Civil Engineering Reference
In-Depth Information
The 50% delay
(often referred to simply as the delay) of a waveform relates
to the amount of time from the instant when a stimulus is applied to when
its response asserts itself. In the context of a digital circuit‚ it is defined as
the interval that elapses between the time when the input waveform crosses a
specified threshold‚ and when the output waveform crosses a given threshold.
Although these two thresholds could‚ in principle‚ be different from each
other‚ they are most commonly set to be at the halfway point‚ also known
as the 50% point‚ of the waveform transition.
The transition time
of a waveform is related to the “slope” of the waveform‚
and is typically defined as the time in which the waveform goes from
to of its final value. Since most such transitions involve exponentials‚
the most commonly used measures correspond to the 10%-90% transition
time and the 20%-80% transition time. However‚ if the signal is modeled
as a piecewise linear function (such as a saturated ramp)‚ a 0% to 100%
transition time is meaningful.
Although finding the most exact values of parameters such as the delay or the
transition time of a waveform requires rigorous circuit simulation‚ quicker and
more approximate estimates may be obtained using timing analysis techniques.
A notable fact is that it is indeed possible‚ under this definition of the 50%
delay‚ to achieve a negative delay! An extremely slow input waveform applied
to the input of a logic stage‚ as shown in Figure 3.2‚ may actually result in
the output rising at a faster rate‚ as a result of which the difference between
the 50% crossing points of the output and the input is negative. This is not
