1.0 Digital Integrated Circuits (ICs) and the TTL 74 series.
1.1 Digital Integrated Circuits (ICs).
Digital ICs are a collection of resistors, diodes and transistors fabricated
on a single piece of semiconductor material usually silicon and referred
to as 'chip'. The chip is enclosed in a protective plastic or ceramic package
with pins extended out for connecting the IC to other devices. The most
common type of package is a dual-in-line package (DIP) as shown in FIGURE
1.1. The pins are numbered counterclockwise when viewed from the top of
the package with respect to an identifying notch or dot at one end of the
chip. The DIP below is a 14-pin package. 16, 20, 24, 28, 40 and 64 pin
packages are also available.
All the ICs that will be used in Laboratory 1 are ICs of the SSI category.
With ICs, electronic circuits become much smaller and less expensive. This
expanded the uses of electronics to various sectors such as industrial
, telecommunication, aerospace, computers, calculators and home appliances.
Modern electronics using ICs is rapidly becoming the 'brains and nerves'
of our complex society.
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Approximate number of gates per chip in commercial products. |
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The TTL 74 series is the most widely used family of digital ICs in the SSI and MSI categories. FIGURE 1.2 shows a standard TTL inverter circuit. Notice that it contains several bipolar transistors and this is how the name TTL (transistor-transistor logic) comes.
The TTL logic family actually consists of several subfamilies or series. TABLE 1.2 lists the name of each TTL series together with the prefix designation used to identify different ICs as being part of that series.
The differences between the various TTL series are in their electrical
characteristics such as power dissipation, delay times and switching speed.
They do not differ in the pin layout or logic operations performed by the
internal circuitry. TABLE 1.2 gives the typical values for some of the
more important electrical characteristics of the different TTL series.
The most commonly used TTL series currently is the LS/ALS series.
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Performance
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Voltage parameters | ||||||
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Logic level voltage ranges
FIGURE 1.3 shows the logic- level voltage ranges for TTL ICs. A 0 logic
is any voltage in the range from 0 to 0.8V and a logic 1 is any voltage
from 2V to 5V. Voltages that are not in either of these ranges are said
to be indeterminate and should not be used as inputs to any TTL device.
Current Limiting
Standard TTL inputs should be current limited. If they are being connected
to VCC to provide a logic high
input, a 1kohm series resistor should be used. However, the resistor is
not necessary if an LS type gate is used. When inputs are connected to
either data or logic switches on digit experimenter, no resistors are needed
since these switches are current limited. Generally, no resistors are needed
for inputs connected to signals.