Digital electronics

Computers are much faster at capturing input data than humans. Our eyes can barely detect the flashing of a fluorescent light operating at 60 flashes per second. But modern computers are many millions of times faster. When we flip a simple switch, we hear only one "click", but a computer will "see" the internal switch contacts vibrating many times before the contacts come to rest. This difference between what we are trying to "tell" the computer and what the computer "hears" can cause serious computer processing problems. Your computer keyboard is a perfect example of this problem. When you type the letter "a", for example, you want to input one and only one "a" into the computer. A normal mechanical switch found under the letter "a" on your keyboard will send many "a's" to your computer before it comes to rest after you type it. To solve this problem, switches need to get rid of the extra vibrations every time they are used. This process is called switch "debouncing."

A counter will consider each bounce pulse separately and count them. Thus one up-down switch operation can produce a count of anywhere between 1 to perhaps 5 to 10 or more when connected to a flip-flop counter trigger input. Four different switch-debouncing circuits are tested, and you will note that they are equally effective in eliminating switch contact bounce.

Flip flop

A flip-flop circuit remembers whether something is supposed to be on or off. The circuit has two inputs and can keep its output stable, in either a high or a low state. Momentarily forcing one of the inputs low, while keeping the other input high, causes the flip-flop to output a steady high signal.

Peripheral devices

Advances in technology resulted in more than just chip manufacturers and computer companies. New industries evolved that produced peripheral devices. Examples of peripherals are: External hard drives to store information Ethernet routers to transmit information between computers Printers to produce legible hard copies Today, microprocessors are found in many everyday consumer products, such as TVs, digital cameras, cell phones, cash registers, cars and trucks, and musical instruments. Many new jobs, such as Network Technician and Systems Engineer, have been created to support these devices.

Bits and Bytes

Bits are usually grouped together into sets of 8 bits, called a byte. 8-bit bytes can handle 256 values. 16-bit bytes can handle 65,536 values. 32-bit bytes can handle 4,294,967,296 values. Early personal computers handled only 8-bit bytes.

Breakdown

Businesses and individuals depend on their computers to function reliably. When a computer breaks down, it must be repaired as quickly as possible. A Computer Repair Technician takes care of hardware problems.

Binary Coding

Computers process numbers much more easily than words. For this reason, coding consists of simple commands and binary numbers. For example, suppose you need to design a thermostat to turn on the air conditioner when the room temperature rises above 80 degrees.

Computers use binary

Computers use binary digits in place of decimal digits. Where decimals have ten possible values ranging from 0 to 9, bits have only two values: 0 or 1. This makes computer operations much easier to design.

Computational devices

Devices also were invented as aids for performing calculations. More than 2,000 years ago, ancient people manipulated beads on an abacus to perform basic arithmetical functions. The first calculator appeared about 300 years ago. Since it required precision gears, it couldn't be manufactured in large quantities. In the 1800s, however, the Industrial Revolution ushered in mass production. Eventually, mechanical calculators became widely available.

Digital-to-Analog

Digital circuits can change digital signals back into analog signals using a digital-to-analog converter. These devices are used in CD players. Digital circuits can also use logic to compare values and make decisions, calculate computations, or sequence events. Not long ago, only humans could do these tasks.

ready-made counter

Digital counters also can be purchased already made in an integrated circuit. Many can be preprogrammed to count either up or down. You can buy the counters to count in binary mode or in binary-coded decimal mode.

Versatility

Flip-flops are quite versatile. You can add a trigger or clock input to divide incoming clock pulses. If you combine these flip-flops in a series, the incoming clock pulses can be divided into any number you choose. Flip-flops used in this way can make digital counters.

One input high

However, if the other output is high, the clock pulses appear at the NAND gate's output. The red arrow shows the pulses. The green arrow shows the output.

Making a clock

If the one pulse per second that we just created were divided by 60 again, we would produce one pulse per minute. By dividing the one pulse per minute by 60, we could produce one pulse per hour. By counting and displaying the second, minute, and hour pulses, you would create a clock! This is the premise on which digital clocks are designed.

Changing lives

In the last 50 years, the lives of average people have changed dramatically due to rapid advances in technology.

pulses from the internal clock

In this way, the NAND gate controls the pulses from the internal clock.

input and output devices

Input and output devices move data into and out of memory. The computer terminal keyboard is the most common input device. Disc drives are also input devices. The most commonly used output device is the printer.

integrated circuits

Integrated circuits led to the creation of the first minicomputers. These computers were small enough to install almost anywhere, including in the average home. Minicomputers paved the way for the personal computer. Advances in the design and construction of integrated circuits allowed more components to be placed on a single chip.

Microprocessors

Intel Corporation invented the first microprocessor in the early 1970s. Many other companies used this technology to produce personal computers. Over the next 10 years, 25 million personal computers were sold in the United States alone!

Use of logic gates

Logic gates are used to make decisions and solve problems. The number of combinations of inputs depends on the number of input gates, as shown in this table. No matter how many inputs an AND logic gate has, there is only one combination of inputs that allows the output to be a 1, or "on".

Memory and feedback

Memory relies on feedback. This means that the output of a gate is fed back into the input. The simplest type of circuit memory is a flip-flop circuit. A flip-flop is a circuit constructed using gates and, typically, two inverters.

The Transistor

Prior to the 1950s, computers used vacuum tubes. Then Bell Laboratories invented the transistor. Transistors were smaller, lighter, and used far less power than vacuum tubes. They also cost less to produce. By the end of the century, entire circuits containing transistors, resistors, and capacitors could be manufactured on a single chip called an integrated circuit.

Punch cards

Punch cards stored and calculated information that could be read and sorted by a machine, rather than by humans. For the first time, people could write programs on punch cards, which allowed customized machines to perform specific tasks.

Binary encoding

Remember that the sum of the bit values equals the decimal number. We would need 7 bits to encode the number 80 for our thermostat. Look at the chart to help you understand why. You can see that the binary number for 80 is 1010000, or 64 + 16. This is called binary encoding. The amplitude of signals on a music CD are encoded this way.

Polarity

Reversing the polarity of the input signals causes the flip-flop to output a steady low signal.

The purpose of some of the first devices invented was to keep track of time. Agricultural societies needed to plant and harvest crops, hunt for food, and keep track of changes in weather. The Maya in southern Mexico created an extremely accurate calendar that divided the year into 18 periods of 20 days each. They added 5 more days at the end of the year to total 365 days.

Sir Isaac Newton invented a water clock to measure the speed and acceleration of objects in his experiments. The clock counted the number of drops that dripped into a container.

accurate readings

The problem is that we wouldn't know if the temperature were 81 or 89 degrees. So the reading wouldn't be a very accurate.

switches

The type of switches required to start and stop the timing device depends on the control circuit design.

AND Gate

This is a diagram of the AND logic gate. It has two or more inputs and one output. The idea is that if both input A and input B are the same--1, let's say--then the output will be 1. Remember that this logic gate is named AND because the output is only "on," or 1, when each and every input is "on," or 1. The information goes into the inputs. The logic gate sends the processed result to the output.

More values

To produce a more accurate thermometer requires more values. To make the thermometer accurate to 1 degree requires 121 values, from 0 to 120. The more carefully a digital device measures a signal, the more accurate the result is.

Toggle Switches

Two push-button switches can implement the same toggle function. The push buttons need to be connected to the set and reset inputs of the simple flip-flop, which you learned about earlier. Look at the truth table shown here to see how it works.

BCD

Unfortunately, most common integrated circuits used to make visual displays, like those on a watch or calculator, are limited to 4 bits and can't handle a 7-bit number. Therefore, you need to encode each decimal place value separately as a 4-bit binary number. This is called binary-coded decimal encoding, or BCD for short.

Constructing flip flops

You can construct a simple flip-flop by wiring two inverters together, as shown in this diagram. You learned how NAND gates can be used as inverters. So now you can make a flip-flop using two NAND gates.

Charles Babbage

A British mathematician and inventor named Charles Babbage first envisioned the programmable calculator. In the mid-1800s, the only power available to Babbage was steam. He spent years trying to develop a working model without ever succeeding.

Using a NAND

A NAND gate might be used inside the circuitry of a rocket launching system. Let's say that a certain level of wind conditions, temperature, fuel pressure, and electric current all need to be present for the automatic launch to take place. If any of these parameters are outside the correct limits, the NAND gate will stop the launch. Therefore, the wind can't be too strong, the temperature can't be too low, and the fuel pressure or the electric current can't deviate from the set levels.

The 555

A timing circuit that continues to function despite interruptions is the 555. This circuit also can be purchased already made. The 555 integrated circuit can deliver timing pulses as slowly as one pulse every few days, or as frequently as hundreds of thousands of pulses per second. Two resistors and a capacitor determine the oscillation frequency of the 555. The two resistors function as a voltage divider, allowing the circuit to function at a constant frequency despite changes in supply voltage or temperature.

Inverters

Although you can use an inverter by itself to solve problems, it is best used in combination with other logic gates. A NOT logic gate is inserted into a circuit to prevent an action from taking place. A good example is the safety circuit on a garage door. If the door encounters an obstacle while being lowered, an increase in the electric current is detected. The NOT gate then prevents the door from closing.

Using flip flops

Escalators are among the many applications for flip-flops. One of the inputs can be connected to a push button labeled "on." The other input can be connected to a push button labeled "off." When the on switch is pushed, the flip-flop will output an on, or high, signal. The escalator will run until the button connected to the off input is pushed. The flip-flop will then assume a low state, and the escalator will remain off until someone pushes the on switch.

Internal clocks can be built in a variety of ways. The simple flip-flop circuit can be modified to vibrate automatically between high and low states. To slow the vibration down, various items can be added, such as: Timing components Resistors Capacitors

Faster clocks must be extremely precise and reliable. A crystal-controlled oscillator provides reliable high-speed timing pulses that can run in hundreds of millions of pulses per second.

Flip flops

Flip-Flop is the unusual name given to a two-state device which offers basic memory for a sequential logic circuit. Flip flops are used for the storage of binary numerical data. The circuit can be made to change state by signals applied to J and/or K inputs and will have one or two outputs. A common type of flip flop is the "jk flip flop". It is the most versatile of the basic flip flops. It operates under the principal that if both J and K are different, then Q takes on the value of J. If J and K are both low, then no change occurs.

Herman holerith

In 1896, the American inventor Herman Hollerith created a company that expanded on Babbage's invention. The company tabulated results and information on punch cards. By this time, electricity had appeared in U.S. cities. Therefore, Hollerith's machines could run on electric motors and relays.

squaring the values

In binary numbers, each number place, or bit, is worth twice as much as the one before it. For example, the first place is worth either 0 or 1. The second place is worth twice as much, either 0 (2 x 0 = 0) or 2 (2 x 1 =2). The third bit is worth twice as much as the second bit, either 0 (2 x 0 = 0) or 4 (2 x 2 = 4). The process of adding bits can be repeated as often as you wish to create the numbers to suit your application. Study this 4-bit binary table so that you can write your own codes.

ENIAC

In the 1920s, the development of the vacuum tube led to the creation of the first true digital computer. In the mid-1940s, ENIAC (Electronic Numerical Integrator And Computer) was built at the University of Pennsylvania as part of a secret military project called Project X. The computer was 150 feet wide and was made from 17,468 vacuum tubes. Computers had arrived!

Clock rate

In the last activity you moved switches manually to send digital signals to the decoder to be turned into digital signals to light up the LED display. You learned that the binary sequence 0001 references the number 1 and that 0010 references the number 2 and so on. We also know that zeros (0) and ones (1) form the basis for all computer operations. The speed of a computer processor, or CPU (central processing units), is determined by the clock cycle or rate which is the frequency of one cycle of the clock. In general terms the higher number of cycles per second, the faster the computer microprocessor will be able to process information. The clock speed is measured in Hz, typically megahertz (MHz) or gigahertz (GHz). For example, a 4GHz processor performs 4,000,000,000 clock cycles per second. That means that for every second, 4 billion changes between 1 and 0 can occur. In the late 1970s CPU clock speeds for early Apple, Atari and Commodore computers were at 1 MHz (1,000,000 cycles per second). In 2015 President Obama signed an executive order for the US to build the world's fastest supercomputer by 2025. It will be capable of making one quintillion (a billion billion) calculations per second - a figure which is known as one exaflop.

Secondary storage

Storing data in primary memory is expensive. In addition, data stored here is easily lost when the computer's power source is turned off. Most data and software programs are kept in secondary storage until needed, even though it may be slower to retrieve. A common secondary storage media remote hard drives located in a central location. This is often referred to as CLOUD STORAGE.

One input low

The NAND gate controls the functions as follows. If the clock pulses are applied to one of the inputs of a NAND gate, and the other input is low, no clock pulses can pass through the NAND gate's output.

Computer processor

The computer processor has two main parts: Primary memory provides temporary storage for the data being processed, as well as software programs. The central processing unit (CPU) contains the electronic circuits that process all the data in the computer. A key component of the CPU is the internal clock inside the CPU which controls how software programs operate. You will learn more about clocks and clock speed later in this module.

Control circuit

The control circuit controls the pulses coming from the internal timing clock that are being sent to the counters.

XOR

The idea behind the EXCLUSIVE OR or XOR gate is that if input A or input B, but not both, are 1, then the output is 1. Interestingly, if all the inputs are 1, or if all the inputs are 0, then, and only then, will the output be low. Here's an example. To make a unanimous vote detector that will indicate when all voters agree to vote for or against some issue, an XOR gate can be used, followed by a NOT gate. This combinational logic gate is called an Exclusive NOR gate (for Exclusive OR and NOT). Like other useful combinational logic circuits, this circuit can be purchased already made in integrated circuit form.

Accuracy of sampling

The second factor that affects the accuracy of the digital signal is how carefully the value of the signal is measured. Let's review the example of the digital thermometer again.

Binary DigITS

The size of the binary number required to do a particular job depends on the range of values of the information to be digitized. Each 0 or 1 is called a bit, which stands for "Binary digIT."

SPDT switches

The start and stop switches need to: Bring the control input of the NAND gate to "high" to start the counters Bring the control input of the NAND gate to "low" to stop the count Adding a single pole double throw switch, or SPDT, can do this. The SPDT acts as an on-off toggle switch to start and stop the timing device.

Inverting the outputs

These are truth tables for two logic gates. Notice that inverting the outputs of an AND gate produces a NAND gate. What do you think would happen if you inverted the outputs of a NAND gate?

Potentiometer

This circuit looks similar to the last one, but there is an important difference. The switch is replaced with a variable resistor, or potentiometer. This input control allows the intensity of the lamp's light to vary from off to extremely bright in a continuous, smooth manner. If you think that sounds like an analog signal, you're right! The point is that not all circuits are digital.

Change the decimal to binary

This situation could be written mathematically as ON = > 80 degrees. Of course, digital circuits can deal only with ones and zeroes. Therefore, we need to change the decimal 80 degrees to its binary equivalent.

The number of functions that a digital circuit can perform depends on the speed at which the circuit is running. The system clock controls the speed. Today's machines are much faster than machines created 10 years ago. An early 8-bit (per byte) machine running at 8 megahertz (8 million bytes per second) could process 2,048,000,000 different values per second. Common 32-bit machines can handle over 2 quadrillion different values per second.

Today's more advanced 64-bit machines are capable of processing far more data at a faster, more efficient rate.

Range of values

We could devise a thermometer that measures only every 10 degrees. Let's say that the range of temperatures falls within 0 and 120 degrees Fahrenheit. This thermometer would have 13 values.

Digital Advantages

When signals change at fast rates, the advantage of digital equipment is obvious. Our eyes can sense changes up to 30 times per second. Our ears can detect changes up to 20,000 times per second. Our eyes and ears can't decipher changes in signals that are faster than either of these. Instead, such signals must be recorded and played back in slow motion--or else sampled at a rate slow enough for us to keep up.

Truth table

When you perform an experiment, you need to record your data in an orderly way. Similarly, a truth table shows the true relationships between the inputs and their resulting outputs. This example shows that when there are two inputs, there are four possible combinations of input switch positions. For the two-input AND logic gate shown here, the output is 1 only when both inputs are 1.

time plus

You could use a simple NAND gate to control the timing pulses coming from the internal clock. NAND gates can turn a stream of pulses on or off. The timing pulses can be added to one of the NAND gate's inputs. If the control input is set to low, the output of the NAND gate will stay high and will not allow the timing pulses to pass through.

A logic gate is a device that processes information in a circuit. Logic gates take information from the input and send the processed result to the output. Logic gates can compare values, perform arithmetic, or sequence and store information. There are three basic types of logic gates: AND gate OR gate NOT gate

You will learn how they can be used independently or in combinations to implement any digital component you require. Review the schematic symbols for each logic gate. These symbols assist Electrical Engineers who design digital circuits.

speed of performance

You've learned that internal clocks on digital devices can operate at different speeds to perform a desired function. A clock runs at 1 pulse per second, or 1 Hz (hertz). This is slow compared to a communications satellite that runs in gigahertz, or billions of pulses per second.