Up counter and Down counter

Mod 4 Up counter

Q0N=Q0'   for every clock 

Q1N  =Q1'   when Q0' :-->0-->1

                                   Q0 :-->1-->0

free running and self starting


Mod 4 Down counter

Q0N =Q0,for every clock


Free running and self starting

Introduction to Asynchronous Counter
  • The asynchronous counter are also called as ripple counter.
  • The basic flipflop is T and basic counting is binary up counting.The up counter are used for implementing incrementation.Due to the simpler design ,asynchronous counter are preferred in IC counter fabrication.
  • IC 7490 is a decade counter  i.e %10 counter
  • IC 7492 is hexadecimal counter .i.e %16 counter

we can constuct up counter ,down counter as well as random counter.


Self starting and free running

A counter is said to be self starting if it is possible to enter counting loops irrespective of the initial state and counter is said to be free running if it contains all possible states in the counting loop.







Relation between self starting and free running:

  1. Every free running counter is also self starting counter.
  2. Every self starting counter is need not be a running counter.
  3. If a counter is not self starting then definetly it is not free running.
Shift counters

Here data is shifted from one Flipflop to next flipflop so this is called Shift Counter.In this counter D flip flop is used.


MOD 2 ring counter


so using this ring counter we can get only mod N counter.i.e

If i have N flip flop in the ring counter then i always get mod N


MOD 4 Johnson counter

since D0=Q1' 


 all the state are involved in counting therefore we got a cycle or loop which is having all the state.So it is act as a mod4 counter.

If i have N Flipflop in twisterring (johnson )counter then i am able to form mod (2N)counter.


Introduction to Counters
  • The counters are used to provide accurate timing and control signals.
  • There are of two types :
    a) Synchronous
    b) Asynchronous
  • In synchronous counters all the flipflops respond to the same clock instances.
  • In asynchronous counters, the output of one flipflop drives the clock of another flipflop.
  • Synchronous counters are faster than asynchronous counters.
  • Due to simplicity of design ,asynchronous counters are used in IC fabrications.
  • The simplified version of synchronous counter  is called shift counters.
  • The basic element in shift counter is D flipflop. and for asynchronous T flip flop is the basic element.
  • The ring counter and johnson counter are further simplified version of shift counter.

Synchronous Counter :


Asynchronous Counter:



Conversion Of Given FlipFlop To Another FlipFlop

1)Get the characteristic table of the target Flipflop.

2)Replace the next state using excitation of the given flipflop.

3)Obtain the expression for the input of given filpflop and realise them.


Example: we have given JK flip flop and we have to convert it into T flip flop




First of all draw the charateristic table for T flip flop


Step 2:

Replace the next state using excitation of the given flipflop(JK).

Step 3:

Obtain the expression for the input of given filpflop and realise them.

now given JK flip flop can be converted with the obatined expression as:


D Flip Flop

It is also called as Delay flipflop.Here we take JK flip flop and join then with not gate between them and named as D.




Characteristic table :


Characteristic Equation:


Qn+1    =DQn' +DQn

            =D(Qn' + Qn)

Qn+1    =D


Exicitation Table:


T Flip Flop

It is also called as toggle flipflop. Here we use JK flip flop and join both the input and name the new input as T.

Charateristic table :

Characteristic equation:

Qn+1 =T'Qn +TQ'



Exciation table:

JK Flip flop

JK Flipflop is same as SR FlipFlop (J=S ,K=R).It is defined for J=K=1 also and the flip flop performs complementation.


Function table For JK Flip Flop

Characteristic Table :


Characteristic Equation:



Exicitation Table:

Clocked FlipFlop

In digital circuits the output of any circuit should not only depends on inputs of the circuit, in-addition to the inputs some sort of triggering mechanism should be provided for synchronized operation digital circuit which enables to provide a real time data at the output of the circuit.

Take a positive level triggered clock


Latch Mode:

If the clock is +ve level triggered then in other then +ve level  output is not depend on input.


Transparent Mode:

If the clock is +ve level triggered then only in this level output will be depend on input.

Based on the Triggred we can classified flip flop into four catogries.


For example if clock is +ve  level Triggered then

SR Flip Flop

To the latch we are providing input i.e

So next state of Q depend on S,R,Qn.

Characterstic table for SR Flip Flop

Characterstic equation

If Qn is already something and if you are trying to changing it into something else i.e Qn+1 then what are the various combination of inputs that can do for you called as excitation table.

Introduction to sequential circuits

The external output of a sequential circuit depends on external inputs and on present contents of "memory element". The present contents of the memory elements are called present state and the new contents of memory elements are obtained by taking external inputs and present state .This is called Next state.



  • A memory element is some medium in which one bit of information can be stored or retained untill necessary , and there after its contents can be replaced by new values.The basic binary or digital memory circuit is known as filpflop.
  • It has 2 stable states which are known as '1' or '0'.so it is also called bistable multivibrator.

Basic Concept used inside Flipflop:

We can use both NAND gate  as well as NOR  gate in implementation of flipflop.



This Device is called as latch or lock because it doesn't have input .It could be latch-0 or latch-1 but if it once there we could not change it.

                   If i want a memeory device which i should be latch it,which means i should able to store something there as well as change it,that is called flip flop.Using latch we can build a flip flop.