YEDITEPE UNIVERSITY
DEPARTMENT OF INFORMATION AND COMPUTER SCIENCE
EXPERIMENT II: INTRODUCTION TO LOGIC IC
2008 Fall
Objective:
An introduction to the logic IC (integrated circuit) families - functional &
electrical characteristics. A review of the basic logic gates
General Information:
Integrated Circuits (IC)
ICs are physical structures that contain the semiconductor circuits.
Logic Families
Logic gates can be built in a variety of ways. Some of the first computing devices
used mechanical relays to implement logic gates. More recent forms include metaloxide semiconductor (CMOS, PMOS, NMOS), emitter coupled circuit (ECL),
resistor-transistor logic (RTL), diode-transistor logic (DTL), and transistor-transistor
logic (TTL).
The members of the logic families are each identified by a unique part number.
Part numbers are in the form 74 XX YYY where YYY is a two or three digit number
uniquely identifying the particular type of chip. For example the part number of the
chip containing four two-input AND gate is 74XX08. The chip containing four twoinput OR gates is 74XX32.
XX are :
LS : Low Power Schottky (TTL)
ALS : Advanced Low Power Schottky (TTL)
HC : High speed CMOS (CMOS=Complementary MOS)
HCT : High speed CMOS - TTL compatible
In addition to the 74 series, there is a 54 series of chips that is functionally
equivalent to the corresponding 74 series of chips but meet more stringent military
specifications.
In the TTL family, a high logic level is represented by a voltage in the range of
2.0V-5.0V., and low logic level by a voltage in the range of 0.0V.-0.8V. Any voltage
greater than 0.8V. and less than 2.00V is undefined.
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TTL became very popular, very soon, although CMOS was also used by many
other people. CMOS spent much less energy because it was high-impedance and it
was much more sensitive to static electricity.
After a while LS (Low Power Scottky) TTL was invented, probably by Scottky. It
used much less energy and was as fast as normal TTL. Everybody started using LS.
(Except for the fast versions: S and FACT and FAST.)
TTL with it's higher power usage, had always been faster than CMOS. Therefore
CMOS was more often used for analogue and TTL for digital circuits, but after some
time, they invented HC (High speed CMOS) and made High speed CMOS, TTL compatible with it (HCT).
This means, that internally a HCT chip is completely High speed CMOS and at the
inputs and outputs are converters, that convert the levels to TTL.
HCT is mostly compatible with TTL, except, you should never leave an input
open, because the circuit can start to oscillate, which costs a lot of energy and can
disturb the working of the rest of the circuit.
We hardly use TTL anymore, since CMOS has become as fast and uses much less
energy and has become as cheap. Most of the former TTL devices, the famous 7400
range, is now being produced in CMOS as HCT (High Speed CMOS).
Wiring and Testing A Circuit
The first requirement in implementing a circuit after it has been designed is
obtaining the necessary parts. The proper IC (integrated circuit) or chips to perform
the logic functions required can be found by looking in databooks (During this
course you can look them up in Datasheets that will be given to you).
Once part numbers are obtained, it is wise to write the pin numbers for each input
and output used on the chip on your logic diagram. In addition to gate connections,
each chip will require two additional connections for voltage supplying : One for 5
volts(Vcc) and one for ground (0V - GND).
Circuits will be wired on CADET logic designer (consult CADET notes for
breadboarding techniques). CADET has :
-
Breadboards for wiring the circuit,
Power supplies,
Switches for inputs,
LEDs for output monitoring.
Wiring a circuit, especially a complex one, can be very frustrating. To make the
process easier and reduce the chance of error, here are a few simple guidelines that
should be followed :
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1. Always start with the power OFF on the logic designer.
2. Insert the required chips on the breadboard (be careful for orientation, Fig.1.1)
3. Connect all power (Vcc) and Ground (GND) pins to +5V power supply and
Ground (0V) respectively.
4. Connect wires between pins, switches, LEDs, and so on as labeled on your
logic diagram.
5. Visually inspect your circuit and correct any mistakes.
6. Turn on the power and test the circuit.
Useful Links
For abbreviations consult : http://ftp.amc.com/chipdir/abbrev.htm#A
For industry products :
- Philips Semiconductor : http://www.philipslogic.com/products/
- National Semiconductor : http://www.national.com
Lab Equipment :
7408 2 input AND
7432 2 input OR
7400 2 input NAND
7402 2 input NOR
Procedure :
1. AND Gate operation.
(a) Look up the TTL part number for a two-input AND gate, and obtain this chip
from your lab instructor.
(b) Connect the inputs of one of the AND gates to the switches on the logic
designer and the output of that gate to an LED on the logic designer.
(c) Verify proper operation of the AND gate for all input combinations by using
the input switches. Red LED indicates a high logic level or 1 and Green LED
indicates low logic level or 0.
(d) Record the TTL part number, produce a truth table based on the results of (c).
2. OR gate operation. Repeat (1) for a two-input OR gate.
3. NAND gate operation. Repeat (1) for a two-input NAND gate.
4. NOR gate operation. Repeat (1) for a two-input NOR gate
5. Can a NAND gate be used as an INVERTER? If so illustrate how with a two-input
NAND gate.
6. What level does an unconnected input appear to be (high or low)? (Hint: Repeat
one of the previous operations with one input left unconnected) What reasoning
did you use to determine your answer.
Questions :
1. Can a NOR gate be used as an INVERTER? If so illustrate how with a three-input
NOR gate.
2. Determine whether the distributive law applies for OR over AND holds true.
Whatever your case prove your assumption by means of a truth table.
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Hint : X + (Y . Z) = (X + Y) . (X + Z)
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