# How is rise/fall time definition

I understand that rise/fall time are related to RC charging/discharging waveforms.

What I don't understand is how is the 0% and 100% points chosen? Do they have to be defined beforehand for a signal?

My oscilloscope will automatically find rise/fall time with 10-90 gating like the picture below for an arbitrary input. How does it automatically choose a 100% point if an RC only asymptotically approaches some value? How does it know to avoid rining at the top of the waveform?

Update 1: Request to check Oscope manual.

"The rise time of a signal is the time difference between the crossing of the lower threshold and the crossing of the upper threshold for a positive- going edge. The X cursor shows the edge being measured."

My confusion is that I never set thresholds. The measurement can be done automatically by just choosing "rise time". Not sure if it just defaults to max and min of what's on screen for 0 and 100%.

My question is more than for just a particular O-scope tho. I'm trying to understand if there is a commonly accepted rigorous definition for this. Even outside an O-scope measurements. If I draw the waveform above, how do you chose the 0 and 100% points?

• What does the o-scope manual tell you? Jul 24 '18 at 8:54
• Hi @Andyaka. Update #1 added. My question is more general tho. I want to know how this is defined even beyond just the measurement of my particular Oscope. Jul 24 '18 at 18:44
• The points are chosen by some algorithm built in scope post processor, which uses certain assumptions. In most cases these assumptions are like "trade secrets" and are not disclosed. So these assumptions may or may not be true for every input signal. That's why I never take these automatic measurements seriously. Sep 25 '18 at 17:24

## 1 Answer

There is an accepted definition for how to calculate settling time such that the waveform has reached the final value within a desired tolerance. https://en.wikipedia.org/wiki/Settling_time

So, after the settling time, the waveform is at 100% +/- the tolerance.

In reality, of course, you take VDDD to be 100% and 0V to be 0% (or whatever is suitable for the type of output you're looking at).