4+5

Question 1

What does a not gate look like? What about an AND, NAND, OR, NOR and XOR??

Answer 1

not: triangle with circle at pointy end, one input one output.
AND: multiple inputs into one semicircular object, one output.
NAND: AND with circle at start of output.
OR:More curved input and more pointy output than AND
NOR: Same as OR but with circle at output.
XOR: Like OR but with line before input entry.

Question 2

What is a combinatorial logic circuit?

Answer 2

One whose outputs are only dependent on the inputs.

Question 3

How can we define a combinatorial circuit?

Answer 3

Truth table(for each of the 2^n possible combinations of input show binary value of output).
Boolean equations
Graphical signals(interconnection of gates).

Question 4

What is a 1 bit half adder? How can we make one?

Answer 4

Gives the result of adding 2 binary digits together. Connect input A and B to a XOR gate and an AND gate. AND gate does carry(2nd bit), XOR gate does sum.

Question 5

How does a full adder work?

Answer 5

Adds together three 1 digit binary numbers, A and B got into a XOR and AND. The XOR goes into a XOR(which outputs the lowest value digit) and AND with C. Then both ANDs go into an OR gate which does the second binary output digit(carry).

Question 6

What are the boolean operators and their precedence?

Answer 6

• (NOT) 1
  • (AND) 2
  + (OR) 3

Question 7

What is De Morgan’s theorem?

Answer 7

The inverse of the result of A OR B is = to the inverse of A AND the inverse of B.
The inverse of the result of A AND B is = to the inverse of A OR the inverse of B

Question 8

What is a transistor?

Answer 8

An electronic switch, it has a base a collector and an emitter. If 0V is applied to B the switch will be open, if +5V is applied to B the switch is closed. If the switch is closed then the signal can travel from the collecter to the emitter, else it cannot.

Question 9

How can we make a NAND gate with transistors?

Answer 9

A power supply supplies a simple circuit with Q being the important part. Two NAND gates allow this path to Q to be short circuited if both are closed.

Question 10

How can we make an electronic switch?

Answer 10

output = A if and only if C = 1. So just do an and gate with C and A as inputs.

Question 11

What is a multiplexer? How about a demultiplexer?

Answer 11

A multiplexer is a digital switch which allows select lines to select which of the inputs gets passed through. The demultiplexer does thjis in reverse, choosing which output to send the signal too based on the select lines.

Question 12

What is a sequential logic circuit? What are the two types?

Answer 12

Output depends on history as well as present inputs.
These can be synchronous:
input changes between clock pulses, state changes at clock pulses
Or asynchronous: State changes occur as changes in inputs occur.

Question 13

What is a Set-Reset latch?

Answer 13

Tie the output of the circuit to one of its input, this allows the circuit to save state. This can be done by combining two NOR gates, one input for each comes from the other ones output. We can then set the latch by setting one of them to 1, or reset the latch by setting the other one to 1.

Question 14

What is an SR- Flip-Flop? What is a D-type flipflop?

Answer 14

We can make a set reset latch which can be turned on and off as we want by adding an enable line which is connected to the set and reset lines seperately with AND gates.
A D-type flipflop is the same but with only one line of input, as S is the inverse of R.

Question 15

How can we use a Data flip flop to store data?

Answer 15

Set or reset D, toggle enable line, storing D, toggle enable line again. Now D can vary without changing stored value.

Question 16

What is special about the JK flip-flop?

Answer 16

Functions like a T-flip-flop(SET-RESET) if J = K or D flip flop if not.

Question 17

What is an asynchronous ripple counter? What is a problem?

Answer 17

A 1-bit counter switches between 0 and 1 each cycle. This essentially divides clock time by 2. A two-bit counter toggles the low bit every full clock cycle and the high bit at half the clock cycle rate. It is made using a JK flip-flop.
The propagation delay is inconsistent, varying based on how many transistors the signal passes through.

Question 18

What is a synchronous counter?

Answer 18

Flip-flops clock at same time by adding logic to next state based on previous state.

Question 19

How can we detect a given input sequence?

Answer 19

Use states represented by flip flops, connecting them logically to allow for state transitions.

Question 20

What is a tristate buffer?

Answer 20

Creates a physical connection or physical disconnection between input and output wires. The output will be the input or a special state called high impedence.