Protecting Inputs and Outputs in Digital Electronics - Page 1
Forum section: electronics / projects

author: pascal_sweden
author_rank: Super Contributor
timestamp: May 08, 2016, 12:54:18 pm
content: I stumbled on the following article this week:http://www.digikey.com/en/articles/techzone/2012/apr/protecting-inputs-in-digital-electronicsDo they apply these techniques in most PCB designs for consumer electronics today?Is it only applicable for direct connections to the inputs of the MCU?Is it only applicable for digital inputs of the MCU? What about analog inputs of the MCU?Are the 74HC series (multiplexer, demultiplexer, selection switch, etc.) already taking up some of this input protection?What about protection of outputs? Can there also be taken some measures?Appreciate if you know about similar articles which are concise and easy to understand.This way I can share the article with my PCB design house for further discussion.

author: Pack34
author_rank: Frequent Contributor
timestamp: May 08, 2016, 02:54:40 pm
content: Anything going off-board needs some level of protection, such as a MAX13206 for basic ESD.The big kicker is what you're interfacing with. Is it something inside an enclosure that's only connecting two boards that are under your control? Is it "customer facing" in that it's an exposed connector that a customer or operator has access to?Typically for stuff that's internal I'll just stick with the MAX13206 and leave it at that.You need to be careful with stuff that's exposed. Especially if you're going for CE mark kind of stuff. Typically I'll try to bead everything and have a very weak RC filter installed (say 100-ohm and 100pF). You can add some zener diodes to clamp the voltage level if you're concerned this could be an issue.An alternative to the zeners would be to have the input digitals go through a unity gain opamp to ensure that the TTL level of the input signal is safe for the FPGA or micro circuit that you're dealing with.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 08, 2016, 08:11:05 pm
content: Absolutely. Inputs and outputs. Analog and digital.What are you protecting against? Do you know? Are you required to meet a standard? Should you reference one, if not?IEC 61000-4 is the most common EMC standard including protective features, being for the European market (CE mark). (In the US, the FCC doesn't regulate susceptibility, but does recommend testing.) Its parts include ESD and surge. ESD is only tested on exposed contacts: metal surfaces, screw heads, connectors, pins, etc. You don't need to apply protective measures to every node in the circuit, or necessarily to every pin in a connector -- if those pins are inaccessible (which is what makes a small, shielded connector like HDMI so handy).It's the simple electrical equivalent of "sanitize your inputs". Don't allow unlimited voltage and current into your circuit! At least, not unless you want those parts of the circuit to survive. Starting from the levels in the standards, you can work backwards and solve for what kinds of resistors, capacitors, diodes, etc. you need.Surge pertains mostly to mains inputs. Any long wires in or between a building can pick up surges from shorted wires or lightning induced fields, though, and can be worth protecting with heavier means (big TVS diodes, MOVs). Home security/automation and industrial controllers are a good example.Tim

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: May 09, 2016, 02:11:03 am
content: On a related topic; you should also protect against the "unpowered board is actually powered by the inputs" thing. There are logic families specifically designed to solve that issue.

author: danadak
author_rank: Super Contributor
timestamp: May 09, 2016, 09:59:30 am
content: http://www.w9xt.com/page_microdesign_pt10_input_protection.htmlhttp://www.kevinmfodor.com/home/My-Blog/microcontrollerinputprotectiontechniqueshttp://www.ti.com/lit/an/slaa593/slaa593.pdfRegards, Dana.

author: Ian.M
author_rank: Super Contributor
timestamp: May 09, 2016, 10:14:28 am
content: Few seem to consider what will happen to the clamping levels under severe overload conditions. Simple Schottky clamps are only safe to use if the maximum possible current through the upper clamp is less than the minimum load on the Vdd rail. Also, if the clamp current is high enough, the clamping voltage can rise high enough to forward bias the internal ESD diodes. If there is no resistor between the external clamps and the pin to limit the ESD diode current this can cause issues. Even a fairly low resistance can make a big difference.Done right, applying grossly excessive input voltage (but less than the flashover voltage) will just blow the input resistor - which should be a fusible type - without damaging the rest of the input protection circuit or the input itself.

author: nowlan
author_rank: Frequent Contributor
timestamp: May 09, 2016, 10:47:57 am
content: That MAX13206 chip seems kind of expensive for 6 diodes.I seem to recall MikesElectric mention he used some usb protection chip. I forget which episode.

author: Pack34
author_rank: Frequent Contributor
timestamp: May 09, 2016, 01:23:15 pm
content: Quote from: nowlan on May 09, 2016, 10:47:57 amJust using it as an example. A PESD1206 would be similar and cheaper.

author: tszaboo
author_rank: Super Contributor
timestamp: May 09, 2016, 01:29:48 pm
content: Yes, but i prefer TVS instead of zeners. And MCU pins should never go outside the product (unlike in the article), there is something in-between, like a 74HCxx, optocoupler, etc. Depends on the application. Unless it is USB.

author: Pack34
author_rank: Frequent Contributor
timestamp: May 09, 2016, 09:07:16 pm
content: Quote from: NANDBlog on May 09, 2016, 01:29:48 pmI never actually though of doing that. So you typically add an OR gate on each output and input line? If the ESD event is significant enough to damage the microcontroller, wouldn't it be able to damage the logic?I've seen some opamps used to act as a buffer and clip the input signals.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 09, 2016, 10:55:55 pm
content: Well, just buffering inputs is a rather tedious use of board area and logic parts.There is some logic behind using a PHY to isolate the brainy parts from the outside world, though. But it still assumes that:1. The board is useful with one or more failed channels, or2. You intend to repair the board, rather than just swap in a new one.If swap-to-repair is the way to go, then it doesn't matter if ESD knocks out a port or the IO bank* of the source device. Fail is fail. So you might as well save the board cost of buffers, if you can run it straight from the main chip.IF, that is, the ESD sensitivity is comparable or equal. Which won't be the case between something like an FPGA versus an HC logic gate, or LVDS transmitter. But probably will be comparable between a microcontroller and an HC logic gate.And if you're going to use protection diodes and filtering anyway, those will give you all your protection, so why bother with a buffer at all?*I've seen an FPGA zapped before, such that one of its eight IO banks went down. Supply current draw was a bit higher, and no pins functioned on that bank, but the rest of the chip was fine. This is probably typical of most complex devices, which have the "brains plus PHY" architecture internally as well. FPGAs more than most, given the diversity of available pin driver types.Tim

author: tszaboo
author_rank: Super Contributor
timestamp: May 10, 2016, 08:08:20 am
content: Quote from: Pack34 on May 09, 2016, 09:07:16 pm(new) MCUs have usually have 2KV HMB ESD performance, while getting a 8KV transceiver is not an issue (should not have written 74HC ,but 74xx). But the more important aspects are the latch up and the non destructive ESD events. I've seen systems reset when an "ESD event happened". The MCU ESD doesnt guarantee that it will not do anything stupid, but it will not break. And people cannot be bothered to write ESD safe code. Yes, that exists.So since I am usually designing high reliability stuff, none of these consumer rubbish, it does actually matter if the MCU resets or not.And latch up. Dave even made a video, showing to power an MCU without power pin. If you connect something external to it, it can power it. If you have something inbetween, with enable pins, you decrease the chance of that happening. Best is that you can make a double footprint and replace the buffers (or gates) with just resistors.

author: Jeroen3
author_rank: Super Contributor
timestamp: May 10, 2016, 11:34:45 am
content: I recently had the same question.Don't worry about ESD. Worry about EFT and (common mode) surges from nearby relays.https://www.eevblog.com/forum/projects/plc-digital-input/msg916753/#msg916753I now know that the simple buffer chips (74HC etc) provide enough clamping when you have sufficiently high input impedance.Notice that you cannot use a normal 0603 to 1206(mil)smd resistor for this because of the power and surge ratings.You could also run it directly to the MCU, but this has less clamping (smaller silicon scale) and you might want to add additional clamping.Main thing is the first capacitor and the first resistor, this attenuates and decouples the signal. Make sure there is an AC path to ground (earth).This topic is often discussed, but always bodged together. Probably because you need to buy the standard. Or because people don't test their circuit. The required test equipment is expensive after all. As is buying a compliance test from a test institution.Note: I have not tested my circuit yet. Still needs to be assembled.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 10, 2016, 03:55:35 pm
content: If you have the opportunity to use shielded cables, do so -- but make sure the shield is tightly grounded to the enclosure and PCB, otherwise it is for naught!I had a situation where the customer insisted on USB, and also wanted to cheap out on the cables (custom assemblies, because water-tight and different supply voltage). It ended up costing them over $20 extra per cable, because we couldn't solve EFT without using shielded cables and metallic connectors.If you think about it, it's very tough indeed to solve. EFT is a 2kV (give or take) pulse, driven from 50 ohms, a couple nanoseconds sharp, that washes over the system like a tsunami. Your only choice is to close the hatches, and let it wash around you. You must shield between connectors, and around the system, with a Faraday cage. (If you have a two layer board, use stitched ground; four layers, use internal ground plane; metallic enclosure, shield and ground to that. Bare boards are okay cages when designed this way, but not ideal.) Even if the shield-to-ground connection has 50dB common mode rejection, that's still ~6V of common mode peak that's blowing out your signal -- even Full Speed USB (3.3V CMOS logic levels) is corrupted by this noise. And you aren't likely to get that much performance, for example on a shielded cable that returns to a header: even if the unshielded length is on 1 cm!If your signals are high bandwidth, shielding is the only option. (Full Speed USB is 12Mbps, and High Speed is even more; with an even smaller amplitude, but the same common mode range.) For lower bandwidth (<1Mbps?), you can remove quite a lot of noise (RFI and most EFT) by filtering. This should be characteristic of any terminal block level signal, and allows very messy wiring and unshielded cabling (what hope do you have of grounding a shield to a terminal block, anyway?).Tim

author: pascal_sweden
author_rank: Super Contributor
timestamp: May 15, 2016, 04:23:35 pm
content: Throwing this topic back in the group. Added Outputs to the title, as I am looking for Protectingboth the Inputs and the Outputs in my Digital Electronics design.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 15, 2016, 09:14:06 pm
content: Outputs depend on the type, obviously. A small LVDS driver will need clamp diodes to increase ESD ratings. LSTTL or HC CMOS output pins might be okay, naked, particularly if you're putting them inside a shielded cable like a parallel port (DB-25). A high current output, like a gate driver, or ULN2003, or something custom (transistor switched, SSR or mechanical relay switched?) might not need any help at all, being beefy enough to handle it, or protected by design (an active-driven output, like a gate driver, has two clamp diodes implicitly, and they are as large as the MOSFETs doing the driving).The nice thing about outputs is, at a certain point, they get more than beefy enough doing what they're supposed to be doing, that you don't have to worry at all about weak surges like ESD and EFT. (Proper 61000-4-5 level surges may still need some assistance, or special cases, like crossed wiring, reversed polarity, and so on.) A 48V, 100A DC motor drive won't even notice ESD, and probably handles surge up to a pretty good range. However, connecting a 48V lead-acid battery to it, the wrong way around, might lead to more interesting results.Tim

author: bitslice
author_rank: Frequent Contributor
timestamp: May 16, 2016, 12:04:56 am
content: The other issue about outputs is that you may not want the output to remain on if your controller has just crashed,some arrangement of pulses to keep a transistor on can be used.

author: nctnico
author_rank: Super Contributor
timestamp: May 16, 2016, 12:33:04 am
content: Quote from: NANDBlog on May 09, 2016, 01:29:48 pmThat is a bit strong. With the right input protection you can use a pin directly but it means there will be a resistor, some capacitance and overvoltage protection involved. I usually limit the upper frequency of the protection circuit just above the maximum frequency the input/output is used for.

author: sarepairman2
author_rank: Frequent Contributor
timestamp: May 16, 2016, 01:04:02 am
content: no, its cheaper to make a new PCB. you want to keep the costs low. blame the customer instead

author: pascal_sweden
author_rank: Super Contributor
timestamp: June 08, 2016, 12:25:04 am
content: If input pins and output pins of the processor and FPGA can not be exposed directly on a connector on the PCB board, what to do with SPI, I2C, JTAG, ISP, or in general a custom feature connector that makes unused processor pins available on a connector for future use?This is not clear to me. Also to sense an analog signal you would expose an analog input pin on a connector.Or for an external interrupt you would expose a digital input pin on a connector.Or to drive an external relais module you would expose a digital output pin on a connector.What about common protection techniques for the below common I/O interfaces on a PCB board?1. SPI connector for external SPI peripherals2. I2C connector for external I2C peripherals3. JTAG connector for Programming and Debugging4. ISP connector for In-Serial-Programming5. Feature connector to expose unused digital input pins, analog input pins, digital output pins, analog output pins and make these unused processor pins available for future use (e.g. extra PCB module)?6. USB port for data communication - Which chips provide good input protection? Do all of them also provide charging functionality? Is there a difference in charging capacitiy? How to implement a fast charger functionality when the USB port will act as a Host? Is Common Mode Choke always needed? FTDI recommends this in their guidelines, but they do not use it themselves in their evaluation boards. This is a bit confusing.7. Serial port for data communication - Does the MAX232 provide enough protection as such?8. Multiplexing I/O pins. What about the 1-to-16 and 16-to-1 multiplexer in the 74HC series?Do these 74HC multiplexer chips specifically, and the 74HC chips in general, provide enough input/output protection? What about ESD, a short, driving output pin with +5V by mistake?Some other interfaces that come to my mind:9. Telephone control application: What about an analog audio input interface on a PCB board, such as a DTMF decoder circuit connected to an input Coax connector to sense an audio signal? Is the DTMF chip as such tolerant to ESD and other issues?10. Drone camera application: What about analog video input and output interfaces on a PCB board, such as a Video input Coax connector and a Video output Coax connector for OSD overlay on an analog CVBS video signal? Is the MAXIM chip for OSD overlay on a CVBS signal, tolerant to ESD and other issues?11. Remote control application: What about an Infrared output circuit with a small connector mounted on the PCB board to drive an external mounted IR LED via a cable connected to the connector?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 08, 2016, 04:12:57 am
content: Quote from: pascal_sweden on June 08, 2016, 12:25:04 amThere are good ideas and bad ideas.Good idea: having foresight in the design process and accommodating possible expansion and stuff.Bad idea: putting an edge connector near a whole in the enclosure and saying "fuck it, let the secondary market fix our problems".Well, not in such condescending terms... but that's the sort of idea.Compare early home PCs. Designs like the ZX Spectrum and C64 had a header or edge connector or whatever, that exposed the processor bus. ESD city right there! It should be no wonder that all those computers needed metal cages to keep all the awful logic switching noise inside; most of them had PCB designs without any ground pour, just logic routed over logic. Brutal, even at 3.58MHz!And not that you can have perfect foresight, but you can at least think of a few possibilities and play with that. A purpose-made expansion slot, instead of an accident. Make it modular, add several (or an expandable option). The IBM PC originally had eight slots!The most important thing, as far as the circuit's survival, is preventing huge transients from blowing out pins and chips. Just putting ESD diodes on a header, and leaving it exposed, is awfully minimal, but likely effective. Adding an RC or LC filter will increase survivability even further, and add EMI filtering.QuoteAt least an RC or LC filter, and diode clamp or zener. You don't want the signal being able to change any faster than you're sampling it at -- unless you're using that as a feature, not a bug.Always filter your signals to the minimum bandwidth required for the application!QuoteA naked interrupt pin would be devastating to an edge triggered device! Apply RF of a few MHz and the poor thing just locks up!Always sanitize your inputs!QuoteThat kind of load would most likely require a buffer anyway, even if it's just a BJT. A protected FET would be excellent, adding short circuit withstanding to the list of features.Quote1. Naked ~HC CMOS level logic: okay for short runs, especially with buffering.2. Long runs require termination. Use source termination for point-to-point links, or a beefy driver with load termination (on the last load in a linear chain). Avoid star configurations. This is typical of modest lengths (low to fractional meters). Sort of related example: the old fashioned MFM/RLL hard drive interface used two ribbon cables, one carrying TTL level control signals, load terminated with a modest value resistor pack. The pack was socketed, so you could plug it into the last hard drive on the port.3. Long runs are even more prone to noise pickup and radiation. Suggest RS-422/485. Signals are unidirectional, so they can be converted to differential and back without any problems. (The MFM drive interface also used RS-422, for the high speed data bits. That was transferred over a single cable per drive, so the controller card had two data ports, and one control port shared between max. two drives.)4. For even greater distances, you can still use an RS-422/485 sort of scheme, but you may want galvanic isolation (to increase common mode range even further), and other features like line loss compensation. It may be more economical to go to an altogether more robust link method, like 10BASE-T, or a radio-over-copper method.Always filter receiver inputs. You don't want RF racking up a dozen extra clock SCK edges between bits! This goes for differential as well as single ended!QuoteRecommend against it. I2C can be carried short distances, but even then, it should use shielded cables. It is difficult to buffer, because of the bidirectional (open collector) arbitration.Even for modest distances, I'd recommend RS-422/485 or CAN instead.QuoteN/A -- only ever used in a laboratory setting. If you've put yourself into a situation where you must use JTAG in the field, you've already lost.I haven't seen a single JTAG header and programmer that will actually withstand any tiny amount of RF. The weak pull-ups are a worry; the lack of controlled signal quality is another (who's source-terminating what? Can it be chained, multi-dropped, or what?), and though most are provided with extra grounds (the 20-pin version is chock full of grounds: in fact, it's every other wire in the ribbon cable, precisely how you want it!), but they aren't usually well used (as soon as the ribbon cable enters the programmer, guess where the grounds go?..).I've also seen a casual disregard for signal quality at the other end of most programmers and dongles: USB ports are so often connected utterly the wrong way, namely, with a ferrite bead from shield to ground. They'reintentionally bad.Good "trap for young players", actually. If you can ever imagine a situation where not being able to use the programmer means certain death to your development or product... try to take as many precautions as possible to avoid that! For example, design a bootloader to work with one of those robust comm channels you'll have.QuoteSimilar, though potentially more troublesome because few-wire programmers may use open-collector arbitration. That is, I2C but with RST being one of the active pins, basically. Depends on the programmer. In any case, I still haven't seen one that's shielded, so you have the same vulnerability during use.Quote1. Don't (see above discussion!).2. Use protection and filtering. Assume every pin is a potential hazard, and design accordingly.3. Use covers and insulation to prevent damage to pins and ICs.4. Use shielded covers and connectors to shunt ESD, short out EMI, and improve signal quality.Good examples:- Old timey game consoles. Bus is exposed, but under a cover, or at the end of a slot. The game cartridge itself may be vulnerable to ESD, but it's replaceable.- PC expansion slots. The connectors cover the pins, so you can't touch them easily. (And if you're charged to the 10kV it takes to jump the remaining distance, frankly you're a retard and deserve the dead motherboard you'll get!) Furthermore, they are internal, and require tools to access (remove covers, mounting screws).- USB, HDMI, basically any high speed data connection today. It's not possible to provide robust ESD protection, while keeping capacitance low enough to carry the data. So, the connector's metal shell protrudes, shunting ESD from outside sources, and being the first to mate with the plug. The sensitive data pins are tucked away inside. The male pins are on the peripheral, where again, they may be more vulnerable, but they're more replaceable too.- Or Ethernet, which is transformer coupled, so can withstand quite a lot of craziness from the outside world.QuoteAgain, protection is somewhat unfeasible on High Speed links, but, if you know it will only be operated up to Full Speed, then the ESD protection and filtering can be quite robust.These features are usually provided by outside parts; you'd have to look and see if any controllers offer ESD > 2kV HBM on the USB pins themselves.QuoteNo, that's software.USB host devices usually have a "protected switch" type device for the 5V output. Often.. they don't even have that, just a polyfuse or something. The peak surge currents through those can be devastating, even if the total energy is limited by the fuse.Ideally, a multilevel "protected switch" is used, to provide a negotiated load current draw.Such is the spec on USB3, which offers more voltage and current after negotiation.QuoteNo, in fact it is almost alwaysworse.USB isnotfully differential. The packet start and end symbols are unbalanced. For these to be communicated with fidelity, the entire connector-to-cable-shield-to-connector signal path must be maintained. A common mode choke of any useful value will deform these pulses, closing the eye and reducing the amount of interference that can be tolerated.Furthermore, USB is source terminated only, so there is no load impedance for a CMC to work against, only the receiver input pin capacitance. You can add some RC damping to fake it, but this unavoidably attenuates the signal some, and still can't be very much (less than 100 ohms, say).Absolutely solid shielding, from end to end, is the only good solution.It should be no surprise that:1. USB always uses shielded cables, even for very short links (except HMI devices usually, but those run at ~UART speeds),2. Maximum link length is only a few meters.QuoteYes-- those are usually on the order of 10kV HBM ESD rating, which is good enough not to need outside protection, even for reliable links.The ISL3280 series (RS-422/485) are even rated for 16.5kV of ESD!RS-232 is quite slow (the pin driver output current, and slew rate, are intentionally limited), but if this is tolerable, it's another physical-layer candidate for SPI. SPI is usually fast enough that you'll want 422 though.QuoteNot recommended, but probably tolerable in practice. CMOS output pins are more robust than input pins, though not by a huge margin. Not sure what the actual rating is (2kV? 4kV?).Again, would recommend diodes and filtering.QuoteTelephone has additional problems. Phone lines are long, and infrequently have been known to carry Zeus's ESD. Lines of this type must be isolated, as you would connect to mains lines: and for much the same reasons!However you'd like to arrange communicating those signals over the isolation barrier... be my guest. (Transformers are common -- phone audio has limited and known bandwidth. The ring tone and DC bias stink, but can be dealt with. Otherwise, your basic DSP/DAQ signal chain, with digital isolators back to the processor, would be a fine way to go.)QuoteUnfamiliar with that chip.Video might be okay, but realize it's intended to be connected to shielded connectors again. They may make the assumption that it'll be okay...Video, and RF in general, often has the bad habit of making things as small, and as vulnerable, as possible. That USB and digital logic pins are as robust as they are; thatthey can be as robust as they are, is pretty remarkable. Not everyone makes the same efforts, though...QuoteWhy not just put the IR LED on the board?An LED drive would be much like a digital logic or RS-232 (single line, odd as that sounds..) application. Limited current, bandwidth.Tim

author: pascal_sweden
author_rank: Super Contributor
timestamp: June 11, 2016, 09:49:01 pm
content: I found two extremely nice interactive tutorials from Texas Instruments about this subject matter:ESD Protection BasicsCircuit Isolation Techniqueshttp://www.digikey.com/en/ptm?f=1431913583&f=549771276The first tutorial contains a table with common I/O interfaces and a specific solution from TI for protection.Note that DigiKey has an extensive library with Product Training Modules (over 2400 interactive tutorials):http://www.digikey.com/en/ptmGood stuff! And all for free! Unbelievable actually.

author: tszaboo
author_rank: Super Contributor
timestamp: June 28, 2016, 03:12:30 pm
content: Quote from: T3sl4co1l on June 08, 2016, 04:12:57 amI agree with everything. Connecting GPIO pins together is fine on an arduino and such. The same design tecniques cannot be used for an end product. This is one of the reason, that those are designed by engineers, not hackers. Have you ever seen a printer with a JTAG port? Or any consumer product? If the signal leaves the product, then no direct path to the MCU. There are some grey areas though. For example a battery pack contacts might be direct, if they are those fingerproof type, and it is covered usually by a removable piece. Stuff like this.

author: dmills
author_rank: Super Contributor
timestamp: June 28, 2016, 04:13:41 pm
content: This tends to be one of the areas that separate the industrial stuff from the domestic kit.It is not at all uncommon to see industrial kit with **extensive** EMC/RFI measures, to the point that half the component count can go on 100pF caps, 100 ohm resistors, ESD diodes, inductors and opto isolators, you typically don't see that in domestic kit to anything like the same extent.I would second the comment about making sure you have a reliable means to push new firmware to the box without needing to take the lid off to find a JTAG or ISP port, we tend to spend a lot of time on this (Also, make sure there is a way to force the bootloader to take the update even is there is an apparently valid main program found, sometimes that main program turns out to be broken).Usually I will include a jtag chain (For production boundary scan as much as anything else), but it is likely only available by a bed of nails on production test and if there are high value parts (BIG fpgas for example) then buffering and protecting this is well worth the parts.Generally an internal use only connector may or may not have much protection, you have to take a view but anything going outside the box gets at least a robust buffer and lowpass filter, and anything coming in gets some sort of ESD clamp and lowpass arrangement if at all possible.Increasingly there are networks (often in really inconvenient 0.4mm DFN type things) for ESD (and to some extent EMC) protection of the modern high speed LVDS/TMDS busses which can be actually worth thinking about for some applications.A nice gotcha with RS485 is the common mode range, particularly if there is a long cable in play with significant current flow, a 60M cable with a 6A DC supply doing remote battery charging for example had me because the screen formed the return and could put the data link outside the common mode range due the IR drop (How we laughed, the other end was 30M underwater).Composhite video is slow enough that you can reasonably protect it with an ESD diode and can limit the bandwidth to below 10MHz (The filter will need to be higher to avoid phase shifting the croma subcarrier), SDI-12G not so much!I have actually seen I2C buffered and taken outside the box, but it always strikes me as being a poor plan (Mind you in my view I2C is usually a last resort anyway), TI IIRC do a bidirectional buffer for it (The thing is kind of weird about exactly how you setup the pullups IIRC but it does work).One of the more annoying interfaces for this stuff is parallel LVDS of the reasonably fast sort (A Few hundred MHz DDR, but lots of pairs), the mobile phone industry has some tempting parts, but 0.4mm DFN, still when the other end is a $1,000 FPGA....73 Dan.

author: prasimix
author_rank: Unknown
timestamp: June 28, 2016, 06:21:36 pm
content: Maybe you can borrow some ideas from discussion postedhere.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 28, 2016, 06:50:59 pm
content: Quote from: dmills on June 28, 2016, 04:13:41 pmIf you're using RS485, do take a close look at the datasheet. Notice the CM range isoutside the supply rails-- receivers have internal divider resistors to sense the line. If you are adding TVS diodes to such an input, don't make the mistake of using unidirectional, say 5.0V parts! 6-12V bidirectional is better. Too low a clamp voltage and you ruin the CM range, making things worse!Most receivers also boast a modest ESD rating themselves, so external protection isn't necessary in many cases. When it is, it's usually because of surge, or excessive ground offset; in which case, a fully isolated receiver may be a better idea.Tim

author: dmills
author_rank: Super Contributor
timestamp: June 28, 2016, 07:57:53 pm
content: Yep, but this was shifting the ground voltage to be outside the +12 -> -7 range that RS485 does, we swore copiously.Ended up that we could finesse it with a pair of L pads inside the plug, halved the signal level but put the signal within the common mode range, and at 9600bps level was not a problem.Regards, Dan.

author: tszaboo
author_rank: Super Contributor
timestamp: June 28, 2016, 09:32:56 pm
content: Quote from: dmills on June 28, 2016, 07:57:53 pmThat means you had more than 7V voltage drop on the cable. Almost no matter how you look at it: that is a lot. It is not the electronics problem, but the electrical guy deciding on the cable.

author: dmills
author_rank: Super Contributor
timestamp: June 29, 2016, 09:57:59 am
content: Actually it was more of a need to re use an existing cable design, and the reworked electronics had a charger that actually worked properly.....Sometimes when you are redoing an existing system you have to work within some constraints.The fact they changed the max cable length from 20M to 100M did not exactly help matters.....Regards, Dan.

author: Gavin Melville
author_rank: Contributor
timestamp: July 02, 2016, 10:01:45 am
content: I work with simply awful noise in industrial environments. If it's only 4kv transients on DC supply rails I'm having a good day. In the end all interfacing is via light. Opto isolators work well, but my standard goto interface is 1mm low cost fibre, which between perfect isolation and low bandwidth keeps noise where it should be. Is it expensive, of course.TVS type diodes need careful design. The turn on is just too slow, often msec. Even small ones start clamping after 10usec.I did get caught recently - LT makes a nice transient stopper, lt4363. Good idea, but if there are so many transients it never turns on then it isn't much use. Perhaps better on well behaved noise.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 02, 2016, 09:00:55 pm
content: Quote from: Gavin Melville on July 02, 2016, 10:01:45 amSurely you mean thyristor type..?Avalanche TVS diodes break down essentially instantaneously (sub 1ns), so that lead inductance dominates their performance.TVS diodes in forward bias are either "normal" or "fast" recovery; I'm not sure what. (I've never seen it in a datasheet, and I don't see any info over a selection of SPICE models, except for bog-standard zeners where they all are given the same token value "TT=50.1n"). In any case, forward and reverse recovery won't be more than a few microseconds, even for the highest voltage devices; you just can't make silicon worse than that.So they're certainly good for surge, no matter how you use them. As long as they're used right, because of that inductance thing, which is especially dominant for fast transients (EFT and ESD).In contrast, thyristor type TVS turn on when the voltage is changing rapidly, or at a peak voltage (they still clamp like regular avalanche TVSs). Turn-on takes some us, but clamping voltage is quite low, even at high currents. They're good for clamping surges on signal lines and (non mains) AC power lines (since, they'll remain on if supplied by DC).QuoteHmm, with response on the order of 5us, it would only be just barely helpful with IEC 61000-4-5 surge (8/20us), but quite suitable for telecoms surge (10/1000us) and load dump. Which is likely the intended application. It's useless for anything faster, including ESD and EFT.Its internal design likely uses a charge pump, so that the turn-on charging / recovery time is rather slow (ms?). It would be a poor choice for extremely intensive environments.Have you considered using aBig Fuckin' Inductor(tm)? If you've gotthat muchdensity of surges, it sounds like you're inside a switching converter! Might as well harness what you can... An inductor is big and pricey, but won't ever fail.Tim

author: Rerouter
author_rank: Super Contributor
timestamp: July 02, 2016, 10:48:39 pm
content: Another question, I will have to expose 12 bidirectional I/O to the outside world, 3.3v in, and 1.8-12.5v out, wondering if there are any good approaches for this, each pins direction will need to be independent and should ideally survive short to ground or short to 32v, (while being capable of 30mA)

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 02, 2016, 11:36:52 pm
content: Not too hard, but sounds interesting if you need particular capabilities (why is it 1.8-12.5, is it analog output? adjustable digital '1'?), or a lot of them while packing it into a tight space!Got any budget for the design?Tim

author: Rerouter
author_rank: Super Contributor
timestamp: July 03, 2016, 12:47:18 am
content: Digital 1 at that voltage. 0 is ground, in essence I am making a diagnostic tool, which will involve talking to devices in the 1.8-5v families while being able to kick out 9-12.5v high voltage programming pulses on 6 of those pins to cover all use cases, budget is ~ $60 for the base hardware for that function, lets say $110 cap, excluding voltage regulators, as it will be used by others in the feild, it will have to survive but not necessarily work when shorted to 32v or ground on any given I/o, each pin should have 12-35mA sink/source to be safeSo essentially level shifting and protection

author: pascal_sweden
author_rank: Super Contributor
timestamp: January 29, 2017, 01:24:28 pm
content: I am still puzzled about the ESD protection for non-edge programming/debugging/expansion connectors.From my perspective, every interface on the PCB board should be protected for ESD protection.Also all the non-edge connectors that don't reside on the edge but in the middle of the PCB board.Such as programming and debugging connectors (JTAG, ISP).And peripheral expansion connectors (SPI, I2C).And connectors that directly interface with the main processor (Feature connectors).Eventually the PCB board will be mounted in a box, where only the edge connectors come out.But even then, one needs to open the box in the field once in a while, to connect a cable to the non-edge connector on the inside, and can touch the connector with his finger during that operation (meaning that ESD risk is also here present).Maybe these non-edge internal connectors don't need the same level of ESD protection as the edge connectors, but at least some basic protection would be helpful.Would a TVS diode work on all of these connectors (JTAG, ISP, SPI, I2C, Feature connector)?Or would that interfer with the JTAG/ISP/SPI/I2C signals/bandwidth/protocol?What solution would work best to have good ESD protection, and at the same time don't interfer with the signals/bandwidth/protocol?I am looking for a solution that provides ESD protection to non-edge connectors, whereby the programming/debugging/expansion features still work the same as before without ESD protection.Could any expert out here please advice? I am working towards a dead-line on a project, and need this input asap.N.B: I have heard stories from some people, that ESD protection on a non-edge connector in the middle of a PCB board, is not very wise, as that would mean that the spike is disposed through the ground plane, and has to travel all the way from the inside of the PCB board to the outside. Basically meaning that it would harm more with actual ESD protection, than without, in the event of a spike. Is that really true? I would like to have a better understanding about this explanation with spikes being disposed through the ground plane.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 29, 2017, 02:27:33 pm
content: Quote from: pascal_sweden on January 29, 2017, 01:24:28 pmMeh. ESD is a risk item.Getting field failures from ESD strikes in the middle of the board? Tell the end users not to stick their grubby ape-fingers on it without basic ESD precautions!Once it's in the box, protected from access by errant appendages, the protection components are no longer necessary.PCs haven't seemed to have much trouble with this -- amateur builders are comfortable following ESD precautions. (Well... whether or not they're necessarily even doing the right thing, but it helps that modern semiconductors are fairly robust, too.)QuoteThere's certainly nothing wrong with it, and I've done it before, for all of the above. The hardest to protect are: anything with really oddball voltage or current requirements (usually only for specialized test equipment, like scope inputs or electrometers, where the protection can be customized anyway), or anything at stupendously high speed (USB2+, PCIe, etc.), which technically shouldn't need ESD protection anyway because of the excellent connector design (USB, IEEE-1394, eSATA, HDMI..), or precautions in use (PCIe is usually for internal board-to-board connections, not hotplugging; though it can be extended on cables, or used for hot-plugging in server situations).QuoteAll of those are slow enough not to worry, and reside within their supply voltage range -- you can use a boring dumb zener, or a diode pair, to handle that no problem.The capacitance might even help out with RFI, which is a related concern.QuoteThe point of a ground plane is to shunt currents and fields around the components. As long as the fields couple well with the traces themselves, it's fine: it acts like a Faraday cage. ESD striking the middle of a board will couple into modest length traces, enough to upset logic values, or trigger the ESD protection circuits of ICs. A poorly designed board might behave quite badly, like the cheap old hardware that was made with two layers of traces going every which way, and no ground plane: it's no better than a jumble of wires in air, and pretty much every single one will get a substantial fraction of a nearby ESD pulse coupled into it (hundreds of volts worth).Still, I can't see why adding TVS diodes wouldn't help, assuming the board is designed to at least work on its own, and isn't an intentionally designed pathological case, like say, looping the TVS diode's return path completely around the board before hitting the main ground connection...So, is this an add-on thing to an existing board? Or a new design? If you'd like some review or consultation, I'd be happy to take a look.Tim

author: wraper
author_rank: Unknown
timestamp: January 29, 2017, 02:40:37 pm
content: Quote from: T3sl4co1l on January 29, 2017, 02:27:33 pmI repaired many hundreds of Raspberry pi2, and there are tons of them with one HDMI line blown out. Likely not because of ESD as such but because of hot plugging and Y caps in SMPS. And probably crappy Chinese HDMI cable which often have connector shell not connected to anything. Low speed USB devices like mouses and keyboards don't require any shield at all, and often there is none.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 29, 2017, 05:03:29 pm
content: Quote from: wraper on January 29, 2017, 02:40:37 pmWait, who the hell would ever pay to have something so cheap repaired?QuoteGood points!Tim

author: wraper
author_rank: Unknown
timestamp: January 29, 2017, 07:30:12 pm
content: Quote from: T3sl4co1l on January 29, 2017, 05:03:29 pmI bought them for cheap and repairs are fast to do (with my throughput optimized process). I can do around 50-60 pieces per day in average. So it pays off well enough.

author: pascal_sweden
author_rank: Super Contributor
timestamp: February 09, 2017, 06:52:44 pm
content: Quote from: pascal_sweden on January 29, 2017, 01:24:28 pmThe picture in attachment illustrates the issue with ESD protection for non-edge connectors.Isn't there any way to solve this problem? I would like to have ESD protection for non-edge connectors, while at the same time not be confronted with the problem as illustrated in the picture.

author: pascal_sweden
author_rank: Super Contributor
timestamp: February 14, 2017, 12:45:07 am
content: How to avoid the traveling of a glitch through the entire ground plane when using ESD protection for non-edge connectors in the middle of a PCB board?Any pointers from experienced PCB board designers?It's hard to believe for me that there really is no solution for this.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 14, 2017, 08:34:46 am
content: Have youreadthis thread? You've received many answers already...Tim

author: pascal_sweden
author_rank: Super Contributor
timestamp: February 15, 2017, 01:50:50 pm
content: Quote from: T3sl4co1l on January 29, 2017, 02:27:33 pmThis makes me conclude that ESD protection can be added on non-edge connectors, as long as the ground plane architecture meets a good standard?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 15, 2017, 05:04:20 pm
content: Fine with me!

author: pascal_sweden
author_rank: Super Contributor
timestamp: February 27, 2017, 07:37:32 pm
content: ESD protection on non-edge connectors it will beJTAG, ISP, SPI, I2C, Feature connector that brings out processor pins

author: pascal_sweden
author_rank: Super Contributor
timestamp: March 10, 2017, 02:28:03 pm
content: Feedback which I got from my electronic design partner in India:"Faraday cage is a concept of EMI not ESD. This cage is created by ground stitch as shown in the picture. It'll help other portion components not to get affected from EMI from it's neighboring portion.For ESD what best we can do make it 4 layer board and add a ground layer in inner layer bottom of top layer and stitch vias as much as possible so that return path will be less I think so."See attached picture.

author: pascal_sweden
author_rank: Super Contributor
timestamp: March 10, 2017, 03:43:25 pm
content: I was searching the Internet today, and found out about this Industry Council regarding ESD.Never heard about them before. But it seems a very good initiative and good source, and I therefore wanted to share hereIndustry Council on ESD Target Levels:http://www.esdindustrycouncil.org/ic/en/aboutThere is a lot of good and free documents there:http://www.esdindustrycouncil.org/ic/en/documentsEspecially White Paper 3 - Part I and Part II:http://www.esdindustrycouncil.org/ic/en/documents/7-white-paper-3-system-level-esd-part-i-common-misconceptions-and-recommended-basic-approacheshttp://www.esdindustrycouncil.org/ic/en/documents/36-white-paper-3-system-level-esd-part-ii-effective-esd-robust-designsIn that White Paper 3 - Part II, they also refer to a good source about EMC from the Ford Motor Company:"In addition to protection strategies for the individual pins of an ECU, the board level design (especially the layout of ground planes) has significant influence on the system level ESD and EMC properties. Ford has published a good introductory guideline document entitled “EMC Design Guide for Printed Circuit Boards” which is available fromwww.fordemc.com."Overview of EMC documents from Ford:http://www.fordemc.com/docs/Downloads.htmEMC Design Guide from Ford:http://www.fordemc.com/docs/download/EMC%20Design%20Guide%20for%20PCB.pdf

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 10, 2017, 05:13:36 pm
content: ESDisEMI. It's considerably more powerful than most conventional sources!Tim