is Switch Bounce BAD? Learn what it is, and How To Fix
it! - In order to fix it, you Have to Know Why it
happens, and its effect on microcontroller inputs. Find
out Everything you Need to Know Here.
Why is switch bounce bad?
Understand the effect of switch bounce on
a microcontroller input.
Find out why a microcontroller may crash
when attaching a button to an input.
Find out the real reason for switch bounce.
Learn three different solutions to switch
Why is switch bounce bad? To answer that first you have
to know what switch bounce is and what causes
reason for a bouncing switch
Whey you hit any button, on any interface, with any
computer you are interacting with a mechanical device.
Mechanical interfaces always have unforeseen actions
such as lag, inertia or even too fast i.e. they don't
operate in exactly the way you thought.
When you push the button down, the button automatically
goes back up using a spring that is always pushing back
on your pushing of that button.
The spring causes the switch bounce bad problem. As you
push down the button the metal contact completes the
circuit but a the same time the spring pushes the button
When you hit the button, energy is put into the spring
so it can contract and expand really fast and while the
button contact is not quite fully pushed down the gap
allows the contact spring to disconnect from the
This causes the push button to generate multiple
connections and disconnections to your circuit i.e. it
oscillates and causes what is known as switch bounce.
is switch bounce bad?: Why you don't think there's a
The thing is that if you attached a light (e.g. an LED) in
circuit with the switch, you would see the light go on
when the button is pushed, and go off when the button is
It's all fine and operates exactly as you would expect.
Except that it isn't - but only when you take a closer
look at the operation of the switch over a smaller time
of a switch bounce problem
Instead of attaching an LED to the switch, if you
attach the switch to the input of a microcontroller and
display the readings from that input pin over time you
would see a logic level something like:
In other words you would see a signal that starts
inactive at '1' (input pulled high) and active '0'
(switch pushed pulling input low), but in between there
would be multiple jumps between the initial state '1'
and the final resting state '0'.
This can happen over a period of 100s of
milliseconds (millisecond = 1e-3s or 1/1000th
of a second).
The problem is that a microcontroller can operate over
a period of microseconds (microsecond = 1e-6s or
happens to a micocontroller when it gets a switch bounce
A microcontroller is a dumb device - it does exactly
what you tell it to do. So if you set the
microcontroller to read the input pin and take action
when the input is active there will be problems.
If the input pin oscillated as above then the
microcontroller has no way of knowing that the
input is bouncing - it just assumes that you are pushing
and releasing the button really fast!
It means you get multiple inputs from a pin when you
only pushed it once!
Why is switch bounce
bad?: Keypad problem
For the case of an LED output it does not really matter
but for everything else it matters a lot. e.g. what if
the button was used as a number input from a keypad - it
is impossible to enter the correct value.
Why is switch bounce
bad?: Interrupt problem
The above example is fairly benign as you would have to
finalise the input by pressing another key, and so could
A far worse problem is if the input is attached to an
interrupt pin. For example perhaps you want an emergency
stop button that reacts quickly to the push button - the
interrupt pin is a logical choice.
The problem here is that the microcontroller could be
locked up in servicing the interrupt so that it never
executes main code i.e. the whole system crashes and
stops working altogether!
are some switches worse than others?
All input buttons cause switch bounce but some cause
more than others. The reason is the physical
construction of the push button and the strength of the
to solve the switch bounce bad problem.
is switch bounce bad?: First observe the switch
There are several techniques to solve switch bounce but
you should observe the bouncing signal on an
oscilloscope to optimise the bounce solution for your
The following buttons have associated bounce
From the screen shots below you can see that switch
bounce varies with the type of switch you use:
A micro button (only really bounces at the rising
The time scale is 1 millisecond.
A physically larger button:
The time here is 20ms per major graticule so bouncing
lasts ~170 milliseconds!
Why is switch bounce
bad?: Choose a solution
There are several solutions to switch bounce and you
need to choose which you use based on the type of switch
bounce you observe.
A filter capacitor RC pair can remove the very fast
switch bouncing but you have to add a capacitor and
resistor the the switch output.
Use a software only delay function. You can see that
the main bouncing events occur near the beginning and
end of the 'push'. So using a delay to ignore the
initial bounce does work. In fact this method is used
a great deal.
There as also a very unusual shift register
debounce method that usesa purely software shift
register - this also works very well and means you
don't need to add any extra hardware.
All these methods are explored in much more detail
(hardware setup and programming and explanation)
in this page: Easy Switch Debounce.