· Web viewWe can get a NAND function in either of two ways. We can simply invert the inputs to the NOR/OR gate, thus turning it into an AND/NAND gate, or we can use the circuit

True History of the Transistor http://www.bn.com.br/radios-antigos/semicond.htmThe transistor was invented in the Beel Telephone Laboratories in December 1947 (not 1948 as is often said) by Bardeen and Brattain. Discovered so to speak (since they were looking for a solid state device equivalent to the vacuum tube), accidentally during studies of surfaces around a point-contact diode. The transistors were therefore of type point-contact "and there is evidence that Shockley, the theorist who headed the research was pissed because the device was not what I was looking for. At the time, he was looking for a semiconductor amplifier similar to what we now call "junction FET. The name transistor was derived from their intrinsic properties "transfer resistor" in English: (transfer resistor). Bell Labs kept the discovery secret until June 1948 (hence the confusion with the dates of discovery). With a estrodosa publicity, they announced their findings publicly, but few people realized the significance and importance of the publication, despite having left the front pages of newspapers. Although it was a great scientific achievement, the transistor not reached immediately, the commercial supremacy. The difficulties of manufacturing added to the high price of germanium, a rare element, kept the price too high. The best transistors costing $ 8 a time when the price of a valve was only 75 cents. Shochley ignored the point-contact transistor and continued his research in other directions. He reoriented his ideas and developed the theory of "transistor junction". In July 1951, Bell announced the creation of this device. In September 1951 they promote a symposium and are willing to license the new technology of both types of transistors to any company that was willing to pay $ 25,000.00. This was the beginning of the industrialization of the transistor. Many firms withdrew the notice of license. Former manufacturers of vacuum tubes, such as RCA, Raytheon, GE and industrial leaders in the market like Texas and Transitron. Many started the production of point-contact transistor, which at that time, worked better in high frequency than the types of joint. However, the junction transistor becomes faster, far superior in performance and is simpler and easier to manufacture. The point-contact transistor was made obsolete by about 1953 in America and later in England. Only a few thousand were manufactured between 120 types, many Americans (not including these numbers, trial versions).The first junction transistor manufactured commercially was primitive compared to modern devices, with a maximum voltage between collector-emitter of 6 volts and a maximum current of a few milliamps. Particularly notable was the Raytheon CK722 transistor 1953, the first device solid state electronic mass produced available to the amateur builder. Various types of transistors have been developed, increasing the frequency response by reducing noise levels and increasing its power capacity. In England, two companies have maintained research labs not so early as in America: Standard Telephones and Cables (STC) and General Electric Company of England "GEC" (no telação with the American GE). Research was conducted in France and Germany without trade effects. In 1950, a shark comes in this small pond: the Dutch PHILIPS by Mullard, its English subsidiary, with a complete plan to industrialize the transistor. The goal was to dominate the Philips 95% of the European market, reaching this goal in few years. The series 'OC' transistor dominated Europe for over 20 years. The former were made of germanium transistors, a semiconductor metal, but soon found that the silicon offered a number of advantages over germanium. The silicon was more difficult to refine because of its high melting point, but in 1955 the first silicon transistor was already sold. Texas Instruments was one of the companies that took part in the initial development of this technology by launching a series of devices known at the time by the letters "900" and "2s". The big turnaround came in 1954 when Gordon Teal perfected a junction transistor made of silicon. The silicon instead of germanium, a mineral is abundant, only losing in the oxygen availability. This fact, coupled with the improvement of production techniques, have significantly decreased the price of the transistor. This enabled him to popularize and would cause a revolution in the computer industry. That only such a revolution would be repeated with the creation and refinement of integrated circuits.

Major innovations in the field of semiconductors

INNOVATION LABORATORY YEAR

POINT OF CONTACT TRANSISTOR Bell Labs-Western Electric 1947

CULTIVATION IN SINGLE CRYSTAL Western Electric 1950

ZONE REFINED Western Electric 1950

TRANSISTOR JUNCTION CULTURED Western Electric 1951

SILICON TRANSISTOR Texas Instruments 1954

MASK OF OXIDE AND DIFFUSION Western Electric 1955

PLANAR TRANSISTOR Fairchild 1960

INTEGRATED CIRCUITTexas Instruments, Fairchild

1961

GUNN DIODE IBM 1963

ResistanceThe electrical resistance of a circuit component or device is

defined as the ratio of the voltage applied to the electric current whichflows through it:

If the resistance is constant over a considerable range of voltage, then Ohm's law, I = V/R, can be used to predict the behavior of the material. Although the definition above involves DC current and voltage, the same definition holds for the AC application of resistors.

Whether or not a material obeys Ohm's law, its resistance can be described in terms of its bulk resistivity. The resistivity, and thus the resistance, is temperature dependent. Over sizable ranges of temperature, this temperature dependence can be predicted from a temperature coefficient of resistance.

Resistivity and ConductivityThe electrical resistance of a wire would be expected to be greater for a longer

wire, less for a wire of larger cross sectional area, and would be expected to depend upon the material out of which the wire is made. Experimentally, the dependence upon these properties is a straightforward one for a wide range of conditions, and the resistance of a wire can be expressed as

The factor in the resistance which takes into account the nature of the material is the resistivity . Although it is temperature dependent, it can be used at a given temperature to calculate the resistance of a wire of given geometry.

The inverse of resistivity is called conductivity. There are contexts where the use of conductivity is more convenient.

Electrical conductivity = σ = 1/ρ

Resistor CombinationsThe combination rules for any number of resistors in series or parallel can be

derived with the use of Ohm's Law, the voltage law, and the current law.

Resistivity CalculationThe electrical resistance of a wire would be expected to be greater for a longer

wire, less for a wire of larger cross sectional area, and would be expected to depend upon the material out of which the wire is made (resistivity). Experimentally, the dependence upon these properties is a straightforward one for a wide range of conditions, and the resistance of a wire can be expressed as

Resistance = resistivity x length/area

For a wire of length L = m = ft

and area A = cm2

corresponding to radius r = cm

and diameter inches for common wire gauge comparison

with resistivity = ρ = x 10^ ohm meters

will have resistance R = ohms.

Enter data and then click on the quantity you wish to calculate in the active formula above. Unspecified parameters will default to values typical of 10 meters of #12 copper wire. Upon changes, the values will not be forced to be consistent until you click on the quantity you wish to calculate.

Commonly used U.S. wire gaugesfor copper wire.

AWGDiameter(inches)

Typical use

10 0.1019 Electric range

12 0.0808 Household circuit

14 0.0640 Switch leads

Resistivities of some metalsin ohm-m(x 10-8) at 20°C.

Aluminum 2.65 Gold 2.24

Copper 1.724 Silver 1.59

Iron 9.71 Platinum 10.6

Nichrome 100 Tungsten 5.65

The factor in the resistance which takes into account the nature of the material is the resistivity . Although it is temperature dependent, it can be used at a given temperature to calculate the resistance of a wire of given geometry.

Resistor-Transistor Logic

Consider the most basic transistor circuit, such as the one shown to the left. We will only be applying one of two voltages to the input I: 0 volts (logic 0) or +V volts (logic 1). The exact voltage used as +V depends on the circuit design parameters; in RTL integrated circuits, the usual voltage is +3.6v. We'll assume an ordinary NPN transistor here, with a reasonable dc current gain, an emitter-base forward voltage of 0.65 volt, and a collector-emitter saturation voltage no higher than 0.3 volt. In standard RTL ICs, the base resistor is 470 and the collector resistor is 640 .

When the input voltage is zero volts (actually, anything under 0.5 volt), there is no forward bias to the emitter-base junction, and the transistor does not conduct. Therefore no current flows through the collector resistor, and the output voltage is +V volts. Hence, a logic 0 input results in a logic 1 output.

When the input voltage is +V volts, the transistor's emitter-base junction will clearly be forward biased. For those who like the mathematics, we'll assume a similar output circuit connected to this input. Thus, we'll have a voltage of 3.6 - 0.65 = 2.95 volts applied across a series combination of a 640 output resistor and a 470 input resistor. This gives us a base current of: 

2.95v / 1110 = 0.0026576577 amperes = 2.66 ma.

RTL is a relatively old technology, and the transistors used in RTL ICs have a dc forward current gain of around 30. If we assume a current gain of 30, 2.66 ma base current will support a maximum of 79.8 ma collector current. However, if we drop all but 0.3 volts across the 640 collector resistor, it will carry 3.3/640 = 5.1 ma. Therefore this transistor is indeed fully saturated; it is turned on as hard as it can be.

With a logic 1 input, then, this circuit produces a logic 0 output. We have already seen that a logic 0 input will produce a logic 1 output. Hence, this is a basic inverter circuit.

As we can see from the above calculations, the amount of current provided to the base of the transistor is far more than is necessary to drive the transistor into saturation. Therefore, we have the possibility of using one output to drive multiple inputs of other gates, and of having gates with multiple input resistors. Such a circuit is shown to the right.

In this circuit, we have four input resistors. Raising any one input to +3.6 volts will be sufficient to turn the transistor on, and applying additional logic 1 (+3.6 volt) inputs will not really have any appreciable effect on the output voltage. Remember that the forward bias voltage on the transistor's base will not exceed 0.65 volt, so the current through a grounded input resistor will not exceed 0.65v/470 = 1.383 ma. This does provide us with a practical limit on the number of allowable input resistors to a single transistor, but doesn't cause any serious problems within that limit.

The RTL gate shown above will work, but has a problem due to possible signal interactions through the multiple input resistors. A better way to implement the NOR function is shown to the left.

Here, each transistor has only one input resistor, so there is no interaction between inputs. The NOR function is performed at the common collector connection of all transistors, which share a single collector load resistor.

This is in fact the pattern for all standard RTL ICs. The very commonly-used µL914 is a dual two-input NOR gate, where each gate is a two-transistor version of the circuit to the left. It is rated to draw 12 ma of current from the 3.6V power supply when both outputs are at logic 0. This corresponds quite well with the calculations we have already made.

Standard fan-out for RTL gates is rated at 16. However, the fan-in for a standard RTL gate input is 3. Thus, a gate can produce 16 units of drive current from the output, but requires 3 units to drive an input. There are low-power versions of these gates that increase the values of the base and collector resistors to 1.5K and 3.6K, respectively. Such gates demand less current, and typically have a fan-in of 1 and a fan-out of 2 or 3. They also have reduced frequency response, so they cannot operate as rapidly as the standard gates. To get greater output drive capabilities, buffers are used. These are typically inverters which have been designed with a fan-out of 80. They also have a fan-in requirement of 6, since they use pairs of input transistors to get increased drive.

We can get a NAND function in either of two ways. We can simply invert the inputs to the NOR/OR gate, thus turning it into an AND/NAND gate, or we can use the circuit shown to the right.

In this circuit, each transistor has its own separate input resistor, so each is controlled by a different input signal. However, the only way the output can be pulled down to logic 0 is if both transistors are turned on by logic 1 inputs. If either input is a logic 0 that transistor cannot conduct, so there is no current through either one. The output is then a logic 1. This is the behavior of a NAND gate. Of course, an inverter can also be included to provide an AND output at the same time.

The problem with this NAND circuit stems from the fact that transistors are not ideal devices. Remember that 0.3 volt collector saturation voltage? Ideally it should be zero. Since it isn't, we need to look at what happens when we "stack" transistors this way. With two, the combined collector saturation voltage is 0.6 volt -- only slightly less than the 0.65 volt base voltage that will turn a transistor on.

If we stack three transistors for a 3-input NAND gate, the combined collector saturation voltage is 0.9 volt. This is too high; it will promote conduction in the next transistor no matter what. In addition, the load presented by the upper transistor to the gate that drives it will be different from the load presented by the lower transistor. This kind of unevenness can cause some odd problems to appear, especially as the frequency of operation increases. Because of these problems, this approach is not used in standard RTL ICs.

Diode-Transistor Logic As we said in the page on diode logic, the basic problem with DL gates is that they rapidly deteriorate the logical signal. However, they do work for one stage at a time, if the signal is re-amplified between gates. Diode-Transistor Logic (DTL) accomplishes that goal.

The gate to the right is a DL OR gate followed by an inverter such as the one we looked at in the page on resistor-transistor logic. The OR function is still performed by the diodes. However, regardless of the number of logic 1 inputs, there is certain to be a high enough input voltage to drive the transistor into saturation. Only if all inputs are logic 0 will the transistor be held off. Thus, this circuit performs a NOR function.

The advantage of this circuit over its RTL equivalent is that the OR logic is performed by the diodes, not by resistors. Therefore there is no interaction between different inputs, and any number of diodes may be used. A disadvantage of this circuit is the input resistor to the transistor. Its presence tends to slow the circuit down, thus limiting the speed at which the transistor is able to switch states.

At first glance, the NAND version shown on the left should eliminate this problem. Any logic 0 input will immediately pull the transistor base down and turn the transistor off, right?

Well, not quite. Remember that 0.65 volt base input voltage for the transistor? Diodes exhibit a very similar forward voltage when they're conducting current. Therefore, even with all inputs at ground, the transistor's base will be at about 0.65 volt, and the transistor can conduct.

To solve this problem, we can add a diode in series with the transistor's base lead, as shown to the right. Now the forward voltage needed to turn the transistor on is 1.3 volts. For even more insurance, we could add a second series diode and require 1.95 volts to turn the transistor on. That way we can also be sure that temperature changes won't significantly affect the operation of the circuit.Either way, this circuit will work as a NAND gate. In addition, as with the NOR gate, we can use as many input diodes as we may wish without raising the voltage threshold. Furthermore, with no series resistor in the input circuit, there is less of a slowdown effect, so the gate can switch states more rapidly and handle higher frequencies. The next obvious question is, can we rearrange things so the NOR gate can avoid that resistor, and therefore switch faster as well?

The answer is, Yes, there is. Consider the circuit shown to the left. Here we use separate transistors connected together. Each has a single input, and therefore functions as an inverter by itself. However, with the transistor collectors connected together, a logic 1 applied to either input will force the output to logic 0. This is the NOR function.

We can use multiple input diodes on either or both transistors, as with the DTL NAND gate. This would give us an AND-NOR function, and is useful in some circumstances. Such a construction is also known as an AOI (for AND-OR-INVERT) circuit.

Transistor-Transistor Logic

Transistor-Transistor Logic

With the rapid development of integrated circuits (ICs), new problems were encountered and new solutions were developed. One of the problems with DTL circuits was that it takes as much room on the IC chip to construct a diode as it does to construct a transistor. Since "real estate" is exceedingly important in ICs, it was desirable to find a way to avoid requiring large numbers of input diodes. But what could be used to replace many diodes?

Well, looking at the DTL NAND gate to the right, we might note that the opposed diodes look pretty much like the two junctions of a transistor. In fact, if we were to have an inverter, it would have a single input diode, and we just might be able to replace the two opposed diodes with an NPN transistor to do the same job.

In fact, this works quite nicely. The figure to the left shows the resulting inverter.

In addition, we can add multiple emitters to the input transistor without greatly increasing the amount of space needed on the chip. This allows us to construct a multiple-input gate in almost the same space as an inverter. The resulting savings in real estate translates to a significant savings in manufacturing costs, which in turn reduces the cost to the end user of the device.

One problem shared by all logic gates with a single output transistor and a pull-up collector resistor is switching speed. The transistor actively pulls the output down to logic 0, but the resistor is not active in pulling the output up to logic 1. Due to inevitable factors such as circuit capacitances and a characteristic of bipolar transistors called "charge storage," it will take a certain amount of time for the transistor to turn completely off and the output to rise to a logic 1 level. This limits the frequency at which the gate can operate.

The designers of commercial TTL IC gates reduced that problem by modifying the output circuit. The result was the "totem pole" output circuit used in most of the 7400/5400 series TTL ICs. The final circuit used in most standard commercial TTL ICs is shown to the right. The number of inputs may vary — a commercial IC package might have six inverters, four 2-input gates, three 3-input gates, or two 4-input gates. An 8-input gate in one package is also available. But in each case, the circuit structure remains the same.

Emmiter-Coupled Logic

Emitter-Coupled Logic is based on the use of a multi-input differential amplifier to amplify and combine the digital signals, and emitter followers to adjust the dc voltage levels. As a result, none of the transistors in the gate ever enter saturation, nor do they ever get turned completely off. The transistors remain entirely within their active operating regions at all times. As a result, the transistors do not have a charge storage time to contend with, and can change states much more rapidly. Thus, the main advantage of this type of logic gate is extremely high speed.

The schematic diagram shown here is taken from Motorola's 1000/10,000 series of MECL devices. This particular circuit is of one 4-input OR/NOR gate. Standard voltages for this circuit are -5.2 volts (VEE) and ground (VCC). Unused inputs are connected to VEE. The bias circuit at the right side, consisting of one transistor and its associated diodes and resistors, can handle any number of gates in a single IC package. Typical ICs include dual 4-input, triple 3-input, and quad 2-input gates. In each case, the gates themselves differ only in how many input transistors they have. A single bias circuit serves all gates.

In operation, a logical ouput changes state by only 0.85 volt, from a low of -1.60 volts to a high of -0.75 volt. The internal bias circuit supplies a fixed voltage of -1.175 volts to the bias transistor in the differential amplifier. If all inputs are at -1.6 volts (or tied to VEE), the input transistors will all be off, and only the internal differential transistor will conduct current. This reduces the base voltage of the OR output transistor, lowering its output voltage to -1.60 volts. At the same time, no input transistors are affecting the NOR output transistor's base, so its output rises to -0.75 volt. This is simply the emitter-base voltage, VBE, of the transistor itself. (All transistors are alike within the IC, and are designed to have a VBE of 0.75 volt.)

When any input rises to -0.75 volt, that transistor siphons emitter current away from the internal differential transistor, causing the outputs to switch states.

The voltage changes in this type of circuit are small, and are dictated by the VBE of the transistors involved when they are on. Of greater importance to the operation of the circuit is the amount of current flowing through various transistors, rather than the precise voltages involved. Accordingly, Emitter-Coupled Logic is also known as Current Mode Logic (CML). This is not the only technology to implement CML by any means, but it does fall into that general description. In any case, this leads us to a major drawback of this type of gate: it draws a great deal of current from the power supply, and hence tends to dissipate a significant amount of heat.

To minimize this problem, some devices such as frequency counters use an ECL decade counter at the input end of the circuitry, followed by TTL or high-speed CMOS counters at the later digit positions. This puts the fast, expensive IC where it is absolutely required, and allows us to use cheaper ICs in locations where the signal will never be at that high a frequency.

Diode Logic

Diode Logic makes use of the fact that the electronic device known as a diode will conduct an electrical current in one direction, but not in the other. In this manner, the diode acts as an electronic switch.

To the left you see a basic Diode Logic OR gate. We'll assume that a logic 1 is represented by +5 volts, and a logic 0 is represented by ground, or zero volts. In this figure, if both inputs are left unconnected or are both at logic 0, output Z will also be held at zero volts by the resistor, and will thus be a logic 0 as well. However, if either input is raised to +5 volts, its diode will become forward biased and will therefore conduct. This in turn will force the output up to logic 1. If both inputs are logic 1, the output will still be logic 1. Hence, this gate correctly performs a logical OR function.

To the right is the equivalent AND gate. We use the same logic levels, but the diodes are reversed and the resistor is set to pull the output voltage up to a logic 1 state. For this example, +V = +5 volts, although other voltages can just as easily be used. Now, if both inputs are unconnected or if they are both at logic 1, output Z will be at logic 1. If either input is grounded (logic 0), that diode will conduct and will pull the output down to logic 0 as well. Both inputs must be logic 1 in order for the output to be logic 1, so this circuit performs the logical AND function.

In both of these gates, we have made the assumption that the diodes do not introduce any errors or losses into the circuit. This is not really the case; a silicon diode will experience a forward voltage drop of about 0.65v to 0.7v while conducting. But we can get around this very nicely by specifying that any voltage above +3.5 volts shall be logic 1, and any voltage below +1.5 volts shall be logic 0. It is illegal in this system for an output voltage to be between +1.5 and +3.5 volts; this is the undefined voltage region.

Individual gates like the two above can be used to advantage in specific circumstances. However, when DL gates are cascaded, as shown to the left, some additional problems occur. Here, we have two AND gates, whose outputs are connected to the inputs of an OR gate. Very simple and apparently reasonable.

But wait a minute! If we pull the inputs down to logic 0, sure enough the output will be held at logic 0. However, if both inputs of either AND gate are at +5 volts, what will the output voltage be? That diode in the OR gate will immediately be forward biased, and current will flow through the AND gate resistor, through the diode, and through the OR gate resistor.

If we assume that all resistors are of equal value (typically, they are), they will act as a voltage divider and equally share the +5 volt supply voltage. The OR gate diode will insert its small loss into the system, and the output voltage will be about 2.1 to 2.2 volts. If both AND gates have logic 1 inputs, the output voltage can rise to about 2.8 to 2.9 volts. Clearly, this is in the "forbidden zone," which is not supposed to be permitted.

If we go one step further and connect the outputs of two or more of these structures to another AND gate, we will have lost all control over the output voltage; there will always be a reverse-biased diode somewhere blocking the input signals and preventing the circuit from operating correctly. This is why Diode Logic is used only for single gates, and only in specific circumstances.

Referências Bibliográficas

DIGITAL ELECTRONIC. Diode Logic. Disponível em:

http://www.play-hookey.com/digital/electronics/dl_gates.html Acessado em 28 Jul.2010.

______________________. Diode Transistor Logic. Disponível em: http://www.play-

hookey.com/digital/electronics/dtl_gates.html Acessado em 28 Jul.2010.

______________________. Emitter-Coupled Logic. Disponível em: http://www.play-

hookey.com/digital/electronics/ecl_gates.html Acessado em 28 Jul.2010.

______________________. Resistor Transistor Logic. Disponível em: http://www.play-

hookey.com/digital/electronics/rtl_gates.html Acessado em 28 Jul.2010.

______________________. Transistor Transistor Logic. Disponível em: http://www.play-

hookey.com/digital/electronics/ttl_gates.html Acessado em 28 Jul.2010.

ETB – Escola Técnica de Brasília. Técnicas de Leitura. Disponível em:

http://ziggi.uol.com.br/site/dwnld/3592 Acessado em 21 jul 2010

HEF4081B – Quadruple 2 – Input and gate. Disiponível em:

www.nxp.com/documents/data_sheet/ HEF4081B .pdf Acessado em 28 jul 2010.

HEF4511B – BDC to 7 – Segment Latch/decoder/driver. Disponível em:

http://www.nxp.com/documents/data_sheet/HEF4511B.pdf Acessado em 28 Jul 2010.

HEF4017B – Stage Jonhson Counter. Disponível em:

http://www.nxp.com/documents/data_sheet/HEF4017B.pdf Acessado em 28 Jul 2010.

HEF4029B – Synchronous up/down counter, binary/ decade Counter. Disponível

em: http://ics.nxp.com/products/hef/datasheet/hef4029b.pdf Acessado em 28 Jul 2010.

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http://ics.nxp.com/products/hef/datasheet/hef4049b.pdf Acessado em 28 Jul 2010.

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http://www.bn.com.br/radios-antigos/semicond.htm Acessado em 28 Jul 2010

Lista de textos

The MicroprocessorThe term microprocessor typically refers to the central processing unit (CPU) of

a microcomputer, containing the arithmetic logic unit (ALU) and the control units. It is typically implemented on a single LSI chip. This separates the "brains" of the operation from the other units of the computer.

An example of microprocessor

architecture.

The microprocessor contains the arithmetic logic unit (ALU) and the control unit for a microcomputer. It is connected to memory and I/O by buses which carry information between the units.

Microcomputer Example

Typical microcomputers employ a microprocessor unit (MPU), a clock, and interfaces to memory and external input/output devices. The units are connected by buses which transfer information between them.

Buses: The exchange of information. Information is transferred between units of the microcomputer by collections of

conductors called buses.

There will be one conductor for each bit of information to be passed, e.g., 16 lines for a 16 bit address bus. There will be address, control, and data buses.

Arithmetic Logic Unit All the arithmetic operations of a microprocessor take place in the arithmetic

logic unit (ALU). Using a combination of gates and flip-flops, numbers can be added in less than a microsecond, even in small personal computers. The operation to be performed is specified by signals from the control unit. The data upon which operations are performed can come from memory or an external input. The data may be combined in some way with the contents of the accumulator and the results are typically placed in the accumulator. From there they may be transferred to memory or to an output unit.

The Accumulator The accumulator is the principal register of the arithmetic logic unit of a

microprocessor. Registers are sets of flip-flops which can hold data. The accumulator typically holds the first piece of data for a calculation. If a number from memory is added to that date, the sum replaces the original data in the accumulator. It is the repository for successive results of arithmetic operations, which may then be transferred to memory, to an output device, etc.

Control Unit of Microprocessor The control unit of a microprocessor directs the operation of the other units by

providing timing and control signals. It is the function of the microcomputer to execute programs which are stored in memory in the form of instructions and data. The control unit contains the necessary logic to interpret instructions and to generate the signals necessary for the execution of those instructions. The descriptive words "fetch" and "execute" are used to describe the actions of the control unit. It fetches an instruction by sending and address and a read command to the memory unit. The instruction at that memory address is transferred to the control unit for decoding. It then generates the necessary signals to execute the instruction.

http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/micropro.html#c1

Number SystemsDigital circuits are inherently binary in nature, but several types of

representations of numerical data are in use.

The representation of an unsigned integer can be done in binary, octal, decimal or hexadecimal. For display purposes, each decimal digit is often represented by a four-bit binary number in a system called binary coded decimal (BCD). Conversions between these representations can be handled in a routine manner.

The representation of signed numbers presents more problems and those problems are addressed in various ways. Some of the codes used are "sign magnitude", "offset binary", "2's complement", "excess-3", "4221", and "Gray". A table can show the display of four-bit integers.

Alphanumeric CodingFor the inherently binary world of the computer, it is necessary to put all

symbols, letters, numbers, etc. into binary form. The most commonly used alphanumeric code is the ASCII code, with others like the EBCDIC code being applied in some communication applications.

ASCIICode

EBCDICCode

Parity ChecksErrors in digital code will result in the changing of a 0 to a 1 or vice versa. One helpful method for determining if a single error of that type has ocurred is to check the evenness or oddness of the sum of the set bits. To facilitate this check an extra bit called the parity bit is added to each word in a data transmission. In the even-parity method the parity bit is chosen so that the total number of 1's including the parity bit is even. The receiver checks the parity to detect any single-bit errors. The same thing can be accomplished with an odd-parity method, so it is necessary to know which is being used in order to communicate with a host computer. It will also be necessary to know how many data bits and how many stop bits are being used.

Serial Communication ProtocolsSerial communication protocols for data include the RS-232 protocol, which

has been used for communication of modems. The MIDI protocol for music and sound applications is also a serial protocol.

Note: This is just a place-holder location for future development. Very little has been done with it to date

RS-232 Serial Communication ProtocolThe most common standard used for serial data transmission is called RS232C. It was set by the Electronics Industry Association and includes an assignment of the conductors in a 25-pin connector. It has also been used widely for data transfer over a modem.

ModemFor serial digital data transmission over telephone lines, the logic levels are

converted to audio tones at one end (modulation) and then back into logic levels at the other end (demodulation). The device which accomplishes this is called a "modem" for "modulator-demodulator". The acoustic modem converts logic 1 to a 2225 Hz sine wave burst and a logic zero into a 2025 Hz tone. As a receiver it treats 1270 Hz as a logic 1 and 1070 Hz as a logic 0. This technique, called frequency-shift keying, allows the same phone line to be used simultaneously for sending and receiving in what is called full-duplex operation. The modem at the other end of the line must receive 2225 Hz as a logic 1 and send 1270 Hz as a logic 1. A basic rate of transmission is 300 baud, but data lines up to 56K baud are in use.

MIDI Communication ProtocolMusical Instrument Digital Interface (MIDI) is a serial data transfer protocol. It

uses one start bit, eight data bits and two stop bits and operates at 31.25 kilobaud. It uses two lines for input devices and three lines for output devices. The controlling device and the instrument controlled are electrically isolated from one another by the use of an opto-isolator and the avoidance of direct common grounds. The controlling device sends a signal through a UART to a 5-pin DIN"MIDI out" connector. On the input side, the signal drives the LED of an optoisolator, and the output of the optoisolator is sent to the UART of the receiving device for conversion to parallel information.

In controlling a device in an integrated music system, the status byte describes the action to be taken while the data bytes provide specific values or other instructions for the type of action requested.

UARTThe conversion of parallel data inside a computer to serial data for use in serial

communication is accomplished by a Universal Asynchronous Receiver/Transmitter (UART). UART chips are used for RS-232 and MIDI communication.

Parallel Communication ProtocolsParallel communication protocols for data include the IEEE-488 protocol, and

the Centronics protocol has been widely used for printers.

Note: This is just a place-holder location for future development. Very little has been done with it to date.

IEEE-488 ParallelHewlett-Packard developed a communication bus which has become the industry standard for laboratory use. It is also known as the GPIB (General Purpose Instrumentation Bus) or the HPIB (Hewlett-Packard Instrumentation Bus). It is a 24 line bus with the following allocation of lines: 16 bi-directional lines (8 data lines and 8 control lines) and 8 additional lines for logical ground returns and shielding. It can connect up to 14 instruments with a computer and operate at a data rate as high as 1 MB per second.

Most manufacturers of research equipment which communicates with a computer offer IEEE-488 devices. Such devices can be classified as 1) listen only, 2) talk only, 3) talk-listen and 4) talk-listen-control.

Analog-to-Digital ConversionThis is a sample of the large number of analog-to-digital conversion methods.

The basic principle of operation is to use the comparator principle to determine whether or not to turn on a particular bit of the binary number output. It is typical for an ADC to use a digital-to-analog converter (DAC) to determine one of the inputs to the comparator.

Digital Ramp ADC

Conversion from analog to digital form inherently involves comparator action where the value of the analog voltage at some point in time is compared with some standard. A common way to do that is to apply the analog voltage to one terminal of a comparator and trigger a binary counter which drives a DAC. The output of the DAC is applied to the other terminal of the comparator. Since the output of the DAC is increasing with the counter, it will trigger the comparator at some point when its voltage exceeds the analog input. The transition of the comparator stops the binary counter, which at that point holds the digital value corresponding to the analog voltage.

Successive Approximation ADCIllustration of 4-bit SAC with 1 volt step size (after Tocci, Digital Systems).

The successive approximation ADC is much faster than the digital ramp ADC because it uses digital logic to converge on the value closest to the input voltage. A comparator and a DAC are used in the process.

Flash ADC

Illustrated is a 3-bit flash ADC with resolution 1 volt (after Tocci). The resistor net and comparators provide

an input to the combinational logic circuit, so the conversion time is just the propagation delay through

the network - it is not limited by the clock rate or some convergence sequence. It is the fastest type of ADC

available, but requires a comparator for each value of output (63 for 6-bit, 255 for 8-bit, etc.) Such ADCs are available in IC form up to 8-bit and 10-bit flash ADCs (1023 comparators) are planned. The encoder logic

executes a truth table to convert the ladder of inputs to the binary number output.

Comparator

The extremely large open-loop gain of an op-amp makes it an extremely sensitive device for comparing its input with zero. For practival purposes, if

the output is driven to the positive supply voltage and if

it is driven to the negative supply voltage. The switching time for - to + is limited by the slew rate of the op-amp.

Comparator Applications

The basic comparator will swing its output to at the slightest difference between its inputs. But there are many variations where the output is designed to switch between two other voltage values. Also, the input may be tailored to make a comparison to an input voltage other than zero.

http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/opampvar8.html#c2

Digital-to-Analog ConversionWhen data is in binary form, the 0's and 1's may be of several forms such as the

TTL form where the logic zero may be a value up to 0.8 volts and the 1 may be a voltage from 2 to 5 volts. The data can be converted to clean digital form using gates which are designed to be on or off depending on the value of the incoming signal. Data in clean binary digital form can be converted to an analog form by using a summing amplifier. For example, a simple 4-bit D/A converter can be made with a four-input summing amplifier. More practical is the R-2R Network DAC.

Four-Bit D/A Converter One way to achieve D/A conversion is to use a summing amplifier.

This approach is not satisfactory for a large number of bits because it requires too much precision in the summing resistors. This problem is overcome in the R-2R network DAC.

R-2R Ladder DAC

The summing amplifier with the R-2R ladder of resistances shown produces the output

where the D's take the value 0 or 1. The digital inputs could be TTL voltages which close the switches on a logical 1 and leave it grounded for a logical 0. This is illustrated for 4 bits, but can be extended to any number with just the resistance values R and 2R.

R-2R Ladder DAC Details

http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/dac.html#c4

The 555 TimerFollowing Forrest Mims in laying out the 555 Timer IC as a block diagram

allows one to focus on the functions of the circuit.

Very popular for its versatility, the 555 Timer IC can operate in either astable or monostable multivibrator mode, resulting in a variety of applications.

This IC contains 23 transistors, 2 diodes and 16 resistors.

Supply voltage: 4.5 to 15Supply current: 3 to 6 mA @5V10 to 15mA@15VOutput current: 200mA maxPower dissipation: 600mW8-pin mini DIP 556 is 14 pin dual 555.

One-Chip RegulatorsMany, if not most, small power supplies today are built with the aid of a family

of one-chip regulators which use zener diodes and several transistors to regulate the output of a rectifier. These remarkable devices provide stable, ripple-free output DC voltages under a wide range of operating conditions.

An example is Fairchild's A7800 series of 3-terminal positive voltage regulators. In a single monolithic package they incorporate two zener diodes, 17 transistors, 21 resistors and a capacitor according to the manufacturer's equivalent circuit. They incorporate internal current limiting and thermal shutdown features and can produce on the order of an ampere of output current.

http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/chipreg.html#c1