Asynchronous

Question 1

What is activity controled by?

Answer 1

The input variables

Question 2

What changes the internal state?

Answer 2

a change in the input

Question 3

When is a state unstable?

Answer 3

If it has no sling

Question 4

What indicates a stable state in a flow table?

Answer 4

circle round it, shows it can be detected by oscilloscope

Question 5

Why do problems arise with the logic in feedback paths?

Answer 5

different time delays within different parts of the logic. Cause a glitch which will make the machine jump to another stable but unwanted state

Question 6

Do all the bits change at the same time?

Answer 6

• Nope, it is very unlikely that all three bits will change at the same time
• during the transition period, other codes will be formed which will correspond to other, unexpected states.

Question 7

What is a zero-hot code?

Answer 7

000000

Question 8

What is a one-hot code?

Answer 8

000001

Question 9

What is a two-hot code?

Answer 9

000011

Question 10

Why would you reduce the number of implicants?

Answer 10

Most devices have a limited number of OR gates

Question 11

What happens if all the inputs change at the same time?

Answer 11

The machine will fail

Question 12

What are the three race hazards?

Answer 12

1) Static
2) Dynamic
3) Essential

Question 13

What is a static race hazard?

Answer 13

• Caused when the signal and its complement are combined together
• results n the machine jumping to an unexpected state
• SOLUTION is to include all prime implicants

Question 14

What is a dynamic race hazard?

Answer 14

• Caused by and interaction between gates.
• result in multiple transitions when only one was intended
• SOLUTION don’t use more than two levels of gating

Question 15

What is an essential race hazard?

Answer 15

• Caused when an input has two transitions and one is less than the total feedback propagation
• Result means the sorter input will probably be ignored
• SOLUTION extending the input signal duration, delay lines

Question 16

What are the steps in asynchronous (and synchronous) FSM design stages?

Answer 16

1) Moore model state diagram
2) flow table
3) iterate round the previous stages
4) determine number of required states
5) code assignment
6) redraw state diagram
7) Inert dummy states if needed
8) Transition table
9) extraction of conical transition equations
10) extraction of prime implication (minimises race hazards)
11) implement

Question 17

What is the golden rule?

Answer 17

• Inputs appear on transitions

- output appear with states

Question 18

If there are N inputs, how many equivalent transitions are there leaving every state?

Answer 18

2^N

Question 19

Can you have 2 of the same transitions leaving the state?

Answer 19

No, only 1

Question 20

Why is it unlikely to have the same condition looping around states?

Answer 20

Would lead to rapid oscillation between states

Question 21

What are truth tables good/bad for?

Answer 21

• good for basic gates

- bad for multi-input multi-output systems

Question 22

What is Sum of products?

Answer 22

• When all the terms use all logical values
• Canonical form
• Minterms

Question 23

What is product of sums?

Answer 23

• would use deMoragn

- there are 2^N maxterms

Question 24

What is the difference between a state table and a flow table?

Answer 24

STATE TABLE
-used to represent every transition in a synchronous finite state machine
-Transitions are implied on clock edges.
-same number of rows as states and the number of destination columns is two to the power of the number of inputs.
FLOW TABLE
-used to represent every transition in an asynchronous finite state machine
-transitions occur on input signal levels so the state is always ‘flowing’ from source-to-destination

Question 25

What is demorgans for f = A.B.C

Answer 25

f(bar) = A(bar) + B(bar) + C(bar)

Question 26

What are the AND rules?

Answer 26

``` A.A(bar) = 0 A.1 = 0 A.0 = 0 A.A = A A.(A+B) = A ```

Question 27

What are the OR rules?

Answer 27

``` A + A(bar) = 1 A + 1 = 1 A + 0 = A A + A = A A + (A.B) = A ```

Question 28

What kind of implicant is a prime implicant?

Answer 28

- It is not an implicant of the function | - It is often the outer or largest group on the k map

Question 29

What is na essential prime implicant?

Answer 29

-Name of a group which includes at least '1' not covered by any other group

Question 30

What is the minimal sum?

Answer 30

-Smallest number of prime implicants requires to describe the function

Question 31

What does the minimal sum =?

Answer 31

minimal sum = essential prime implicant + some prime implicants

Question 32

Why do asynchronous FSMs only used AND, OR and INVERTERS?

Answer 32

Flip-flops require a clock signal and gates can change instantaneously without a clock signal

Question 33

YooHoo

Answer 33

Boobies