Virtual Lab - IIT Kharagpur

Computer Organisation and Architecture.

Design of Registers and Counters :

In a sequential circuit the present output is determined by both the present input and the past output. In order to receive the past output some kind of memory element can be used. The memory element commonly used in the sequential circuits are time-delay devices. The block diagram of the sequential circuit-

A circuit with flip-flops is considered a sequential circuit even in the absence of combinational logic. Circuits that include flip-flops are usually classified by the function they perform. Two such circuits are registers and counters:

  1. Register is a group of flip-flops. Its basic function is to hold information within a digital system so as to make it available to the logic units during the computing process.
  2. Counter is essentially a register that goes through a predetermined sequence of states.

There are various different kind of Flip-Flops. Some of the common flip-flops are: R-S Flip-Flop, D Flip-Flop, J-K Flip-Flop, T Flip-Flop. The block diagram of different flip-flops are shown here -


  • RS flipflop If R is high then reset state occurs and when S=1 set state.the both cannot be high simultaneouly. this input combination is avoided.
  • JK flipflop 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.

  • D flipflop The D flip-flop tracks the input, making transitions with match those of the input D. It is used as data store.
  • Tflipflop The T or "toggle" flip-flop changes its output on each clock edge,
  • Types of Registers:

    4-bit Serial-in Serial-out

    4-bit Serial-in Parallel-out

    4-bit Parallel-in Serial-out

    4-bit Parallel-in Parallel-out

    Types of Counters:

    4-bit Synchronous Binary Counter

    4-bit Synchronous Ring Counter

    4-bit Synchronous Johnson Counter

    Design Issues :

    The four different types of flip-flops are supplied here. One can easily build any register or counter using those flip-flop and different logic gates. But the clock input is under development, so it is not possible now to build any register or counter completely.