QQQ 2224. Lecture 2

1.

Basic Logic Gates
Lecture 2
Digital Electronics

2.

Course Materials
All needed Software and course materials will
be located on Canvas.
◻ Materials that are used in this slides are taken
from the textbook “Digital Electronics A
Practical Approach with VHDL” by William
Kleitz
◻

3.

Announcements
Office Hours
Thursday
11:00 - 13:00
5th Floor, Open Space

4.

Basic Logic Gates
Basic building blocks
◻ Has one or more input terminals
◻ One output terminal
◻
◻ HIGH (1) or LOW (0) depending on input
◻
Basic logic gates:
◻ AND
◻ OR
◻ NAND
◻ NOR
◻ Inverter

5.

The AND Gate

6.

The OR Gate

7.

Timing Analysis
◻
can be observed on an oscilloscope or a logic
analyzer

8.

(a) Change U1 to a two-input OR gate (OR2).
(b) Change U1 to a three-input AND gate (AND3) and add the third input
waveform.

9.

Enable and Disable Functions

10.

Enable and Disable Functions

11.

The word generator is
used to create the enable
signal (En) and the
clock oscillator (Cp).
Notice that whenever
En is HIGH, the AND
gate passes Cp to the
output at X.
When En is LOW, the
OR gate passes Cp to
the
output
at
Y,
otherwise Y is HIGH.

12.

Review Questions
1.
2.
3.
What input conditions must be satisfied for
the output of an OR gate to be LOW?
A ___________ (HIGH/LOW) level is
required at the input to an AND gate to
enable the signal at the other input to pass to
the output.
What is the purpose of a truth table?

13.

Using IC Logic Gates
◻
gates are available as Ics
◻ 7408 (74LS08, 74HC08) - quad two-input AND
gate
◻ 7411 (74LS11, 74HC11) - triple three-input AND
gate
◻ 7421 (74LS21, 74HC21) - dual four-input AND
gate
◻ 7432 (74LS32, 74HC32) - quad two-input OR
gate
◻
Pin 1 is identified by a small circle next to it or
by a notch cut out between pin 1 and 14

14.

Using the 7408 TTL IC in
the clock enable circuit

15.

Introduction to Troubleshooting
ICs and digital electronic circuits can go bad
◻ Troubleshooting - procedure used to find the
fault, or trouble, in the circuits
◻ understand theory and operation of the
circuit…
◻ two simple tools used to test ICs and digital
circuits
◻
◻ logic pulser – generates HIGH, LOW levels
◻ logic probe – measures levels and indicates them

16.

(a) misalignment of pin 14;
(b) cracked board;
(c) solder bridge;
(d) burned transistor

17.

The Inverter
used to complement, or invert, a digital signal
◻ sometimes referred to as the NOT gate
◻

18.

The NAND Gate

19.

The NOR Gate

20.

Review Questions
1.
2.
3.
4.
Which pins on the 7408 AND IC are used for
power supply connections, and what voltage
levels are placed on those pins?
What is the purpose of an inverter in a digital
circuit?
The output of a NAND gate is always HIGH
unless all inputs are made ___________
(HIGH/LOW)?
The output of a two-input NOR gate is
___________ (HIGH/LOW) If A = 0, B = 1

21.

Logic Gate Waveform
Generation
◻
Johnson shift counter - general-purpose
repetitive waveform generator
Generating a 3-ms HIGH pulse (between
2ms - 5ms) using an AND gate and a
Johnson shift counter

22.

23.

24.

25.

26.

27.

Using IC Logic Gates

28.

IEEE/IEC Standard Logic
Symbols

29.

Combinational Logic
combinations of gates (AND, OR, NAND,
NOR…)
◻ automobile warning buzzer example:
◻
◻ the buzzer activates if the headlights are on and
the driver’s door is opened or if the key is in the
ignition and the door is opened

30.

Q&A
Any Questions?