Propagation delay Flashcards
What is propagation delay?
This is the delay in time of a digital circuit as the sum of the propagation delay of all of its components and the time a signal takes to traverse between components
What can propagation delay affect?
The frequency that a circuit can operate at
What is t_PHL?
The time between a transitions on the input and a transition on the output when the output is changing from HIGH to LOW
What is t_PLH?
This is the time between transitions on the input and a transition on the output when the output is changing from LOW to HIGH
What is the reference point at which t_PHL and t_PLH are taken?
at 50% between the LOW voltage and the HIGH voltage
What is the difference in time for t_PHL and t_PLH
They are not necisarity equal
What is a race condition?
This is when the output of a gate depends on nearly simultaneous events occuring on its input but whichever signal arrives first will change the output
Explain an erroneous output
This is when the output flickers between two values because a race condition has occured
How do we solve eroneous output?
We wait until the output is valid (this is the point when all race conditions have ended)
How is the validity time calcualted?
By determining the time for the critical path
What is the critical path?
This is the signal path with the maximum propagation delay
How is the maximum frequency calculated?
f_max = 1 / prop_delay
What are ways that we can avoid large propagation delays causing issues?
> Reducing the clock frequency
> Breaking the logic up into stages (called pipelining)
Describe pipelining
> To reduce propagation delays, we break up the circuit into smaller blocks with lower propagation delays
Registers are added between blocks
What is the penulty of pipelining?
It requires multiple clocks to complete a single process
