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Digital Electronics : Sequential Circuits Part 1 This tutorial introduces Sequential Circuits and Flip-Flops. Here's a quick outline of the topics which will be covered :

A sequential circuit is a combinational circuit with some feedback from the outputs. In a sequential circuit, the output state depends on both the inputs and the outputs. The term “sequential” comes from the fact that the output depends not only on the current states, but on the states immediately preceding. A flip-flop is usually shown as a rectangle with two or more inputs and two outputs.

### FLIP-FLOP

The most important and basic type of a sequential circuit is a FLIP-FLOP.

There are 4 kinds of Flip-Flops namely—

• RS flip-flop
• D flip-flop
• T flip-flop
• JK flip-flop

### RS flip-flop

The inputs of an R-S flip-flop are labeled R (reset) and S (set) and the outputs are labeled as Q and Q’.

### D flip-flop

A D flip-flop has only one input (D) and two outputs (Q and Q’). In a D flip-flop, the output Q sets its value similar to the value of the input D.

### T flip-flop

A T flip-flop has only one input (T) and two outputs (Q and Q’). In a T flip-flop, the output Q toggles its value depending on the value of the input T.

### J-K Flip-Flop

The J-K flip-flop is the most versatile of the basic flip-flops. It 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.

### Counters / Timers

A timer or counter is a series of bistables or flip-flops that change state once for every input signal, thus one of these circuits would divide the input frequency by a factor of two. If this signal is then fed into the next bistable, the output is 1/4 of the original frequency. The next circuit would have an output of 1/8, then 1/16 and so on. Counters exploit the toggling behavior of JK flip-flops which is observed when J and K are set to 1. In counters, the JK flip-flops are arranged one after the other feeding at the output of the preceding flip-flops. Counters can be either Asynchronous or Synchronous.

#### An Example of Asynchronous Counters : Ripple Counters

In a ripple counter, the flip-flop output transition serves as a source for triggering other flip-flops. In other words, the CP inputs of all flip-flops (except the first) are triggered not by the incoming pulses, but rather by the transition that occurs in other flip-flops.

#### An Example of Synchronous Counters : Binary Counter

In a synchronous counter, the input pulses are applied to CP inputs of all flip-flops. The change of state of a particular flip-flop is dependent on the present state of other flip-flops. The common pulse triggers all the flip-flops simultaneously, rather than one at a time in succession as in a ripple counter. The decision whether a flip-flop is to be complemented or not is determined from the values of the J and K inputs at the
time of the pulse. If J = K = 0, the flip-flop remains unchanged. If J = K = 1, the flip-flop complements.
The Binary Counter - In a synchronous binary counter, the flip-flop in the lowest-order position is complemented with every pulse. This means that its J and K inputs must be maintained at logic-1. A flip-flop in any other position is complemented with a pulse provided all the bits in the lower-order positions are equal to 1, because the lower-order bits (when all 1s) will change to Os on the next count pulse. The binary count dictates that the next higher-order bit be complemented.

These counters have been covered in a reasonably detailed manner in this tutorial document

## Complete Tutorial document with truth-tables, examples and examples of circuits :

### Here's a list of all the tutorials we currently have in this area - Introductory Digital Electronic Circuits and Boolean logic

 Introduction to the Number System : Part 1 Introducing number systems. Representation of numbers in Decimal, Binary,Octal and Hexadecimal forms. Conversion from one form to the other. Number System : Part 2 Binary addition, subtraction and multiplication. Booth's multiplication algorithm. Unsigned and signed numbers. Introduction to Boolean Algebra : Part 1 Binary logic: True and false. Logical operators like OR, NOT, AND. Constructing truth tables. Basic postulates of Boolean Algebra. Logical addition, multiplication and complement rules. Principles of duality.  Basic theorems of boolean algebra: idempotence, involution, complementary, commutative, associative, distributive and absorption laws. Boolean Algebra : Part 2De-morgan's laws. Logic gates. 2 input and 3 input gates. XOR, XNOR gates. Universality of NAND and NOR gates. Realization of Boolean expressions using NAND and NOR. Replacing gates in a boolean circuit with NAND and NOR. Understanding Karnaugh Maps : Part 1 Introducing Karnaugh Maps. Min-terms and Max-terms. Canonical expressions. Sum of products and product of sums forms. Shorthand notations. Expanding expressions in SOP and POS Forms ( Sum of products and Product of sums ). Minimizing boolean expressions via Algebraic methods or map based reduction techniques. Pair, quad and octet in the context of Karnaugh Maps. Karnaugh Maps : Part 2Map rolling. Overlapping and redundant groups. Examples of reducing expressions via K-Map techniques. Introduction to Combinational Circuits : Part 1Combinational circuits: for which logic is entirely dependent of inputs and nothing else. Introduction to Multiplexers, De-multiplexers, encoders and decoders.Memories: RAM and ROM.  Different kinds of ROM - Masked ROM, programmable ROM. Combinational Circuits : Part 2 Static and Dynamic RAM, Memory organization. Introduction to Sequential Circuits : Part 1 Introduction to Sequential circuits. Different kinds of Flip Flops. RS, D, T, JK. Structure of flip flops. Switching example. Counters and Timers. Ripple and Synchronous Counters. Sequential Circuits : Part 2ADC or DAC Converters and conversion processes. Flash Converters, ramp generators. Successive approximation and quantization errors.