Digital Electronics Basic Knowledge

vvLogic gates: A logic circuit the outputs depends upon the inputs according to some logic rules.

Types of logic gates:

ð  Basic gates/Primary/Fundamental (AND, OR, NOT, BUFFER)

ð  Compound gates (NAND, NOR, XOR, XNOR)

vvUniversal Gate: A gate that can perform all the basic logical operation is called Universal Gate. NAND, NOR gates are Universal Gate.

¯¯ Middle of 19^th century, an English mathematician George Boole developed rules for manipulations of binary variables, known as Boolean algebra.ion.

vvSign-Magnitude Representation: Digital circuit understands only two symbol 0 and 1.Thus those are used to indicate the sign of a number. Normally an additional bit is used as the sign bit and it is placed at the most significant bit (MSB) location. A 0 is used to a positive number and A 1 is used to a negative number. This kind of representation is called a Sign –Magnitude –Representation.

vvBipolar Junction transistor:  A Bipolar Junction transistor (BJT) consists of a silicon crystal in which either thin layer of p- type silicon is sandwiched between two layers of n- type silicon or a layer of n- type is sandwiched between two layers of n- type silicon.

vvTransistor Configurations: A transistor can be used in any one of the three configurations,

ð  The common base (CB)

ð  The common-emitter (CE)

ð  The common-collector (CC) configurations

Field-Effect Transistor: The field-effect transistor is another three-terminal semiconductor device which can be used as a last operating switch.

¯¯ Metal-Oxide-Semiconductor Field Effect Transistor (MOSFET).

There are two types of MOSFET,

ð  Enhancement MOSFET

ð  Depletion MOSFET.

vvFET Switches: The JFET and MOSFET devices can be used as switches. These devices are unipolar, i.e the current flows due to the majority charge caners only and are voltage controlled devices.

Semiconductor Devices: There are two types of semiconductor devices

ð  Bipolar and

ð  Unipolar

Bipolar: There are two types of operations in Bipolar ICs,

ð  Saturated and

ð  Non-Saturated.

In saturated Logic, the transistors in the IC are driven to saturation, where as in the case of non-saturated                        logic, the transistors are not driven into saturation.

Saturated Bipolar Logic Families are:

ð  Resistor-Transistor Logic (RTL).

ð  Direct-Coupled Transistor Logic (DCTL).

ð  Integrated-Injection Logic (I²L).

ð  Diode- Transistor Logic (DTL).

ð   High-Threshold Logic (HTL) and

ð   Transistor-Transistor Logic (T²L).

vvNon-Saturated Bipolar Logic Families are:

ð  Emitter-Coupled Logic (ECL).

vvUnipolar Logic Families: The MOS devices are unipolar devices and only MOSFET’s are employed in MOS logic circuits. The MOS logic Families are;

ð  PMOS,

ð  NMOS and

ð  CMOS (Complementary Metal-Oxide-Semiconductor).

vvDiode- Transistor Logic (DTL): The Diode- Transistor Logic (DTL) is somewhat more complex than RTL but because of its greater fan-out and improved noise margins it has replaced RTL. Its main disadvantage is slower speed.

¯¯ Programmable Logic Devices (PLD).

¯¯Field-Programmable Gate Arrays (FRGA’s).

¯¯Computer Aided Design (CAD).

Digital Circuit’s are divided into two;

ð  Combinational Circuit’s

ð  Sequential Circuits.

vvCombinational Circuits: In Combinational Circuit the outputs at the any instant of time depends upon the inputs presents at that instant of time. This means there is no memory in these circuits.

 vvSequential Circuits: In Sequential Circuits the outputs at the any instant of time depends upon the inputs presents at that instant of time. As well as previous input/outputs. This means that there are some element  which are used to store the fast information.

The following method can be used to simplify the Boolean function,

ð  Algebraic Method

ð  Karnaugh-map technique.

ð  Quine-McCluskey method  and

ð  Variable Entered Mapping (VEM) technique.

¯¯Sum of product (SOP)

¯¯Product of sum (POS)

    Product of sum (POS): A logic expression in the form ORed terms ANDed together.

    Product of sum (POS): A logic expression in the form ANDed terms ORed together.

   Standard POS A POS form of logic expression consisting of only maxterms.

   Standard SOP ASOP form of logic expression consisting of only minterms.

Half adder: A logic circuit for the addition of two one bit numbers is referred to as a Half Adder.

Half Subtractor: A logic circuit for the subtraction of two one bit numbers is referred to as a Half subtractor.

Full adder: A logic circuit for the addition of two one bit numbers and the carry from the previous stage is referred to as full adder.

vvFull Subtractor: A logic circuit for the subtraction of two one bit numbers and the borrow from the previous stage is referred to as full subtractor.

vvMultiplexer (MUX): The multiplexer is a logic circuit that gates one out of several inputs to a single output. The input selected is controlled by a set of select inputs. For selecting one out of n inputs for connection to output a set of m select inputs is required where 2^m=n

Types of Multiplexer:

ð  2:1

ð  4:1

ð  8:1

ð  16:1

Advantage of MUX:

ð  Simplification of logic expression is not required.

ð  It minimizes the IC package count.

ð  Logic design is simplified.

vvD-Multiplexer: The D-Multiplexer performs the reverse operation of multiplexer. It accepts the single input and distributes it over several outputs. The select input code determines to which output the data is to be transferred or transmitted. The number of output line is an m where n=2^m

Memory Devices:

ð  Flip-flop

ð  Register and

ð  Counter.

vvFlip-flop: A Flip-flop is binary cells capable of storing one bit of information.

Types of Flip-flop:

ð  RS latch

ð  RS flip-flop

ð  D Flip-flop

ð  JK Flip-flop

ð  T Flip-flop

ð  Master slave Flip-flop

ð  Triggering of Flip-flop

ð  Edge- Triggered Flip-flop

Application of Flip-flops: Some of the common uses of Flip-flop are as,

ð  Bounce elimination switch

ð  Latch

ð  Registers

ð  Counters

ð  Memory

¯¯Clock Cycle: The interval between successive positive or negative transitions in a clock.

¯¯D Flip-flop: A flip-flop whose output follows the input when triggered.

¯¯Edge- Triggered Flip-flop: A flip-flop whose state changes on the string (positive) or falling (negative) edge of a clock pulse.

vvRegister: A register is a group of binary storage cells suitable for holding binary information. A group of flip-flop constitutes a register since each flip-flop is a binary cell capable of storage one-bit of information. An n bit register has a group of n flip flops.

Types of register:

ð  Shift Register

ð  Serial register and

ð  Parallel Register (MAR, MBR, I/O, ACC)

vvMemory Unit (MU): A memory Unit is a collection o storage cells together with associated circuits needed to transfer information in and out of storage. Example: RAM, ROM

vvCounter: A sequential circuit that goes through a prescribed sequence of a states upon the application of inputs pulses is Called Counter.

Types of Counter:

ð  Binary Counter

ð  BCD Counter

ð  Up counter

ð  Down Counter

ð  Up-Down counter

ð  Ripple Counter

ð  Johnson Counter and

ð  Synchronous counter.

Registers are classified depending upon the way in which data are entered and retrieved. There are four possible modes of operation;

ð  Serial-in, serial out (SISO)

ð  Serial-in, parallel out (SIPO)

ð  Parallel-in, serial out (PISO)

ð  Parallel-in, parallel out (PIPO)

vvAsynchronous Sequential Machine Modes: There are two modes of operation of Asynchronous Sequential Machine depending upon the type of input signals. These are;

ð  Fundamental Mode

ð  Pulse Mode

vvFundamental Mode: Fundamental Mode operation assumes that the input signals will be changed only when the circuits are in a stable state and that only one variable can change at a given time.

Fundamental Mode circuits are of two types;

ð  Circuits without latches

ð  Circuits with latches.

There are three types of Multivibrators. These are;

ð  Astable (or free-running) multivibrator

ð  Monostable multivibrator (for one shot), and

ð  Bistable multivibrator (or flip-flop)

vvAstable (or free-running) multivibrator: A multivibrator circuit having both the states as quasi-stable states. It is a square wave oscillator. Also known as free-running multivibrator.

vvBistable multivibrator (or flip-flop): A multivibrator circuit that has both the states as stable. It goes from one stable to another when triggered. It is same as the FLIP-FLOP.

vvSpecifications for D/Converters: The characteristics of a D/A converter, which are generally specified by the manufacturers are,

ð  Resolution

ð  Linearity

ð  Accuracy

ð  Settling time and

ð  Temperature sensitivity.

¯¯Dual-slope A/D Converter has four major blocks,

ð  An integrator

ð  A comparator

ð  A binary counter and

ð  A switch driver.

vvSpecifications for D/Converters: The following specifications are usually specified by the manufacturers of A/D converters,

ð  Range of input voltage,

ð  Input impedance

ð  Accuracy,

ð  Conversion time, and

ð  Format of digital output.

¯¯Complementary Binary Input (CBI)

¯¯Complementary Coded decimal (CCD)

¯¯Complementary Offset Binary Code (COB)