1. Chapter 3 (part 1)
Basic Logic Gates 1

2. Introduction
Logic gates are the basic building blocks for forming digital electronic circuitry.
A logic gate has
One output terminal; One or more input terminals
The outputs is either HIGH(1) or LOW(0) depending on the digital levels at the input terminals and the property of the gate
Five basic logic gates
AND, OR NAND, NOR and inverter

3. 3-1 The AND Gate
The output, X, is HIGH if input A AND input B are both HIGH.
If A = 1 and B = 1, then X = 1.
If A = 0 or B = 0, then X = 0.
Schematic symbol of two-input AND gate

4. Truth table for AND Gate
Resultant output
All possible input combinations

5. Figure 3-2 Use AND gate to activate a burglar alarm
The output of the AND gate (A) goes HIGH to turn on the alarm, if the alarm activation key (K) is in the ON position AND the front door (D) is opened.

6. Figure 3.3 Electrical analogy for an AND gate
(a) using manual switches; (b) using transistor switches.

7. Boolean equation for AND Gate
Boolean Equation: X = A•B or X = AB
Read as “X equals A AND B”
Can have more than two inputs
Equation 3-1
Number of combinations in truth table = 2N
where N = number of inputs

8. Figure 3-4 Multiple-input AND gate symbols

9. Column A lists eight 0s, then eight 1s;
Column B lists four 0x, four 1s, four 0s, four 1s.

10. 3-2 The OR Gate
The output at X will be HIGH whenever input A OR input B is HIGH or both are HIGH
If A = 1 and/or B = 1, then X = 1
If A = 0 and B = 0, then X = 0
Schematic symbol of two-input OR gate

11. Truth table for the OR Gate

12. Figure 3.6 Electrical analogy for an OR gate
(a) using manual switches; (b) using transistor switches.

13. Boolean equation for the OR Gate
Boolean Equation: X = A + B
Read as, “ X = A or B”
Can have more than two inputs
Number of combinations = 2N
N = number of input bits

14. Figure 3-7 Multiple-input OR gate symbols
8 input OR gate has 256 entries.

15. Figure 3.8 eight-input OR gate symbol

18. 3-3 Timing Analysis
Timing diagrams are used to analyze the output response to varying inputs
Oscilloscope
Display plots voltage versus time
Dual-trace can display two waveforms at a time.
Logic analyzer
Can display state tables that show binary levels on signal lines
Can display up to 16 waveforms at a time

22. 3-4 Enable and Disable Functions
AND and OR gates can be used to enable or disable a waveform from being transmitted from one point to another.
Example: Use a 1-MHz clock oscillator to transmit only four pulses
Solution: Enable four clock pulses to be transmitted, and then disable the transmission from then on.

23. Enable function using AND gate Figure 3-16
Clock frequency is 1MHz
The circuit and waveforms to enable four clock pulses. Low -> disable

24. Disable function using OR gate Figure 3-17
High -> disable

25. 3-5 Using IC Logic Gates AND and OR gates are available as ICs.
Data Manual from manufacturer includes
IC pin layout, Logic gate type, Technical specifications
Example ICs
7408(74LS08,74HC08): quad two-input AND gate
7411(74LS11,74HC11):dual four-input AND gate
7432(74LS32,74HC32):quad two-input OR gate

26. Common prefixes and suffixes
DIP
Family designation
LS – Lower power TTL family
HC – High-speed CMOS family
(74LS11,74HC11):dual four-input AND gate
Manufacturer
SN: Texas Instruments. E.g., SN7400
Package style (suffix)
N: plastic dual-in-line package (P-DIP), SN7400N
M: Small-outline Integrated Circuit (SOIC)
SOIC

27. Figure 3.19 The 7408 quad two-input AND gate IC pin configuration.
The top side typically has
Indented circle / notch cut

28. Figure 3.20 Using the 7408 TTL IC in the clock enable circuit of Figure 3–17.
The clock oscillator signal passes on to the receiving device when the switch is in the enable (1) position, it stops when in the disable (0) position.

29. Figure 3.21 Pin configurations for other popular TTL and CMOS AND and OR gate ICs:
(a) 7411 (74HC11);
(b) 7421 (74HC21);
(c) 7432 (74HC32).

30. 3-6 Introduction to Troubleshooting Techniques
The procedure used to find the fault or trouble in a circuit.
Tools to test ICs and digital circuits
Logic Probe shows digital levels: Low, High, and floating - open circuit, neither high nor low
Logic Pulser provides digital pulses to a circuit being tested.
Both tools allow you to tell if the pulse signal is getting through the IC as expected.
Indicator lamp glows strongly if high
Lamp is off if output is low
Glows dimly if floating

31. Apply power to Vcc and GND.
Check each AND gate with the pulser/probe.
When pulser is on pin 12, Vcc is connected with pin 13, the light in the end of the probe flashes at the same speed as the pulser -> good.
Reverse the connections to pins 12 and 13 and check the probe again.

32. Apply power to Vcc and GND.
Check each OR gate with the pulser/probe.
When pulser is on pin 2, GND is connected with pin 1, the light in the end of the probe flashes at the same speed as the pulser -> good.
Reverse the connections to pins 1 and 2 and check the probe again.

33. Gate 2
Gate 3
Gate 2
Gate 3

34. Gate 1
Gate 4
Gate 1
Gate 4