Digital Circuit Design

[Bartie Spalitto]

Basicallycircuit design interest me a lot. I like the idea of working with AND OR NOT etc gates and building things with them. I've been wanting to build a CPU for quite a few years now, but I lack the knowledge. I'm fairly decent at programming however, so I can think "logically", but with circuit design it all is very difficult for me to understand past simple adders and such.

So, I'm looking for a beginner's book on the subject. I plan on doing all design and testing in a simulator such as Logisim, but being shown how to actually put circuits together on a breadboard out of gates(or even transistors and such) would be a definite plus, but I wouldn't want for that to be the focus of it all.

I'm not familiar with the content of the 2009 version (having taken it back when it was breadboards & modules) but later used the 2004? OCW version (java iirc simulation) as a reference, then implemented that processor architecture in verilog and subsequently in an FPGA kit.

Take a look at . I consider this the "Arduino" of FPGAs. They have a whole system that includes an inexpensive development board, easy to use software, and tutorials for Verilogue and even a new language they are developing called Lucid. It is really a great resource. I found it easy to use and would recommend it for anyone wanting to try digital circuit design!

There are many Logic families that follow the principle of Digital Signals. Examples of such Logic families consider 3.5V to 5V voltage as high logic and 0V to 1V as low logic. This means voltage lying anywhere between 3.5V to 5V would be represented by 1 and voltage lying anywhere between 0V to 1V would be represented by 0. The actual value of voltage is not important in digital signals.

Representation for a range of voltages as 1 or 0, makes digital circuit operation simpler than analog or fuzzy circuits. Operating only in two states, either high or low, makes these signals fast, and less susceptible to noise, temperature, and irrespective of the aging components.

Digital Circuits are designed using logic gates, diodes, transistors, inductors, capacitors, and resistors. As Digital Circuits follow Boolean Laws, the logic expressions should be simplified for a small circuit.

The inputs of the first half adder are two single binary digits A and B. The output of the first half adder sum S is fed to the input of the second half adder terminal 1 on K. The sum output of the second half adder is obtained across X.

The inputs of the first half subtractor are two single binary digits A and B. The output of the first half subtractor difference D is fed to the input of the second half subtractor terminal 1 on K. The difference output of the second half subtractor is obtained across X.

The output across the second half subtractor for Difference X is a direct XOR operation of D of the first half subtractor at input terminal 1 and the borrow bit P at input terminal 2 of the second half subtractor.

CircuitLab provides online, in-browser tools for schematic capture and circuit simulation. These tools allow students, hobbyists, and professional engineers to design and analyze analog and digital systems before ever building a prototype. Online schematic capture lets hobbyists easily share and discuss their designs, while online circuit simulation allows for quick design iteration and accelerated learning about electronics.

Digital electronics is a field of electronics involving the study of digital signals and the engineering of devices that use or produce them. This is in contrast to analog electronics which work primarily with analog signals. Despite the name, digital electronics designs includes important analog design considerations.

Digital electronic circuits are usually made from large assemblies of logic gates, often packaged in integrated circuits. Complex devices may have simple electronic representations of Boolean logic functions.[1]

The binary number system was refined by Gottfried Wilhelm Leibniz (published in 1705) and he also established that by using the binary system, the principles of arithmetic and logic could be joined. Digital logic as we know it was the brain-child of George Boole in the mid 19th century. In an 1886 letter, Charles Sanders Peirce described how logical operations could be carried out by electrical switching circuits.[2] Eventually, vacuum tubes replaced relays for logic operations. Lee De Forest's modification of the Fleming valve in 1907 could be used as an AND gate. Ludwig Wittgenstein introduced a version of the 16-row truth table as proposition 5.101 of Tractatus Logico-Philosophicus (1921). Walther Bothe, inventor of the coincidence circuit, shared the 1954 Nobel Prize in physics, for creating the first modern electronic AND gate in 1924.

Mechanical analog computers started appearing in the first century and were later used in the medieval era for astronomical calculations. In World War II, mechanical analog computers were used for specialized military applications such as calculating torpedo aiming. During this time the first electronic digital computers were developed, with the term digital being proposed by George Stibitz in 1942. Originally they were the size of a large room, consuming as much power as several hundred modern PCs.[3]

The Z3 was an electromechanical computer designed by Konrad Zuse. Finished in 1941, it was the world's first working programmable, fully automatic digital computer.[4] Its operation was facilitated by the invention of the vacuum tube in 1904 by John Ambrose Fleming.

At the same time that digital calculation replaced analog, purely electronic circuit elements soon replaced their mechanical and electromechanical equivalents. John Bardeen and Walter Brattain invented the point-contact transistor at Bell Labs in 1947, followed by William Shockley inventing the bipolar junction transistor at Bell Labs in 1948.[5][6]

At the University of Manchester, a team under the leadership of Tom Kilburn designed and built a machine using the newly developed transistors instead of vacuum tubes.[7] Their "transistorised computer", and the first in the world, was operational by 1953, and a second version was completed there in April 1955. From 1955 and onwards, transistors replaced vacuum tubes in computer designs, giving rise to the "second generation" of computers. Compared to vacuum tubes, transistors were smaller, more reliable, had indefinite lifespans, and required less power than vacuum tubes - thereby giving off less heat, and allowing much denser concentrations of circuits, up to tens of thousands in a relatively compact space.

While working at Texas Instruments in July 1958, Jack Kilby recorded his initial ideas concerning the integrated circuit (IC), then successfully demonstrated the first working integrated circuit on 12 September 1958.[8] Kilby's chip was made of germanium. The following year, Robert Noyce at Fairchild Semiconductor invented the silicon integrated circuit. The basis for Noyce's silicon IC was the planar process, developed in early 1959 by Jean Hoerni, who was in turn building on Mohamed Atalla's silicon surface passivation method developed in 1957.[9] This new technique, the integrated circuit, allowed for quick, low-cost fabrication of complex circuits by having a set of electronic circuits on one small plate ("chip") of semiconductor material, normally silicon.

In the early days of integrated circuits, each chip was limited to only a few transistors, and the low degree of integration meant the design process was relatively simple. Manufacturing yields were also quite low by today's standards. The wide adoption of the MOSFET transistor by the early 1970s led to the first large-scale integration (LSI) chips with more than 10,000 transistors on a single chip.[23] Following the wide adoption of CMOS, a type of MOSFET logic, by the 1980s, millions and then billions of MOSFETs could be placed on one chip as the technology progressed,[24] and good designs required thorough planning, giving rise to new design methods. The transistor count of devices and total production rose to unprecedented heights. The total amount of transistors produced until 2018 has been estimated to be 1.31022 (13 sextillion).[25]

An advantage of digital circuits when compared to analog circuits is that signals represented digitally can be transmitted without degradation caused by noise.[29] For example, a continuous audio signal transmitted as a sequence of 1s and 0s, can be reconstructed without error, provided the noise picked up in transmission is not enough to prevent identification of the 1s and 0s.

In a digital system, a more precise representation of a signal can be obtained by using more binary digits to represent it. While this requires more digital circuits to process the signals, each digit is handled by the same kind of hardware, resulting in an easily scalable system. In an analog system, additional resolution requires fundamental improvements in the linearity and noise characteristics of each step of the signal chain.

With computer-controlled digital systems, new functions can be added through software revision and no hardware changes are needed. Often this can be done outside of the factory by updating the product's software. This way, the product's design errors can be corrected even after the product is in a customer's hands.

Information storage can be easier in digital systems than in analog ones. The noise immunity of digital systems permits data to be stored and retrieved without degradation. In an analog system, noise from aging and wear degrade the information stored. In a digital system, as long as the total noise is below a certain level, the information can be recovered perfectly. Even when more significant noise is present, the use of redundancy permits the recovery of the original data provided too many errors do not occur.

In some cases, digital circuits use more energy than analog circuits to accomplish the same tasks, thus producing more heat which increases the complexity of the circuits such as the inclusion of heat sinks. In portable or battery-powered systems this can limit the use of digital systems. For example, battery-powered cellular phones often use a low-power analog front-end to amplify and tune the radio signals from the base station. However, a base station has grid power and can use power-hungry, but very flexible software radios. Such base stations can easily be reprogrammed to process the signals used in new cellular standards.

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