Introduction To Digital Electronics:
Digital electronics is a sub-field of electronics engineering. Digital electronics 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.
Key Questions:
An overview of digital systems
Define logic levels
Define digital waveforms
Logic families
Building blocks of digital circuits
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
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.
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| Logic levels in digital circuits |
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: in a nonideal waveform the transition 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: the waveform that doesn't represent itself after fixed intervals and may have pulses with different pulse widths.
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| Periodic and nonperiodic waveforms |
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/waveform transition from high to low.
Leading edge: waveform/ signal edge that occurs first that 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.
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| Positive going pulse and negative going pulse |
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
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| Ideal and nonideal pulses |
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. Logic gates 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):
It 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):
It 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.
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