Table Of Contents |

Overview of digital systems |

Logic levels |

Digital waveforms |

Logic families |

Building blocks of digital circuits |

Digital electronics is a sub-field of electronics engineering. These circuits use digital signals instead of analog signals, and use binary numbers 1 and 0 for representing ON/OFF states respectively. Boolean Algebra is the basis of digital electronics. Today digital technology is not confined to computer systems. It is applied to a wide range of daily use appliances like such as TV, radar, medical instrumentation, military systems, and other consumer electronics.

## Digital Systems Overview:

The two binary digits are used to represent anything in digital systems like numbers, letters, symbols, instructions, logic, etc

Most systems use 1 as a high-level voltage and 0 as a low-level or ground voltage. This is called positive logic.

High=1, Low =0

Negative logic is less common in which systems use 1 as low-level or ground-level voltage whereas 0 represents high-level voltage.

High=0, Low=1

click here to know number system

## Logic Levels In Digital Electronics

The binary 1 & 0 are specified by some voltage level. In a practical circuit, a HIGH (1) is voltage in between a specified minimum and specified maximum voltage range. There is a high voltage range for CMOS this range is between 3.5V-5V. Similarly, a LOW (0) is not the ground level or 0V. A LOW is a voltage in between a specified minimum voltage range and a specified maximum voltage range. For CMOS low voltage range is between 0V to 1.5 V.

## Digital Waveforms:

There are many kinds of waveforms, but if we are working with digital electronics we always deal with digital waveforms, that switch between only two logic levels 0 & 1.

It represents the two states of Boolean logic (high or low, true or false). At this point, we have to discuss some important parameters and concepts related to waveforms. I also tried to give pictorial representation as well where is possible.

### Ideal waveforms:

in an ideal waveform the transition from low to high or high to low is instantaneous.

### Non-Ideal waveforms:

The waveform in which transition occur from low to high or high to low takes some time due to stray capacitance and inductance.

### Periodic waveforms:

** **The waveform which repeats itself after a fixed interval (termed as Period T). And have an equal pulse width (tw)

### Non-Periodic waveforms:

Waveform that doesn’t represent itself after fixed intervals and may have pulses with different pulse widths.

#### Pulse train:

Digital signals are in form of series that are repeated and called pulse trains.

#### Rising edge:

Signal/waveform transition from low to high.

#### Falling edge:

signal or waveform transition from high to low.

#### Leading edge:

The waveform or signal edge that occurs first is at t = 0.

#### Trailing edge:

Waveform/signal edge that occurs at t = 1.

#### Positive going pulse:

The pulse that goes from LOW logic level (0) to HIGH logic level (1) or leading edge is a rising edge and the trailing edge is a falling edge.

#### Negative going pulse:

** **The pulse that goes from HIGH logic level (1) to LOW logic level (0) or leading edge is the falling edge and the trailing edge is the rising edge.

#### Rise time (tr):

The time required for a waveform to rise from 10% of its amplitude to 90% of its amplitude.

#### Fall time (tf):

The time required for a waveform to fall from 90% of its amplitude to 10% of its amplitude.

#### Pulse width (tw):

Duration of a pulse, measured in between 50% of rise time to 50% fall time

#### Amplitude:

The height of the waveform or the intensity of the waveform.

#### Period:

Time in which waveform repeats itself. T=1/f

#### Frequency**:**

How many times a waveform repeats itself within one second f=1/T

#### Duty cycle:

Ratio of pulse width (tw) to the period (T) and expressed in percentage

D.C = (tw/T) %

## Logic Families:

Logic families include bipolar and metal oxide semiconductors.

### Bipolar ICs include

- DTL (Diode Transistor Logic)
- TTL (Transistor Transistor Logic)
- ECL (Emitter Coupled Logic)
- IIL (Integrated Injection Logic)

### Metal Oxide semiconductors (MOS) ICs include

- CMOS (Complementary MOS)

- NMOS (N-channel MOS)

- PMOS (P-channel MOS)

- QMOS (Quick MOS)

## Building Blocks of Digital Electronics:

### Logic gates:

Logic gates perform basic logic operations on one or more binary inputs and produce a single binary output. They implement Boolean logic. They are fundamental building blocks of digital Integrated circuits. There are three types of logic gates: AND, OR, NOT. The other logic gates are derived from primary gates. Like NOR, NAND, XOR, and XNOR.

### Latches and Flip-flops:

A latch is a simple memory element that can hold a bit as long as power is supplied. It has two stable states (as in a bistable multivibrator) that can use to store information. They are asynchronous and work on clock levels.

Latches and flip-flops are basic building blocks of sequential circuits.

Flip-flops are similar to latches. But they are synchronous and work on clock edges.

The output of both latches and flip-flops depends not only on current input but also on previous inputs and outputs.

### Timers:

Timers are used in industries or used in other appliances to control process, and automatically starts/stops appliances after a predetermined interval of time. Quartz timers are more accurate than others.

### Counters:

As the name implies counter is for counting electronic events, such as pulses. It usually consists of several flip-flops. There are synchronous and asynchronous counters.

### Encoders:

An encoder is a device that converts information from one format to another. In digital electronics, it is a combinational circuit that converts information into a coded form, such as binary or BCD. It has 2n input lines and n-bit output lines.

### Decoders:

It has the opposite functionality as the encoder. It converts information from n-bit coded input lines and produces 2n unique outputs.

### Multiplexers (Mux):

Multiplexers is also a combinational circuit and has many input lines and a single output. It is used to select one input from many and feed it to the output. It has 2n input lines and n select lines. And only one output line.

### Demultiplexer (Demux):

Demultiplexer is the exact opposite of multiplexers. It is also called a data distributor. It has a single input. 2n output and n select lines. Select lines decide which output line will be available for incoming input. Input is routed to the selected output lines.

### Adders:

A digital circuit that performs addition. There are two types of adder circuits. Half adder and full adder.