Chapter 1: Logic gates Flashcards
What is an analogue signal?
-It gives an an analogy-copies the behaviour of a physical quantity. A temperature sensor outputs a voltage which copies the behaviour.
-It can have any value between the minimum and maximum of the power supply
What is a digital signal?
-It carries information in the form of a number. Electronic systems usually employ the binary number system
-It has only 2 possible values: 1 or 0.
When a digital signal is at minimum voltage it is referred to as what logic signal?
Logic 0
When a digital signal is at maximum voltage it is referred to as what logic signal?
Logic 1
What are logic gates?
Logic gates are the basic building block of all digital electronic systems. They are the decision making units in electronic systems and there are different types for different
situations.
State the truth table for a NOT gate
A Q
0 1
1 0
State the Boolean expression for the NOT gate
Q=A
State the truth table for an AND gate
B A Q
0 0 0
0 1 0
1 0 0
1 1 1
State the Boolean expression for the AND gate
Q=A.B
State the truth table for an OR gate
B A Q
0 0 0
0 1 1
1 0 1
1 1 1
State the Boolean expression for the OR gate
Q=A + B
State the truth table for NAND gate
B A Q
0 0 1
0 1 1
1 0 1
1 1 0
State the Boolean expression for the NAND gate
Q=Ā.
State the truth table for a NOR gate
B A Q
0 0 1
0 1 0
1 0 0
1 1 0
State the Boolean expression for the NOR gate
State the truth table for a two input XOR gate
B A Q
0 0 0
0 1 1
1 0 1
1 1 0
State the truth table for a 3 input XOR gate
C B A Q
0 0 0 0
0 0 1 1
0 1 0 1
0 1 1 0
1 0 0 1
1 0 1 0
1 1 0 0
1 1 1 1
The output Q is logic 1 when either A OR B OR C is logic 1, or A AND B AND C are all logic 1 but NOT
when any two inputs are logic 1.
State the Boolean expression for the XOR gate
Q=A⊕B
State the truth table for a 2 input XNOR gate
Inverted form of the XOR gate
B A Q
0 0 1
0 1 0
1 0 0
1 1 1
State the truth table for a 3 input XNOR gate
Exact opposite of the 3 input XOR gate
C B A Q
0 0 0 1
0 0 1 0
0 1 0 0
0 1 1 1
1 0 0 0
1 0 1 1
1 1 0 1
1 1 1 0
State the Boolean expression for the XNOR gate
What counts as logic 1? What counts as logic 0?
70% of the power supply. 30% of the power supply. Anything in between is a faulty chip
What is a sourcing current?
When a load is connected to a logic gate in such a way that the logic gate supplies current to the load, the logic gate is said to be sourcing current.
What is a sinking current?
When a load is connected so that current flows from the power supply through the load to the logic gate, then the logic gate is said to be sinking current.
A.1 =
A.1=A
A.0=
A . 0 = 0
A.A=
A . A = A
A. Ā =
A . Ā= 0
A+1=1
A + 1 = 1
A+0=
A + 0 = A
A+A=
A + A = A
A + Ā=
A + Ā = 1
What is de Morgan’s Theorem?
Break the line, change the sign or vise versa
Each square in a Karnaugh map corresponds to what?
A cell in a truth table
How would you complete a Karnaugh map using a truth table?
First find all the ‘cells’ where the output is high. Place ones into the corresponding Karnaugh map squares. Then fill the remaining squares with 0.
How would you complete a simplified Boolean expression using a Karnaugh map?
Identify groups of 2,4,8 which contain logic 1. Then finds the terms to each group and then combine them. This gives a simplified expression of Boolean.
How would you create an AND using NAND
NAND gate followed by a NAND inverter with its two inputs connected together
How would you create a NOT gate using NAND gates?
A NAND gate with its two inputs connected together
How would you create an OR gate using NAND gates?
Two NAND gates, connected in parallel, with their outputs feeding into another NAND gate
How would you create a NOR gate using NAND gates?
Two NAND gates, connected in parallel, with their outputs feeding into another NAND gate, which then gets inverted
How can some systems of just NAND gates been simplified?
By eliminating redundant gates?
What is a Multiplexer?
A device which is able to transfer a signal from any one of a number of inputs to an output, using a combination of digital signals applied to its input select pins
What are the S inputs connected to in a Multiplexer?
C,B,A
One disadvantage of a multiplexer?
Much more expensive than logic gates
One advantage of a multiplexer?
They can be reprogrammed to produce the output of any logic gate.
On a multiplexer, what does input 0000 select?
D0
On a multiplexer, what does input 1111 select?
D7
On a multiplexer, D inputs are connected to what?
Logic 1 or logic 0
What is the region between Logic 1 and logic 0?
Open band
If a circuit is in the region between logic 1 and logic 0, how does this effect the overall circuit?
It results in highly unpredictable behaviour.
