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Overview of Last Lecture
• Difference between Latch and Flip Flop? • Latch is level sensi:ve • Flip Flop is edge sensi:ve
• Construc:on of Flip Flop – Master Slave arrangement – Using customized circuit
• Posi:ve Edge Triggered Flip Flop • Nega:ve Edge Triggered Flip Flop
Quiz : Timing Diagram
• Other type of Flip Flops • JK Flip Flop • T Flip Flop
– Characteris:c Table of Flip Flop? – Characteris:c Equa:on of Flip Flop?
Notes on Flip Flops
• The timing of the response of a flip flop to input data and clock must be taken into consideration when using edge-triggered flip flops.
• There is a minimum time, called setup time, for which the D input must be maintained at a constant value prior to the occurrence of the clock transition.
• There is a minimum time, called hold time, for which the D input must not change after the application of the positive transition of the clock.
• The propagation delay time of the flip flop is defined as the time interval between the trigger edge and the stabilization of the output to a new state.
Other Flip Flops
• Other types of flip flops can be constructed by using the D flip flop and external logic. The two most commonly used are: – JK flip flops – T flip flops
JK Flip Flop
• The JK flip flop performs three operations: – set it to 1 – reset it to 0 – complement the output
• The J input sets the flip flop to 1. • The K input resets the flip flop to 0. • When both J and K are enabled, the output
JK Flip Flop Logic
Analysis of the JK Circuit
• The circuit applied to the D input is D = JQ’ + K’Q
– If J = 1 and K = 0, D = Q + Q’ = 1, set to 1 – If J = 0 and K = 1, D = 0, reset to 0 – If J = K = 1, D = Q’, complements the output – If J = K = 0, D = Q, leaving the output unchanged
JK Characteristic Table
Q(t+1) = J(t)Q’(t) + K’(t)Q(t)
T Flip Flop
• The T (Toggle) flip flop is a complementing flip flop and can be obtained from a JK flip flop when inputs J and K are tied together.
• The T flip flop can be obtained from a D flip flop by using an XOR as the input for D. – The expression for D input is D = T ⊕ Q = TQ’ + T’Q – When T = 0, ( j = k = 0 ) then D = Q and there is no change in the output – When T = 1, ( j = k = 1 ) then D = Q’ and the output complements
T Flip Flop Logic
• Characteristic tables define the logical properties of a flip flop by describing its operations in tabular form. – They define the next state as a function of the inputs and the present
state. – Q(t) refers to the present state prior to the application of a clock edge. – Q(t + 1) refers to the next state one clock period later. – Clock edges are not listed as inputs but are implied by the transition from
t to t + 1.
T Flip Flop Characteristic Table
• The D flip flop can be expressed as: – Q(t + 1) = D
• The JK flip flop can be expressed as: – Q(t + 1) = JQ’ + K’Q
• The T flip flop can be expressed as: – Q(t + 1) = TQ’ + T’Q
Direct Input to Flip Flops
D Flip Flop with Asynchronous Reset
• Make changes in circuits of Master‐Slave D Flip Flop with NAND Gates into D flip flop with Clear and Preset inputs