ECE380 Digital Logic Flip-Flops, Registers and Counters: Flip-Flops Electrical & Computer Engineering Dr. D. J. Jackson Lecture 25-1 Flip-flops • The gated latch circuits presented are level sensitive and can change states more than once during the ‘active’ period of the clock signal • Circuits (storage elements) that can change their state no more than once during a clock period are also useful • Two types of circuits with such behavior – Master-slave flip-flip – Edge-triggered flip-flop Electrical & Computer Engineering Dr. D. J. Jackson Lecture 25-2 1 Master-slave D flip-flop • Consists of 2 gated D latches –The first, master, changes its state while clock=1 – The second, slave, changes its state while clock=0 Master Slave Q Q m s D D Q D Q Q Clk Q Clk Q Clock Q 38 transistors Electrical & Computer Engineering Dr. D. J. Jackson Lecture 25-3 Master-slave D flip-flop • When clock=1, the master tracks the values of the D input signal and the slave does not change – Thus Qm follows any changes in D and Qs remains constant • When the clock signal changes to 0, the master stage stops following the changes in the D input signal • At the same time, the slave stage responds to the value of Qm and changes states accordingly •Since Qm does not change when clock=0, the slave stage undergoes at most one change of state during a clock cycle • From an output point of view, the circuit changes Qs (its output) at the negative edge of the clock signal Electrical & Computer Engineering Dr. D. J. Jackson Lecture 25-4 2 Master-slave D flip-flop Clock D Q m Q = Q s D Q clock Q Electrical & Computer Engineering Dr. D. J. Jackson Lecture 25-5 Edge-triggered flip-flop • A circuit, similar in functionality to the master-slave D flip-flop, can be constructed with 6 NAND gates 1 P3 P1 2 5 Q D Q Clock clock P2 6 Q 3 Q Positive-edge-triggered D type flip-flop D 4 P4 24 transistors Electrical & Computer Engineering Dr. D. J. Jackson Lecture 25-6 3 Edge-triggered flip-flop • The previous circuit responds on the positive edge of the clock signal • A negative-edge triggered D flip-flop can be constructed by replacing the NAND with NOR gates D Q D Q clock Q clock Q Positive-edge-triggered Negative-edge-triggered D type flip-flop D type flip-flop Electrical & Computer Engineering Dr. D. J. Jackson Lecture 25-7 Comparing D storage elements D D Q Qa clock clk Q clock D Q Qb D Qa Q Q b D Q Q c Q c Q Electrical & Computer Engineering Dr. D. J. Jackson Lecture 25-8 4 Clear and preset inputs • It may be desirable to specifically set (Q=1) or clear (Q=0) a flip-flop • Practical flip-flops often have preset and clear inputs – Generally, these inputs are asynchronous (they do not depend on the clock signal) Preset’ D Q As long as Preset’=0, Q=1 clock Q As long as Clear’=0, Q=0 Clear’ Electrical & Computer Engineering Dr. D. J. Jackson Lecture 25-9 T flip-flop • Another flip-flop type, the T flip-flop, can be derived from the basic D flip-flop presented • Feedback connections make the input signal D equal to the value of Q or Q’ under control of a signal labeled T D Q Q T Q Q Clock Electrical & Computer Engineering Dr. D. J. Jackson Lecture 25-10 5 T flip-flop • The name T derives from the behavior of the circuit, which ‘toggles’ its state when T=1 – This feature makes the T flip-flop a useful element when constructing counter circuits TQ(t+1) Clock 0Q(t) T 1Q’(t) Q T Q clock Q Positive edge triggered Electrical & Computer Engineering Dr. D. J. Jackson Lecture 25-11 JK flip-flop •The JK flip-flop can also be derived from the basic D flip-flop such that D=JQ’+K’Q • The JK flip-flop combines aspects of the SR and the T flip-flop – It behaves as the SR flip-flop (where J=S and K=R) for all values except J=K=1 – For J=K=1, it toggles like the T flip-flop Electrical & Computer Engineering Dr. D. J. Jackson Lecture 25-12 6 JK flip-flop J D Q Q K Q Q Clock JKQ(t+1) 00 Q(t) J Q 01 0 clock 10 1 K Q 11 Q’(t) Positive edge triggered Electrical & Computer Engineering Dr. D. J. Jackson Lecture 25-13 JK flip-flop timing diagram Complete the following timing diagram 1 Clk 0 1 K 0 1 J 0 1 Q 0 1 Q 0 Time Electrical & Computer Engineering Dr. D. J. Jackson Lecture 25-14 7.