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True History of the Transistor
http://www.bn.com.br/radios-antigos/semicond.htm
The 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
LABORATORY
INNOVATION
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 CIRCUIT
Texas Instruments,
Fairchild
1961
GUNN DIODE
IBM
1963
Resistance
The 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 Conductivity
The 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 Combinations
The 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 Calculation
The 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 =
and area A =
m=
ft
cm2
corresponding to radius r =
and diameter
cm
inches for common wire gauge comparison
with resistivity = ρ =
will have resistance R =
x 10^
ohm meters
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 gauges
for copper wire.
AWG
Diameter
(inches)
Typical use
10
0.1019
Electric range
12
0.0808 Household circuit
14
0.0640
Switch leads
Resistivities of some metals
in 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 reamplified 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.playhookey.com/digital/electronics/dtl_gates.html Acessado em 28 Jul.2010.
______________________. Emitter-Coupled Logic. Disponível em: http://www.playhookey.com/digital/electronics/ecl_gates.html Acessado em 28 Jul.2010.
______________________. Resistor Transistor Logic. Disponível em: http://www.playhookey.com/digital/electronics/rtl_gates.html Acessado em 28 Jul.2010.
______________________. Transistor Transistor Logic. Disponível em: http://www.playhookey.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.
HEF
4049
–
Hex
Inverting
Buffers.
Disponível
em:
http://ics.nxp.com/products/hef/datasheet/hef4049b.pdf Acessado em 28 Jul 2010.
PHILIPS – Integrated Circuits – HE 4000B- Logic Family CMOS. In: Philips
Eletronics North America Corporation. Printed in U.S.A. 1996.p. 211; 267; 343; 429;
485.
RESISTORS. Disponível em:
http://hyperphysics.phytr.gsu.edu/hbase/electric/resis.html#c1 Acessado em 26 jul 2010.
SEMICONDUTORES. A verdadeira História do Transistor. Disponível em:
http://www.bn.com.br/radios-antigos/semicond.htm Acessado em 28 Jul 2010
Lista de textos
The Microprocessor
The 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 Systems
Digital 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 Coding
For 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.
ASCII EBCDIC
Code Code
Parity Checks
Errors 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 Protocols
Serial 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 Protocol
The 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.
Modem
For 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 Protocol
Musical 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.
UART
The 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 Protocols
Parallel 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 Parallel
Hewlett-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) talklisten and 4) talk-listen-control.
Analog-to-Digital Conversion
This 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 ADC
Illustration 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 Conversion
When 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 Timer
Following 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 15
Supply current: 3 to 6 mA
@5V
10 to 15mA@15V
Output current: 200mA
max
Power dissipation: 600mW
8-pin mini DIP
556 is 14 pin dual 555.
One-Chip Regulators
Many, 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