Tuesday, December 3, 2013


  • Sequential logic is a type of logic circuit whose output depends not only on the present value of its input signals but on the past history of its inputs.
  • Sequential circuits must have a storage element in order to remember their previous inputs.
  • The storage element can be found in flip-flop where the state of this element is a function of the previous inputs to the circuit.
  • in other words, sequential logic is combinational logic with memory.
  • The memory is in a present state and will advance to a next state on a clock pulse at any given time as determined by conditions.
Memory Elements And Flip Flops

  • The fundamental circuit is the RS (Reset-Memory) element. 
  • The JK flip-flop has an RS flip-flop at its core.
  • It adds circuitry that synchronizes output transitions to a clock signal.

Flip flop

Synchronous & Asynchronous Inputs
  • Synchronous inputs are those whose effect on the flip-flop output is synchronized with the clock input.
  • Asynchronous inputs are those that operate independently of the synchronous inputs and the input clock signal.
  • These are in fact override inputs as their status overrides the statusof all synchronous inputs and also the clock input.
  • They force the flip-flop output to go to a predefined state irrespective of the logic status of the synchronous inputs.
  • When active, the PRESET and CLEAR inputs place the flip-flop Q output in the ‘1’ and ‘0’ state respectively.
  • When it is desired that the flip-flop functions as per the status of its synchronous inputs, the asynchronous inputs are kept in their inactive state.
JK Flip Flop
  • The J-K flip-flop is the most versatile of the basic flip-flop.
  • It has the input- following character of the clocked D flip-flop but has two inputs,traditionally labeled J and K. If J and K are different then the output Q takes the value of J at the next clock edge.

  • If J and K are both low then no change occurs.
  • If J and K are both high at the clock edge then the output will toggle from one state to the other. 
  • It can perform the functions of the set/reset flip-flop and has the advantage that there are no ambiguous states.

  • Above is the simplified version of the versatile JK Flip-Flop. Note that the outputs feed back to the enabling NAND gates. This is what gives the toggling action when J=K=1.

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Nor Aini binti Ramlan

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