Digital Electronics: Sequential Logic Circuits Study Notes

By Yash Bansal|Updated : September 9th, 2021

 In this article, you will find the Study Notes on Digital Electronics: Sequential Logic Circuits Study Notes which will cover the topics such as Designing Digital Electronics: Sequential Logic Circuits 



















































1. Introduction

Description: seq_block

The sequential circuit is of two types.

  • Synchronous Sequential Circuit: Change in input signals can affect memory elements only upon activation of clock signals.
  • Asynchronous Sequential Circuit: Change in input signals can affect memory elements at any instant of time. These are faster than the synchronous circuit.

2. Flip Flops

  • It is a one-bit memory cell which stores the 1-bit logical data (logic 0 or logic 1).
  • It is a basic memory element.
  • The most commonly used application of flip flops is in the implementation of a feedback circuit.
  • As a memory relies on the feedback concept, flip flops can be used to design it.
  • In the synchronous sequential circuit, Memory elements are clocked flip flops and generally edge triggered.
  • In the asynchronous sequential circuit, Memory elements are unclocked flip flops/time delay elements which are generally level triggered.
  • Flip flop circuit is also known as bistable multivibrator or latch because it has two stable states (1 state, 0 states).

For the electronic circuits, there are mainly four types of flip flops present.

  • S-R Flip Flop (Basic Flip Flop)
  • Delay Flip Flop (D Flip Flop)
  • J-K Flip Flop
  • T Flip Flop


Basic SR Flip Flop

  • The Set-Reset (SR) flip flop is designed with the help of two NOR gates or two NAND gates.
  • SR Flip Flop is also called as SR latch.

SR Latch Implementation Using NAND Gates:

Logic diagram of SR latch using NAND gates

Truth Table of Logic Diagram

SR Latch Using NOR Gates:

Logic diagram of SR latch using NOR gates

Truth Table of Logic Diagram

Clocked SR Flip Flop Implementation using NAND Gates:

It is also called a Gated S-R flip flop. The main problem with S-R flip flops is using NOR and NAND gate in the invalid state. By using a bistable SR flip-flop this problem can be overcome. This can change outputs when certain invalid states are met, regardless of the condition of either the Set or the Reset inputs.

  • SR Flip Flop Using NOR Gates:

SR flip flop using NOR gates

Truth Table of SR Flip Flop

With both S=1 and R=1, the occurrence of a clock pulse causes both outputs to momentarily go to 0. When the pulse is disabled (removed), the state of the flip-flop become indeterminate, depending on whether the set or reset input of the flip-flop remains at 1 longer than the transition to 0 at the end of the pulse.

Characteristic Table

Characteristic equation of SR flip flop

Excitation Table

JK Flip Flop

A JK flip-flop eliminates the indeterminate state of the SR type. Inputs J and K is similar to the inputs S and R to set and clear the flip-flop (In JK flip-flop, the letter J is set and the letter K is for clear). When logic 1 are applied to both J and K inputs simultaneously, the flip-flop switches to its complement state. If Q=1 then it switches to Q=0 and vice versa.

  • JK flip flop using SR flip flop:

S = JQ'

R = KQ

  • JK flip flop using NAND latch:


  • JK flip flop using NOR latch:


Characteristic Table

Excitation Table

  • Characteristic equation for JK flip flop:

D-Flip Flop

D flip flop is also known as a Transparent latch, Delay flip flop or data flip flop. The D input goes directly to the S (J) input and the complement of the D input goes to the R (K) input. 

  • The D-flipflop has only a single data input (D).
  • If D = 1, the flip-flop is switched to the set state (unless it was already set).
  • If D = 0, the flip-flop switches to the clear state.


Truth Table

Characteristic Table

Excitation Table

  • Characteristic equation for D-flop flop

Qn + 1 = D

T – Flip Flop

  • The T flip-flop is a single input version of the JK flip-flop where T is connected to both J and K inputs directly.
  • When T = 0, the flip flop enters into Hold mode, which means that the output, Q is kept the same as it was before the clock edge.
  • When T = 1, the flip flop enters into Toggle mode, which means the output Q is negated after the clock edge, compared to the value before the clock edge.

Truth Table

Characteristic Table

Excitation Table

  • The characteristic equation of T-Flip Flop:

Qn + 1 = T ⊕ Qn

  • Race Around Condition:
    • The race around condition will occur in JK flip flop when J = = 1 and tpd (FF) < tpw.
    • To avoid race around condition.

tpw < tpd (FF) < TCLK

3. Master Slave (MS) Flip Flop

  • A master-slave flip-flop is constructed from two separate flip-flops. One circuit serves as a master and the other as a slave. Input clock is applied to master and Inverted clock applied to slave.

  • In Master Slave, flip flop output is changed only when slave output is changing.
  • The master flip-flop is enabled on the positive edge of the clock pulse and the slave flip-flop is disabled by the inverter.
  • The information at the external J and K inputs is transmitted to the master flip-flop.
  • When the pulse returns to 0, the master flip-flop is disabled and the slave flip-flop is enabled. The slave flip-flop then goes to the same state as the master flip-flop.
  • Master is level triggered, and Slave is edge triggered
  • No race around condition occurs in Master Slave flip flop.
  • It stores only one bit.

4. Flip Flop Conversions

The flip flop conversions are classified into different types which are:

  • SR-FF to JK-FF Conversion
  • JK-FF to SR-FF Conversion
  • SR-FF to D-FF Conversion
  • D-FF to SR-FF Conversion
  • JK-FF to T-FF Conversion
  • JK-FF to D-FF Conversion
  • D-FF to JK-FF Conversion

Procedure for Flip Flop conversion:

  1. Conversion Table: Construct the characteristic table of required flip flop (unknown), and fill available (known) flip flop excitation.
  2. Solve K map for given (known) flip flop as input and required flip flop as output.
  3. Implement the required flip flop using the known flip flop.

Example: Conversion from JK flip flop to D flip flop is shown below.

Characteristic Table


5. Registers

When a group of the flip flop is used to store a word ( a group of bits) then it is called register. To store bits, flip flops are cascaded in the register. If in a register, the binary information can be moved from stage to stage, this type of registers is called shift registers. According to data movement in a register, shift registers can be classified as

  • Serial Input Serial Output (SISO)
  • Serial Input Parallel Output (SIPO)
  • Parallel Input Serial Output (PISO)
  • Parallel Input Parallel Output (PIPO)

Serial Input Serial Output (SISO)

  • In registers edge trigger circuit used to make circuit synchronous.
  • If no clock is applied then get same data which is stored.
  • In N bits SISO registers to provide N bits data, Serially in require N clock pulse, and Serially out require (N-1) clock pulse.

Serial Input Parallel Output (SIPO)

  • To provide N-bit data: Serial in requires N clock pulse, and Parallel out requires no clock pulse.
  • SIPO can provide n × tCIK delay to the input.
  • SIPO can convert serial data or temporal code to parallel or serial code.

Parallel Input Serial Output (PISO)

  • If control = 0 then it acts as parallel input;
  • If control = 1 then it acts as serial output;
  • To provide parallel input, one clock pulse is required.
  • To provide N bits serial output, it requires (N-1) clock pulse.
  • PISO can convert special code to temporal code.

Parallel Input Parallel Output (PIPO)

  • In PIPO register for parallel input number of pulse required is 1 clock pulse.
  • In PIPO register for parallel output number of pulse required is 0 clock pulse.
  • PIPO register cannot be used as a shift register.
  • It is used for temporal storage of data in microcontroller, DSP, CPU etc.

Summary of Registers


6. Counter

  • A counter is a sequential logic circuit capable of counting the number of clock pulses arriving at its clock input.
  • The sequence of count may be ascending, descending or non-sequence.
  • For a counter circuit having flip flops, Maximum possible states (N) = 2n
  • If N = 2n , the counter acts as a binary counter.
  • If  N < 2 , the counter the non-binary counter.
  • It counter is capable to count from 0 to 2n-1.
  • MOD number is the Number of states present in a counter is known as modulus count or MOD number.
  • For n-flip flops, the counter will have 2n different states then this counter is said MOD- 2n counter.

MOD-N Counter

  • MOD number indicates frequency division obtained from the last flip flops.

  • Cascaded two counters:

  • MOD-MN counter:
    • Overall states of combined counter = MN
    • Input frequency = f
    • Output frequency f = f/(MN)

7. Classification of Counters

Based upon the applying clock pulse, counters are classified into two categories.

  • Synchronous counter
  • Asynchronous counter (ripple counter)

8. Toggle Mode Circuit

These are frequency dividers circuit.

Other Toggle Mode Circuit

9. Asynchronous Counter (Ripple counter)

  • A different clock pulse is applied to different flip flops.
  • All flip flops are operating in toggle mode.
  • In asynchronous counter flip flop applied with external clock acts as LSB bit.

3-bit Ripple Up Counter

  • Input clock is applied at LSB bit.
  • It n-bit ripple counter maximum possible states are 2n.
  • Bit ripple up counter counts from 0 to 2n - 1.
  • If all states are used then with input frequency f, then output frequency will be f/2n
  • Calculation of Time Period of Flip Flop: In n-bit ripple counter if propagation delay of each flip flop is tpd(FF), then the time period of the clock is:

  • Maximum Clock Frequency:

  • Due to propagation delays of flip flops decoding errors are present.
  • Clear and preset are known as asynchronous input to flip flop.
  • In any ripple counter, the following conditions will fulfil
    • Negative edge trigger and Q as clock ⇒ up counter
    • Positive edge trigger and Q as clock ⇒ up counter

3-bit Ripple Down Counter

  • Positive edge trigger and Q as clock ⇒ down counter
  • Negative edge trigger and Q as clock ⇒ down counter

Non-binary Ripple Counter

Decode counter or BCD counter is an example of a non-binary counter. It requires 4 flip flops.

  • Used state = 10 and unused states = 6 → (24 -10)
  • Output frequency of BCD counter = f/10
  • For making non-binary counter clear (clr) signal is used.
  • c1r is active high, and (clr)' is active low.

10. Synchronous Counters

In this type of counter, there are no connections of the first flip flop output to a clock input of the next flip flop.

Ring Counter: It is a circular shift register with only flip flop being set at any particular time, all others are cleared. It is a shift register with feedback.

  • In-ring counter, if the feedback is used the number of states is reduced.
  • With n flip flops maximum states = n.
  • Number of unused states in-ring counter = 2n – n
  • Maximum Clock Frequency: If the input frequency is fthen at the output of every flip flop we get f/N frequency. In-ring counter, if the propagation delay of each flip flop is tpd(FF) then

Johonson Ring Counter: Jhonson ring counter is also called as a Twisted ring counter, Switch tail counter, Creeping counter, or Mobies counter.

  • In n - bit Jhonson counter maximum used states = 2n, unused states = 2n - 2n.
  • If the input clock frequency is f, the output frequency of each flip flop is f /2n and the duty cycle is 50%.
  • A disadvantage of Jhonson Ring Counter: Lockout may occur. To decode each state one, two-input AND or NOR gate is used.


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Yash BansalYash BansalMember since Nov 2018
Content Manager (GATE)
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