Asynchronous Flashcards
What is activity controled by?
The input variables
What changes the internal state?
a change in the input
When is a state unstable?
If it has no sling
What indicates a stable state in a flow table?
circle round it, shows it can be detected by oscilloscope
Why do problems arise with the logic in feedback paths?
different time delays within different parts of the logic. Cause a glitch which will make the machine jump to another stable but unwanted state
Do all the bits change at the same time?
- 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.
What is a zero-hot code?
000000
What is a one-hot code?
000001
What is a two-hot code?
000011
Why would you reduce the number of implicants?
Most devices have a limited number of OR gates
What happens if all the inputs change at the same time?
The machine will fail
What are the three race hazards?
1) Static
2) Dynamic
3) Essential
What is a static race hazard?
- 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
What is a dynamic race hazard?
- 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
What is an essential race hazard?
- 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
What are the steps in asynchronous (and synchronous) FSM design stages?
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
What is the golden rule?
- Inputs appear on transitions
- output appear with states
If there are N inputs, how many equivalent transitions are there leaving every state?
2^N
Can you have 2 of the same transitions leaving the state?
No, only 1
Why is it unlikely to have the same condition looping around states?
Would lead to rapid oscillation between states
What are truth tables good/bad for?
- good for basic gates
- bad for multi-input multi-output systems
What is Sum of products?
- When all the terms use all logical values
- Canonical form
- Minterms
What is product of sums?
- would use deMoragn
- there are 2^N maxterms
What is the difference between a state table and a flow table?
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
What is demorgans for f = A.B.C
f(bar) = A(bar) + B(bar) + C(bar)
What are the AND rules?
A.A(bar) = 0 A.1 = 0 A.0 = 0 A.A = A A.(A+B) = A
What are the OR rules?
A + A(bar) = 1 A + 1 = 1 A + 0 = A A + A = A A + (A.B) = A
What kind of implicant is a prime implicant?
- It is not an implicant of the function
- It is often the outer or largest group on the k map
What is na essential prime implicant?
-Name of a group which includes at least ‘1’ not covered by any other group
What is the minimal sum?
-Smallest number of prime implicants requires to describe the function
What does the minimal sum =?
minimal sum = essential prime implicant + some prime implicants
Why do asynchronous FSMs only used AND, OR and INVERTERS?
Flip-flops require a clock signal and gates can change instantaneously without a clock signal
YooHoo
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