Rising/falling of input signal

What is the problem when the rising or falling of an input signal is sluggish?

A sluggish rising or falling of an input signal can be caused by any of the following three problems.

(1) Timing variation
Generally, threshold voltage variation occurs in semiconductor products due to variation in manufacturing processes or use conditions.
If rising or falling of an input signal is sluggish, it causes considerable variation in the timing for judging the signal's high level and low level.

In the example shown below, an input signal has a different transition time when compared to another input that has the same threshold voltage variation.
Even though the threshold voltage variation is the same for these two inputs, "Variation 1" shows the slight variation in the timing of judging the low level when the falling edge is not slowed, while "Variation 2" shows a much variation in this timing when the falling edge is slowed.

In addition, "Variation 3" shows the effect when viewed from the perspective of the original signal's transition timing.


Timing variation between two input signals
Almost all systems are designed based on signal timing.
When a large amount of variation occurs in the timing of input signal detection, the timing design must be slowed down enough so that such variation can be ignored.

(2) Problems caused by noise near the threshold voltage
When an input signal changes near the threshold voltage, noise may be superimposed on the signal.

Under such conditions, even a small amount of noise can cause an input signal to exceed the threshold voltage.
In this event the noise is regarded as part of the input signal's fluctuation, and this can lead to problems such as an inverted input sequence when several signals are input or when very short signals are input.
In other words, the system's noise margin becomes very small.


Effects of noise on slowing signal transition
(3) Problems caused by through current
When an input signal reaches an intermediate potential, a through current may flow through the input buffer.
If the input signal has a sluggish rising and/or falling, it may spend a longer time at this intermediate potential, which increases the likelihood of a flowing through current.

The effects of a through current cannot be safely ignored when using a device in standby mode to conserve power, when using a device that has a small operating current, or when otherwise trying to minimize a system's power consumption.

In any case, this problem can have a major impact on the system's design and operations, so we recommend avoiding use of input signals with sluggish rising or falling transitions, unless absolutely necessary.

What standard rise time should I use for the rising of an input signal whose slow rising edge will trigger an interrupt?

Devices do not have standard rise times for edge detection.
For rising edges, it suffices if the level changes from a level that can be detected as the low level to a level that can be detected as the high level.

However, from the system's perspective, if there is also variation in the timing by which this slow rising edge is detected, you should carefully check whether that variation is causing any problems.

Also, a slow rise time means that the signal is more susceptible to noise.
As is shown in the figure below, what should have been a rising edge may be detected as multiple rising edges due to noise effects.

It is also important to check for noise effects because when edge detection fails for a falling edge, if the falling edge is sluggish then noise effects may cause it to be detected as a rising edge, which can trigger interrupts at unexpected times.


Edge detection for signal with sluggish transition