# Logic Overshoot

Discussion in 'General Electronics Chat' started by DexterMccoy, Apr 13, 2014.

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1. ### DexterMccoy Thread Starter New Member

Feb 19, 2014
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What cause this overshoot voltage?

It's from logic switching

Here is the schematic

look at the overshoot voltage

But what is causing the overshoot voltage?

from switches or logic switches?

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2. ### Wendy Moderator

Mar 24, 2008
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Why the sideways schematic? It does matter to be able to read it clearly you know.

3. ### DexterMccoy Thread Starter New Member

Feb 19, 2014
429
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Just how my phone uploads the pictures im not sure why yet

If the Overshoot is greater than -5 volts, the overshoot is bad

My question is what causes the overshoot? is this capacitance?

Why does switching or logic switching cause overshoot?

Feb 28, 2009
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5. ### BillB3857 Senior Member

Feb 28, 2009
2,402
348
Switching causes transients. Distributed capacitance and inductance can set up a resonant circuit that will tend ring at a given frequency. That is only one reason.

6. ### Wendy Moderator

Mar 24, 2008
20,772
2,540
Pretty lame excuse. You never use a computer to access this site? It takes about 1 minute to post clearly, and it does bother people who are trying to help you.

I resized and rotated this image after I wrote this post.

Overshoot is commonly caused by inductance. Just like you can light up an LED that will not respond to 1½VDC, inductance (and in a view other cases, capacitance) can boost voltage as a spike. It requires a fast transient response for inductance to do this though.

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7. ### DexterMccoy Thread Starter New Member

Feb 19, 2014
429
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Where is the inductance or capacitance coming from , this boost of voltage as a negative spike from switches or logic switching?

8. ### BillB3857 Senior Member

Feb 28, 2009
2,402
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Any wire or conductor has inductance. Any conductor has capacitance formed between it and any other conductive surface.

9. ### Wendy Moderator

Mar 24, 2008
20,772
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And a wire at a specific length has a resonant frequency. This is basic antenna theory.

10. ### DexterMccoy Thread Starter New Member

Feb 19, 2014
429
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So it has nothing to do with the switching transition from high to low ? the switching Voltages

11. ### BillB3857 Senior Member

Feb 28, 2009
2,402
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The switching action creates a change which starts the resonance. Hit a bell and it keeps ringing. Same thing.

12. ### DexterMccoy Thread Starter New Member

Feb 19, 2014
429
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So when switching from low to high or from low to high, this change creates resonance and capacitance? inside the Logic IC chip or a switch

13. ### vk6zgo Active Member

Jul 21, 2012
677
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God,Mother Nature,fate,the Laws of Physics---take your pick!

14. ### Wendy Moderator

Mar 24, 2008
20,772
2,540
Only physical parts create inductance or capacitance. Signals respond to it.

What level are you at in school?

15. ### DexterMccoy Thread Starter New Member

Feb 19, 2014
429
2
So not Logic Switching causing Overshoot voltages?

Only Physical Manual Switches do this?

So it's the Internal poles and wafer that is causing the inductance and capacitance? overshoot voltage?

16. ### BillB3857 Senior Member

Feb 28, 2009
2,402
348
Anything that causes voltage or current to change quickly cause it.

17. ### Wendy Moderator

Mar 24, 2008
20,772
2,540
Resonance, like capacitance and inductance, exists because of physical layout and components. Even printed circuit board leads are components. These are all physical characteristics.

Signals react to components. They do not create anything but electrical activity. Why is this a hard concept for you?

You have been accused of trolling. You have claimed to be a high level tech, while asking questions whose answers I knew at 13 years old. This is OK, we allow beginners questions, but you don't seem to get how low level some of these questions truly are.

That's OK, you are in my home. I will answer each and all your questions to the best of my ability, but I will point out when you are repeating yourself, such as now.

Just curious, do you ever build electronic projects? I personally consider it great fun, and have learned a lot doing so.

18. ### MrChips Moderator

Oct 2, 2009
12,625
3,451
You are ignoring one very important design feature of logic outputs.

All components have capacitance. Capacitance has the ability to store charge.

The outputs of logic gates are commonly designed with two switching transistors in a push-pull arrangement known as "totem-pole" outputs. The capacitance at the output is critical in achieving high switching speeds. High capacitance slows down the transition speeds of logic outputs.

The output switching transistors in a totem-pole arrangement operate in complimentary fashion. When one switch is open the other one is closed.

If the switches operate in "break-before-make" mode the output capacitance causes the logic output to linger and thus slows down the signal.

In order to speed up the switching of the logic output, the switches are designed to "make-before-break". This creates a temporary short across the power supply rails, increases the current flow and reduces the switching time.

However, there is an adverse consequence of having high switching speeds. The "make-before-break" creates a temporary short across the power supply rails and increases the supply current. This creates a problem called "ground bounce" and supply glitches. In other words, the supply rail will experience a momentary dip in voltage while the ground will experience a momentary rise.

This is a particularly significant problem with byte-wide bus drivers when all eight bits can be expected to switch simultaneously in the same direction occasionally, from logic low-to-high or logic high-to-low.

For this reason, placing a 0.1μF capacitor across the power supply pins is mandatory for all types of logic circuits.

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19. ### DexterMccoy Thread Starter New Member

Feb 19, 2014
429
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So this doesn't have any TTL or CMOS logic switching overshoot voltage?

So this creates the overshoot voltage during TTL and CMOS logic switching?

20. ### DexterMccoy Thread Starter New Member

Feb 19, 2014
429
2
Mr. Chips I'm really confused

Are you saying that inside , internally inside the TTL or CMOS logic chips it is either switching make before break or break before make?

Cause the TTL and CMOS internally switching causes transients to get bleed into the power rails if you don't add decoupling caps