Discussion of Key Concepts
 

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.
 

 
 
 
 
FIGURE 1.1  (a) Dual-in-line package
 
 
 
                 FIGURE 1.1(b) Top view showing pin numbers.
       Digital ICs are often categorized according to their circuit complexity as measured by the number of equivalent logic gates in an IC. There are currently five standard levels of complexity as in TABLE 1.1.

       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.
 

 
 Complexity
Approximate number of gates per chip in commercial products.
 Typical products
Small-scale integration (SSI)
Less than 12
Logic gates, flip-flops
Medium-scale integration (MSI)
12 to 99
Counters, multiplexers, adders
Large-scale integration (LSI)
100 to 9999
8 bit microprocessor, ROM, RAM
Very large-scale integration(VLSI)
10,000 to 99,999
16 and 32 bit microprocessors, sophisticated peripherals
Ultra large-scale integration (ULSI)
100,000 or more
64 bit microprocessors, special processors, real time image processing
TABLE 1.1
        1.2    TTL 74 Series

        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.

FIGURE 1.2 TTL Inverter Circuit

        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.
 

 
 
74
74s
74LS
74AS
74ALS
74F
Performance 
rating
Progation delay(ns)
9
3
9.5
1.7
4
3
Power dissipation(mW)
10
20
2
8
1.2
6
Speed-power product(pJ)
90
60
19
13.6
4.8
18
Max. clock rate(MHz)
35
125
45
200
70
100
Fan-out(same series)
10
20
20
40
20
33
Voltage parameters
VOH (min)
2.4
2.7
2.7
2.5
2.5
2.5
VOL (max)
0.4
0.5
0.5
0.5
0.4
0.5
VIH (min)
2.0
2.0
2.0
2.0
2.0
2.0
VIL (max)
0.8
0.8
0.8
0.8
0.8
0.8
TABLE 1.2 Typical TTL Series Characteristics
 
        1.2.1    TTL Inputs
 
        Power and Ground
        Referring to FIGURE 1.1 all TTL ICs have a dcpower supply voltage connected to one of their pins labeled as GND. VCC in the range of 4.5V to 5.5V is required. Typically 5V power supplies are used. TTL ICs will not work reliably with dc supply voltages outside of this recommended maximum of 5.5V is exceeded.

        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.

 FIGURE 1.3 Logic level voltage ranges for TTL ICs.
        Unconnected (floating) Inputs
        If a TTL input is left unconnected, the gate generally reacts as though the input was high. However, an open TTL input is noise-prone. So the high state is not reliable. If only gate is being used on a multiple gate chip, the inputs to the unused gates may be left open in the breadboard construction in the lab.

        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.