4 SR Latches with NAND gates

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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

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

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

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8 Graphic symbols for Latches

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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.

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

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

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12 Graphic Symbols

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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.

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

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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

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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.

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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

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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

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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

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

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Sequential circuits The digital circuits considered thus far have been combinational, where the outputs are entirely dependent on the current inputs. Although.

Sequential circuits The digital circuits considered thus far have been combinational, where the outputs are entirely dependent on the current inputs. Although.

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