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ELCouz
Joined: 18 Jul 2007
Posts: 427
Location: Montreal,Quebec
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| iButton ESD help! |
 Posted: Tue Jun 17, 2008 6:33 pm |
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Dear members,
I have a simple question about iButtons...
How do you protect them from ESD ?
I mean on the IC side ... someone can touch the socket for the iButton...
So ESD are likely to happen...
How do i protect the PIC from high voltage spike ?
I'm using a pic18f2550 with touch.c from CCS in case you are wondering 
Have a nice day!
Laurent |
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newguy
Joined: 24 Jun 2004
Posts: 1903
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| Posted: Tue Jun 17, 2008 8:31 pm |
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My standard procedure for protecting a digital line would be:
PIC pin ---- cap to ground ----- series resistor ----- varistor/TVS to ground ---- device PIC is communicating with
From the outside looking back toward the PIC, an ESD strike would first encounter the TVS (transient voltage suppressor)/varistor. When these types of device conduct, they'll still develop a relatively high voltage across them, which is the reason for the series resistor. If the strike is particularly nasty, the cap to ground will help to "level out" the wave of charge that makes it to the PIC. By this point, any strike has been attenuated to the point that the PIC's internal I/O pin clamp diodes should be able to handle the event. From the outside looking in, it's really just a low pass filter. The trick is to properly size each component to handle the worst anticipated ESD event while not negatively affecting the signal.
I typically use Littelfuse TVS's, for example these: http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail?name=F2182CT-ND. The resistor doesn't have to be able to handle a large power dissipation - I typically use 100 - 1,000 ohm 0603 surface mounts. I usually use 10nF 50V 0603 caps.
This type of arrangement, if properly designed, will handle almost anything. A customer of a customer actually struck one of my "babies" with an arc welder and the only damage was to the connector itself. Once the connector was replaced, the unit functioned normally. If you anticipate a really nasty environment, then an optoisolator would do the trick. |
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ELCouz
Joined: 18 Jul 2007
Posts: 427
Location: Montreal,Quebec
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| Posted: Wed Jun 18, 2008 10:14 am |
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Thank you for your reply!
| Quote: | | This type of arrangement, if properly designed, will handle almost anything. A customer of a customer actually struck one of my "babies" with an arc welder and the only damage was to the connector itself. Once the connector was replaced, the unit functioned normally. If you anticipate a really nasty environment, then an optoisolator would do the trick. |
Wow! Never seen that before! It's nice to see that you are very careful about your electronic design without neglecting any details.
Is this mean also that your design :
| Code: | | PIC pin ---- cap to ground ----- series resistor ----- varistor/TVS to ground ---- device PIC is communicating with |
...can survive vandalism (ex: someone trying to fry the door lock via the ibutton socket with a high voltage) ?
It is not required in my design but it would be a «good bonus»
Regards,
Laurent |
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crystal_lattice
Joined: 13 Jun 2006
Posts: 164
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| Posted: Wed Jun 18, 2008 10:51 am |
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| Check the "Application of iButton standards.pdf" have a zener across the input, but might have more info if you read between the lines... |
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newguy
Joined: 24 Jun 2004
Posts: 1903
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| Posted: Wed Jun 18, 2008 11:22 am |
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| ELCouz wrote: | Is this mean also that your design :
| Code: | | PIC pin ---- cap to ground ----- series resistor ----- varistor/TVS to ground ---- device PIC is communicating with |
...can survive vandalism (ex: someone trying to fry the door lock via the ibutton socket with a high voltage) ?
It is not required in my design but it would be a «good bonus»
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Depending on the sizing of the resistor (resistance, power rating), yes, as long as the final resistor value doesn't interfere with the comms PIC <-> iButton. There is a microchip app note somewhere regarding directly connecting a PIC pin (via a resistor) to the mains for zero-crossing detection. Same principle. |
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ELCouz
Joined: 18 Jul 2007
Posts: 427
Location: Montreal,Quebec
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| Posted: Wed Jun 18, 2008 12:02 pm |
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| Quote: |
Check the "Application of iButton standards.pdf" have a zener across the input, but might have more info if you read between the lines... |
Thanks, I've read many section of this application note... very useful but it's says that they explain in deep , in a technical document about ESD and ibuttons which i can't find on their site! The schematic they talk about in the app note is a bare minimum when it comes to protect from ESD. (one zener diode)
I would like to read more about ESD, protecting device from them...
It is the first time that i must develop a device which humans are «touching it». So if you know literature & applications notes who explain the fundamental basis of ESD, it would be greatly appreciated if you could refer me some examples.
| Quote: | | Depending on the sizing of the resistor (resistance, power rating), yes, as long as the final resistor value doesn't interfere with the comms PIC <-> iButton. There is a microchip app note somewhere regarding directly connecting a PIC pin (via a resistor) to the mains for zero-crossing detection. Same principle. |
Yes, I've seen it too, IIRC it was about how to detect mains frequency...
EDIT: newguy, i was close !  http://ww1.microchip.com/downloads/en/AppNotes/00521c.pdf
Thank you for your tips everyone !
Regards,
Laurent |
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crystal_lattice
Joined: 13 Jun 2006
Posts: 164
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| Posted: Wed Jun 18, 2008 2:52 pm |
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| Yes, the zener is minimum, but like i said they might explain more in the text, which document are you referring to? If you can give me an outdated/broken link I might just be able to help you, filename can also be used but takes long. |
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ELCouz
Joined: 18 Jul 2007
Posts: 427
Location: Montreal,Quebec
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| Posted: Wed Jun 18, 2008 3:09 pm |
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| Quote: | | which document are you referring to? If you can give me an outdated/broken link I might just be able to help you, filename can also be used but takes long. |
Sure...http://pdfserv.maxim-ic.com/en/an/appibstd.pdf
PAGE 95:
I've searched through all app. notes from maxim-ic here http://www.maxim-ic.com/appnotes10.cfm/ac_pk/1
and cannot find any others docs who talk about ESD with 1-Wire devices!
Hope this helps!
Regards,
Laurent |
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SherpaDoug
Joined: 07 Sep 2003
Posts: 1640
Location: Cape Cod Mass USA
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| Posted: Wed Jun 18, 2008 3:14 pm |
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| Code: | | PIC pin ---- cap to ground ----- series resistor ----- varistor/TVS to ground ---- device PIC is communicating with |
Well put newguy, I really like this! Too many designers leave out the series resistance, which means the parallel element has to work perfectly. With this the TVS eats most of the shock and the RC snubs the rest.
_________________
The search for better is endless. Instead simply find very good and get the job done. |
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crystal_lattice
Joined: 13 Jun 2006
Posts: 164
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| Posted: Thu Jun 19, 2008 12:22 am |
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The link "http://pdfserv.maxim-ic.com/en/an/appibstd.pdf " points to the "Application of iButton standards.pdf" i was actually asking for the link for the other file you can't find.
What hardware are you using for the 1-wire interface, the chip solution or the budget pull-up resistor solution. I'm not sure but the chip solution might have built-in ESD protection might be worth your while to upgrade to them if they do? |
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ELCouz
Joined: 18 Jul 2007
Posts: 427
Location: Montreal,Quebec
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| Posted: Thu Jun 19, 2008 12:35 am |
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| Quote: | The link "http://pdfserv.maxim-ic.com/en/an/appibstd.pdf " points to the "Application of iButton standards.pdf" i was actually asking for the link for the other file you can't find.
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Actually when i said i can't find the file.... i mean i can't find the technical datasheet they're talking about in the Application of iButton standards PDF ... sorry for that misunderstanding!
| Quote: | | What hardware are you using for the 1-wire interface, the chip solution or the budget pull-up resistor solution. |
Still don't know ... but the newguy design is tempting. It's cheap, simple... and it had made it's proof ...
Thanks for replying crystal_lattice, newguy and SherpaDoug !
Regards,
Laurent |
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crystal_lattice
Joined: 13 Jun 2006
Posts: 164
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ELCouz
Joined: 18 Jul 2007
Posts: 427
Location: Montreal,Quebec
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| Posted: Thu Jun 19, 2008 12:45 am |
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| Quote: | In case of abnormal
ESD hits beyond its maximum ratings the DS9503 will eventually fail “short” thus preventing further
damage. |
This is nice, $0.36 per 1k, cost less in overall than the newguy design ... i will try both and put them to ESD testing...
| Quote: | if used with circuits that already have a strong ESD–protection at their I/O port, the
ESD protection level is raised to more that 27 kV (IEC 801–2 Reference model). |
Already have a strong ESD–protection means ? Like newguy's design?? ... a reference schematic in this case would be helpful
Have a nice day!
Regards,
Laurent |
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crystal_lattice
Joined: 13 Jun 2006
Posts: 164
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| Posted: Thu Jun 19, 2008 1:02 am |
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No, don't have any ESD protection, have never really experienced any ESD related problems and none of my iButton/1-wire devices i've designed went into 'production' yet, just prototyping, so can't say i've had any 'come backs'.
I have however recently (well only on one day) experienced some bad static buildup, not sure if it was just the clothes combo, or the different shoes i was wearing(work in safety shoes all day) , will have to check the clothes' tags... Maybe i can even use this 'suite' to test the protection devices...
Good luck with the ESD, will post my findings when i test them... |
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