How to reduce static in a CPU based ite
How to reduce static in a CPU based ite
(OP)
We are using a 89c52 cpu in a machine and we are having horrible static issues. When the user touches the machine and discharges static to it it resets the machine. We have tried a lot of grounding throughout the machine and still no luck . It only seems to happen when the humidity drops low ( less then 30 percent or so )and the problem ' fixes ' itself when it gets more humid and less static in the air. Any help would be aprrecaited . Is there anything that can be done now ,after the intitial enginerring has been done , to modify this board to help out ?





RE: How to reduce static in a CPU based ite
Use a static discharge gun to simulate the effect in order to make fault finding easier. This way you get a deterministic repeatable test so you know what changes are better.
RE: How to reduce static in a CPU based ite
Dan - Owner

http://www.Hi-TecDesigns.com
RE: How to reduce static in a CPU based ite
If there are no user controls, then isolating the PCB or having a single grounding point may help.
RE: How to reduce static in a CPU based ite
I wouldn't say so. I had a similar problem with an operator's panel mounted in a control desk. The only way to make that particular system work without resets was to cround its 0 V rail directly to the panel steel with a short wire. Doing this created a path for ESD discharge directly to ground - without passing through circuit ground and creating resetting or otherwise disturbing voltage peaks while finding its way to control desk ground.
Of course, controlling humidity or ionizing air is another way of avoiding the problem. Sometimes a semi-conducting mat solves the problem.
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: How to reduce static in a CPU based ite
RE: How to reduce static in a CPU based ite
The microfarad capacitors have nothing to say when ESD hits. Use small, fast ceramic capacitors.
Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: How to reduce static in a CPU based ite
Dan - Owner

http://www.Hi-TecDesigns.com
RE: How to reduce static in a CPU based ite
As far as the caps on the panel cables , please explain . The cables right now do direct from the controls ( joystick , buttons... ) direct to the main board and in through a connector . What would you recommend that a resistor / cap solution go in this area ? This is set up like an RC network , so they go in paralell with each other , right ? What values should we try .
Thanks
Gary
RE: How to reduce static in a CPU based ite
The reset can also take its source from the spike irradiating energy from the cables inside of your box. So filtering at the panel level would be the way to go.
RE: How to reduce static in a CPU based ite
RE: How to reduce static in a CPU based ite
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RE: How to reduce static in a CPU based ite
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RE: How to reduce static in a CPU based ite
The classic is the standard 2-Line LCD displays. They have a metal bezel around them. If this bezel is not deliberately grounded when someone touches it or a spark jumps thru an overlay to it they always crash the CPU running them by conducting the pulse thru the data lines to the CPU's I/O.
This problem has nothing to do with your power supply or any caps associated with it.
You need to deal with that conducted charge.
The best method is to have the controls be either insulated or directly grounded. Barring this possibility you need to limit the voltage spikes from getting to the CPU. This is likely not possible via add on stuff but should be part of the I/O board/hardware.
You you put a healthy ground plane everywhere you can on the board.
Use a semiconductor transient voltage suppression(TVS) protection device like a SIDACtor or Thyristor, Transorb, MOVs, zeners or voltage suppressors from the circuits leading from the offending control appendages to your ground plane. (my favorite = bidirectional Transorb)
Look in a Digikey Catalog.
As previously mentioned (by felixc) you run resistors of some value between these controls and the CPU on all lines except the grounds.
It is best if you can also put some resistance between the aforementioned appendages and the suppression devices as that gives the suppression devices a huge boost in protection ability.
You may be able to retro some of these solutions to your existing boards but it won't be pretty.
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: How to reduce static in a CPU based ite
Dan - Owner

http://www.Hi-TecDesigns.com
RE: How to reduce static in a CPU based ite
Nature of the beast. Most the time they want a million possibilities covered so they add the kitchen sink then use half the stuff. I can imagine those dipsw for which product are we today.
hmmm you got the CPU, RAM, ROM and 2 PIOs and some jungle logic to address them all.. That's par for the course!
Good to see a forest of opto's too.(unless they're bridges)
They aren't getting it done though if static is still blitzing things.
Nice idea to use a junker PC supply because they are potent but cheap.
Thanks for the pictures balagary!
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: How to reduce static in a CPU based ite
I saw several of these in your pictures. It is the exact way that the earthing is done which is critical.
I also noticed a lot of wood inside. Wood is not a good RF conductor. Remember that a static discharge has a risetime of less than 1ns; that is seriously fast! You need a metallic shell around your circuit board. If you don't have one you need to make one.
RE: How to reduce static in a CPU based ite
What happens if the PC supply or your control board catches fire? All that wood... Is that a problem or the way things work in that industry?
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: How to reduce static in a CPU based ite
Dan - Owner

http://www.Hi-TecDesigns.com
RE: How to reduce static in a CPU based ite
Next - If the static is coming in to the machine is being introduced from the player into the control panel, is it more likely that it is coming through something like the joystick i/o and into the main computer ( being that the joy stick is plastic ) or maybe it is more likely that it is getting in the computer from the coin acceptor which is a metal / metal plated plastic which is mounted to the metal panel ? The reason I am asking this is that maybe we can get away with only pretecting just the inputs that are more likely to be carrying the static problem to the main pc instead of having the problem of trying to put the fix into all of the inputs.
Next - What is the best way to trouble shoot this problem? I mean to make the static and put it to the machine to make it be a problem ? We currently just put on a sweatshirt and rub up and down on some bubble wrap and then go and touch the game . Any better ways ?
Part of above - In trying to see if we are making any progress in fixing the problem , if we remove all of the wires coming from the control panel and just leave 1 or 2 attached and then work on shocking the machine, can we just put the mods on these 1 or 2 inputs instead of doing the 10 or so that are there all the time. Once we figure what fix works , then we implement it to all of the inputs ? Am i looking at this the right way?
Next - Is there a way to 'watch' the static and actually see where it is going ? I was thinking of using a scope and watching differnet parts of the cpu or the i/o chips or is the static so crazy it goes all over and it is impossible to watch ?
There are a bunch of these machines in the field that are experiencing these problems so thanks all of you for helping out here to try and get this solved
RE: How to reduce static in a CPU based ite
It is very well that you want to understand what's going on. And that is a very good beginning. But wanting to understand doesn't solve your problem.
You need to find someone who thinks, eats and breathes ESD and EMI. Such a person has lived with the phenomena so long that she/he has developed not only a sound scientific foundation, but also an intuitive feel for what to do. Such persons are usually 40+ years old. They also have the instrumentation (ESD gun, fast digital scope etc) that is needed.
Such a person is hard to find. It is even harder to verify that it is the right person you have found. One tip: If you hear lots of bragging - dont hire that person. If you get references and they say she/he is good - then hire.
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: How to reduce static in a CPU based ite
Yeah you can remove most the controls and try isolating to a single control. Then work a fix on it. That is an excellent method of attack.
While a scope sounds like a good idea they often confuse static problems by adding new ground paths and capacitance.
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: How to reduce static in a CPU based ite
I worked on a system where a dc to dc converter was regularly burning out.
It transpired that static was the problem in causing latchup of an A2D ic.
Fixed that with caps & transzorbs & such.
Then it started blowing up the clock chip instead.
Ho hum.
RE: How to reduce static in a CPU based ite
RE: How to reduce static in a CPU based ite
Does any one want to suggest any quick and dirty fixes for this problem that would save us sometime and effort while we are fine tuning the exact components ?
Any part numbers anyone wants to suggest for those
suppression devices ? Or just scan digikey for these ?
Thanks
RE: How to reduce static in a CPU based ite
ESD is typically tested to around 8 to 15 kV for air discahrge. A common model is a 150 pF cap discharged through a 1 kOhm reistor. There are also specs for modeling furniture ot object discharge. There, the capacitance can be up to 500 pF or so, with resistance as low as a few ohms.
A good reference I've used is Electrostatic Discharge: Understand, Simulate and Fix ESD Problems by Michel Mardiguian. ISBN 0-932263-27-5
RE: How to reduce static in a CPU based ite
RE: How to reduce static in a CPU based ite
Its more components but destructive reset static just cant get through this arrangement thanks to essentially back to back diodes to ground. A good ground that is
RE: How to reduce static in a CPU based ite
RE: How to reduce static in a CPU based ite
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: How to reduce static in a CPU based ite
"When the user touches the machine and discharges static to it it resets the machine."
Try this:
Before touching the machine, user or operator should wear ESD wrist strap. In order to work properly, the wrist strap must make good contact at both ends (with his skin at one end, and with the chassis/grounding point at the other).
Also check the continuity (Cold Test) between isolated points of your system with the body of the machine.
WARNING: The wrist strap is intended for static control only. It will not reduce or increase your risk of receiving an electric shock from electrical equipment. Follow the same precautions you would use without a wrist strap.
RE: How to reduce static in a CPU based ite
Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: How to reduce static in a CPU based ite
1. If the user touches the metal control panel and the problem does not occur , were would the next step be ? Would the user go and try and touch something closer to the main pcb ?
2. What are we proving by doing this ? Or maybe disproving is this doesnt work ?
<agreed kids will not wear a wire strap - kids will never do what you want them to do , but if we told them NOT to wear it - they just might do it :) )
RE: How to reduce static in a CPU based ite
Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: How to reduce static in a CPU based ite
RE: How to reduce static in a CPU based ite
The purpose of the using ESD wrist strap is to discharge the static charge into ground directly through grounding point. Your problem resolves if the charge discharges to ground not through the machine. Its all the PATH we give to the charge to be discharged.
RE: How to reduce static in a CPU based ite
RE: How to reduce static in a CPU based ite
Can you see yourself putting on an ESD wrist strap before adjusting your TV each time??
Of course your solution will work! But it is totally unrealistic.
You can save all the passenger deaths in aviation if you just make sure you never have any planes leave the ground. Not too useful though.
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: How to reduce static in a CPU based ite
Dan - Owner

http://www.Hi-TecDesigns.com
RE: How to reduce static in a CPU based ite
Before doing any further modification in design, I would suggest a thorough Cold Test of the system.
RE: How to reduce static in a CPU based ite
RE: How to reduce static in a CPU based ite
RE: How to reduce static in a CPU based ite
Dan - Owner

http://www.Hi-TecDesigns.com
RE: How to reduce static in a CPU based ite