Are these USB series resistors important? - Page 1
Forum section: electronics / projects

author: zapta
author_rank: Super Contributor
timestamp: February 08, 2015, 07:28:56 pm
content: This diagram is from the datasheet of the LPC11U35 (USB full speed)I tried it with and without the series 33 ohm resistors and the signals looks just the same and so is the functionality. How important is it to have those series resistors?I am not interested in formal USB compliance, just want to thing to work reliably.

author: paulie
author_rank: Frequent Contributor
timestamp: February 08, 2015, 07:44:37 pm
content: In stripping down your Arm-Mini design I came to pretty much the same conclusion: excess baggage. In many cases I found 90% of circuitry superfluous. A lot of so called engineering these days would best be described as religious ritual. For example the impedance of those resistors is inconsequential compared to the relative drive characteristics of the MCU. Almost exactly the same situation with some AVR designs like USBASP. For super high speed USB with special low impedance dedicated devices it's a different story.There are some who argue the zeners are required even for 3.3v circuits thereby demonstrating almost complete lack of understanding of how these really work.

author: langwadt
author_rank: Super Contributor
timestamp: February 08, 2015, 08:05:17 pm
content: Quote from: zapta on February 08, 2015, 07:28:56 pmtogether with the resistance of the output drivers it forms a series termination of the ~100R differential twisted pairprobably not terrible critical, but is it worth the risk to save two resistors?

author: DanielS
author_rank: Frequent Contributor
timestamp: February 08, 2015, 08:15:12 pm
content: Quote from: zapta on February 08, 2015, 07:28:56 pmIf you want stuff to work reliably, you usually have to stick with reference designs. If the reference design includes 33 ohms resistors, it most likely means the chip's IO driver requires them for proper cable impedance termination. Without these resistors, you might be compromising signal integrity on the USB cable and this may translate into fussy behavior based on specific cable build quality and length - it might work fine using a cheap 2' cable but be unusable on a quality 6' cable.If you want to test your luck, you could put the resistors on your PCB design and then use a solder bridge to see if you can get away without one. Or you could put both of the SMD resistors' pads on the same net to have a trace between pads and cut that trace if you find out you really need the resistors.

author: free_electron
author_rank: Super Contributor
timestamp: February 08, 2015, 08:17:47 pm
content: There are multiple reasons for those two resistors1) it is bloody hard to meet the impedance on the board in terms of track width and clearance to meet the usb spec. putting these resistor directly at the pin allows for an easier way to do the pcb layout. if your stackup allows you to easily create the 90 ohms differential you don't need them. fact is most stackups with standard pre-preg material on a 4 or 6 layer board are in the 60 to 75 ohm...tack on 22 or 33 ohm and you are there.2) these resistor limit current for an esd strike. integrating these resistors is a no-go becasue of 1 above and because they cannot handle a lot of power nor standoff voltage in integrated form.go read this if you want to know why and how to do it properly ( as opposed to believing in ritual )http://www.usb.org/developers/docs/hs_usb_pdg_r1_0.pdf

author: free_electron
author_rank: Super Contributor
timestamp: February 08, 2015, 08:19:06 pm
content: Quote from: DanielS on February 08, 2015, 08:15:12 pmthe problem is that this 'test' involves you pulling eye-patterns... most hobbyists don't have a scope fast enough to pull that off nor do they have access to the test criteria.

author: paulie
author_rank: Frequent Contributor
timestamp: February 08, 2015, 08:31:59 pm
content: Quote from: DanielS on February 08, 2015, 08:15:12 pmThat's exactly what I did on one of the boards zapta sent me. Then ran it over the coals with a variety of PCs and cables. For one particularly tricky ASUS and my longest and shortest unshielded cables it worked fine so the resistors were left out of the remaining builds.These "rituals" do serve a purpose in military, medical, etc applications but in many cases more for insurance and liability reasons than religious. This is HOBBY world guys, much as you might like to pretend otherwise.

author: langwadt
author_rank: Super Contributor
timestamp: February 08, 2015, 08:54:53 pm
content: Quote from: free_electron on February 08, 2015, 08:17:47 pmsticking a series resistor at the end of a transmission line does not change the impedancebut 2x33R + 2x impedance of an output makes a roughly ~100R termination

author: zapta
author_rank: Super Contributor
timestamp: February 08, 2015, 09:18:42 pm
content: Quote from: langwadt on February 08, 2015, 08:05:17 pmIt's an exercise in keeping things simple.

author: miguelvp
author_rank: Super Contributor
timestamp: February 09, 2015, 07:00:57 am
content: Find a way to induce noise on the USB cable, without those resistors I will think your differential information would be too degraded in the other end depending on the distance and noise picked up by the cable.For example, if the cable runs in parallel near an AC power cable, without the proper termination your communication distance might degrade.At least that's my understanding but I'm a noob at this.

author: tggzzz
author_rank: Super Contributor
timestamp: February 09, 2015, 10:41:51 am
content: Quote from: paulie on February 08, 2015, 07:44:37 pmTranslation: I don't understand why X is there, therefore there is no reason to have X there.A very unimpressive example of the Dunning-Krueger effect.

author: senso
author_rank: Frequent Contributor
timestamp: February 09, 2015, 12:35:55 pm
content: But also using the "its a HOBBY" argument, a pair of resistors will cost you next to nothing, and you are paying PnP time to place them, soldering those is a 15 seconds job, and the space occupied in a pcb is irrelevant.

author: zapta
author_rank: Super Contributor
timestamp: February 09, 2015, 01:04:46 pm
content: Quote from: senso on February 09, 2015, 12:35:55 pmIt adds another SKU. Trying to keep the design simple. Only 2 resistor values.

author: zapta
author_rank: Super Contributor
timestamp: February 09, 2015, 01:11:45 pm
content: Quote from: free_electron on February 08, 2015, 08:17:47 pmThat intel document seems to focus on high speed USB (480mpbs), my board is full speed only (12mpbs). Does it matter than much in the lower speeds?

author: Pjotr
author_rank: Frequent Contributor
timestamp: February 09, 2015, 01:12:49 pm
content: Anyway, a proper design is by default 90% crap by the new proposed definition of "proper"So also leave out proper ESD protection and rely on the small on-chip diodes only.

author: zapta
author_rank: Super Contributor
timestamp: February 09, 2015, 03:11:01 pm
content: Quote from: Pjotr on February 09, 2015, 01:12:49 pmThanks for the suggestion.

author: tggzzz
author_rank: Super Contributor
timestamp: February 09, 2015, 03:21:38 pm
content: Quote from: Pjotr on February 09, 2015, 01:12:49 pmGood joke!

author: zapta
author_rank: Super Contributor
timestamp: February 09, 2015, 03:22:52 pm
content: Quote from: tggzzz on February 09, 2015, 03:21:38 pmI expect mature communication here.

author: Pjotr
author_rank: Frequent Contributor
timestamp: February 09, 2015, 03:53:43 pm
content: Hi Zapta,Leaving out the series resistors will probably not corrupt USB 1.1 but they are there also for EMC reasons (slew limiting) sometimes. A QAD design that works in a particular occasion is not always a good design.There is a load of good application notes about USB implementation available on the www. So why not doing it the standard right way and as advised by the manufacturer, instead of making short cuts?

author: donotdespisethesnake
author_rank: Super Contributor
timestamp: February 09, 2015, 04:06:42 pm
content: Quote from: zapta on February 08, 2015, 07:28:56 pmDefinitely leave them in.I and many others have been bitten by "hobby" designs that take shortcuts, and then spend a lot of time and effort tracking down why my board resets when the fridge turns on (or other common equipment). Turns out for the cost of a few SMDs, the problem is easily fixed, but the amount of frustration caused was ridiculous. The designer says similar thing "wanted to save money/keep it simple/didn't think it was needed".Typically, the Chinese take these shortcuts, most of the time they get away with it, many times users get bitten.Beginners, or even professionals, trying to develop embedded code can find it tricky at the best of times, having the CPU reset randomly does not help!I would therefore argue for more protection on USB, rather than less. Best practice is not to connect shield directly to ground, but use an RC pair.I also use a buffer chip like USBUF01W6, which has series resistors and 1.5k pull up built in and claims to have ample ESD protection , so replaces at least 3 components with 1. There are several other similar chips, although downside is that they are usually a quite small component if you are doing DIY.In general, negative experience due to unreliability tends to create more of a bad rep, than simplicity can create a good rep.

author: free_electron
author_rank: Super Contributor
timestamp: February 09, 2015, 04:16:28 pm
content: Oh it's one of them 'hobby' things eh ?Why don't you just forget about all of the rules and simy wire it up with a bunch of recycled power cords. After all, those work reliably with 10 to 15 ampere at 220 volt so surely that measly few milliamps at 3 volt of usb will be no problemhopeless. We are all doomed. Nobody takes pride in what they do anymore.

author: zapta
author_rank: Super Contributor
timestamp: February 09, 2015, 04:39:59 pm
content: Quote from: donotdespisethesnake on February 09, 2015, 04:06:42 pmThat's USBUF01W6 looks interesting except that I need to switch the 1.5k resistor. Do you know of a similar device with an open ended 1.5k resistor or even better a switched 1.5k resistor?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 09, 2015, 05:23:42 pm
content: Quote from: Pjotr on February 09, 2015, 03:53:43 pmActually very little to do with EMC; that's why the shield is there, and why it's supposed to be grounded as directly as possible to the circuit ground plane.Although the bulk of a USB packet (at 1.1 Full Speed) is balanced, the signals are essentially equivalent to 74HC CMOS drivers, and the line status / negotiation signals are explicitly unbalanced (both lines high/low, one open, etc.). Which is a good reason not to route it as full differential -- it isn't! Likewise, common mode chokes on the data pair won't save you much trouble.I'm still waiting to hear what Zapta's oscilloscope bandwidth is.Tim

author: senso
author_rank: Frequent Contributor
timestamp: February 09, 2015, 06:36:01 pm
content: Quote from: zapta on February 09, 2015, 04:39:59 pmIf the only thing really using the 3.3v then you can just use and IO pin from the micro to the 3.3v pin in the package, needs to be tested, but it might work.

author: zapta
author_rank: Super Contributor
timestamp: February 09, 2015, 07:43:29 pm
content: Quote from: T3sl4co1l on February 09, 2015, 05:23:42 pmIt's an old 60Mhz tektronix DSO, similar to that batch of Military DSO's Dave scored a few months ago. I used one probe with standard earth crocodile, and look at D+ and at D- while communicating with an Apple Macbook Air notebook.

author: donotdespisethesnake
author_rank: Super Contributor
timestamp: February 09, 2015, 08:00:55 pm
content: Quote from: zapta on February 09, 2015, 04:39:59 pmI switch the 3.3V input on the chip to a 3.3V rail via a P-Mosfet, so the MCU can control it.Another couple of devices to consider, NUF2042 (no pullup) and STF202.I am still looking for similar devices in a slightly bigger package.

author: tggzzz
author_rank: Super Contributor
timestamp: February 09, 2015, 08:05:32 pm
content: Quote from: zapta on February 09, 2015, 07:43:29 pmSo full speed = 12Mb/s, therefore the edge rate should be between 4ns and 20ns. (http://www.ti.com/sc/docs/apps/msp/intrface/usb/emitest.pdf) You won't be able to adequately visualise a 4ns edge with a 60MHz scope, however good the probe is.In addition, the 140nH inductance will resonate with the probe tip capacitance and any capacitance on the signal lines. That will undoubtedly distort the waveform you see.Note also that the TI document above answers your original question: why are those resistors there in the first place.

author: ralphd
author_rank: Frequent Contributor
timestamp: February 09, 2015, 08:32:01 pm
content: Quote from: zapta on February 09, 2015, 01:11:45 pmAs others have said, if you're anal about emissions, you'll use them even at 12mbps. However if the chip you're using has slew rate control, you don't need them.There was some comments on this subject on my blog post about AVR USB interfacing.http://nerdralph.blogspot.ca/2015/01/usb-interfacing-for-avr-microcontrollers.html#comment-formAn easy way to actually check the EMI is to use a RTL-SDR dongle. I tested an Arduino pro mini board and found it had significant emissions at multiples of the 16Mhz clock frequency.

author: zapta
author_rank: Super Contributor
timestamp: February 09, 2015, 09:13:55 pm
content: Quote from: free_electron on February 09, 2015, 04:16:28 pmThe pride of copying from the reference design?;-)

author: free_electron
author_rank: Super Contributor
timestamp: February 09, 2015, 09:58:55 pm
content: The pride of doing it right as opposed to slap it together

author: Neilm
author_rank: Super Contributor
timestamp: February 09, 2015, 10:06:39 pm
content: Quote from: free_electron on February 09, 2015, 09:58:55 pmSlapping it togethermightbe OK for a hobbyist, assuming that it works. It is certainly not for a professional designer.

author: ralphd
author_rank: Frequent Contributor
timestamp: February 09, 2015, 10:07:27 pm
content: Quote from: free_electron on February 09, 2015, 09:58:55 pmAnd some people take pride in efficiency/minimalism. Waste not want not...

author: Pjotr
author_rank: Frequent Contributor
timestamp: February 09, 2015, 10:48:49 pm
content: Quote from: ralphd on February 09, 2015, 10:07:27 pmI wonder when they manage to get a good working USB interface with just one copper molecule in the endOn the other end EE's tend to be perfectionists and over-engineer. That is true as well. The real truth, a reliable working thing, is somewhere in between.

author: wraper
author_rank: Unknown
timestamp: February 09, 2015, 11:09:54 pm
content: Quote from: ralphd on February 09, 2015, 10:07:27 pmAnd in result making devices which are waste by themselves. IMO one can have pride about minimalism only if find how to achieve the same thing in some smart way that requires less parts. Not stupidly cutting "unnecessary" parts.

author: tggzzz
author_rank: Super Contributor
timestamp: February 09, 2015, 11:49:30 pm
content: Quote from: wraper on February 09, 2015, 11:09:54 pmYup. Everything should be "as simple as possible, but not simpler".Removing componentsmerelybecaue you don't understand why they are there is the Dunning-Kruger effect in action!

author: SL4P
author_rank: Super Contributor
timestamp: February 10, 2015, 12:10:46 am
content: I love these blind simplifications - I've even written legal disclaimers to customers - putting the risk on them for 'following' their demands for simplification of a circuit.A good example is a cost cutting measure that saved around $4 per device. But in my opinion was a huge mistake.With 150 units in the field at the time - mostly rural sites on sheep stations and other remote locations 50-150km from the nearest town - let alone support...A fix or upgrade would cost at minimum between $200 and $1000 per unit if needed 'after the fact' when you factor in the time, travel, accommodation etc (no cost of redesign or purchase new modules included)...Average that out at $600 per site x 150 sites == $90K minimum to correct a simple reliability & OH&S risk - onexistinginstallations. i'm not paying!

author: tggzzz
author_rank: Super Contributor
timestamp: February 10, 2015, 12:50:37 am
content: Quote from: SL4P on February 10, 2015, 12:10:46 amI once wrote a proposal for a machine to do X. The legal Ts&Cs conained the clause that we did not warrant the device to be suitable for X. The covering letter drew attention to that clause.The client was happy to sign, since they realised their reputation was at stake and therefore they had to ensure it was suitable for X.Happy days.

author: ralphd
author_rank: Frequent Contributor
timestamp: February 10, 2015, 01:27:53 am
content: Quote from: wraper on February 09, 2015, 11:09:54 pmMost of economics/business comes down to doing more with less. While over-engineering makes sense for transportation or medical devices, for consumer devices it is the minimalist designs that succeed.You seem to have a rather big ego to equate the suggestion of dropping the series resistors with stupidly cutting parts.Stupid would be dropping the parts when you know they are required. The OP's question was whether they are required or not. I think it is stupid to assert that they are required without having tested the circuit or at least a similar one.

author: c4757p
author_rank: Super Contributor
timestamp: February 10, 2015, 01:40:08 am
content: I suggest we make a whole bunch of flamebait argument honeypot threads that are only visible to Certain People, so the rest of us can discuss USB series terminations in peace. Should keep 'em busy for a while, as long as nobody tells them...oops

author: wraper
author_rank: Unknown
timestamp: February 10, 2015, 01:58:21 am
content: Quote from: ralphd on February 10, 2015, 01:27:53 amSo how Topic starter will test are they required or not? My guess all he will be able to test is it kinda works (it does) or does not. Nothing about long term reliability, ESD or surge robustness, electromagnetic emissions, compatibility and other possible issues.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 10, 2015, 02:00:04 am
content: I wouldn't say the average EE overdoes it. 90/10 rule works for everything. 90% of EEs are just smart enough to graduate, and use reference designs slapped together. 90% of them don't understand the inner workings, the EMC, limitations of their instruments... the list goes on. Not that, few if any, of the 10% (even the 10% of the 10%) have total knowledge, but in the space of practical "need to know" bits of knowledge, there's a big difference.You will often get excessive ratings when the level of knowledge is poor, to help account for the error in knowledge. But it swings both ways; the sigma is large, and new designs will fail as often as they'll end up overrated. Difference being, the failed ones never see the light of the marketplace, so how would you know?Truly excellent engineering is done down to every level, including cost and reliability. It takes a lot of experience to be able to do that all at once, on a short (one proto) project schedule at that. It can absolutely be done by accident, the difference being it has to be done a hundred times over. Which is what you get from Chinese products; a thousand companies attempt some product, a hundred of those succeed (and probably a hundred go under in the attempt), and ten of those wind up with the cheapest usable products that end up on eBay, etc. and make a little profit.Tim

author: zapta
author_rank: Super Contributor
timestamp: February 10, 2015, 03:36:49 am
content: Quote from: donotdespisethesnake on February 09, 2015, 08:00:55 pmThe NUF2042 looks very good and I like the small size (0.5mm pitch) . I think I will incorporate it the design. The series internal resistors are 17.6 to 26.4 ohm and I don't plan to add external resistors, I hope it's ok. The datasheet has a note to contact a sales person for 33 ohm resistance, do you know if there is anything of the shelf with 33 ohms?http://www.onsemi.com/pub_link/Collateral/NUF2030XV6-D.PDF

author: Niklas
author_rank: Frequent Contributor
timestamp: February 10, 2015, 06:47:43 am
content: Quote from: zapta on February 10, 2015, 03:36:49 amThe NUF2030* seems to be obsolete and is replaced by NUF2230*. NUF2042* is listed as "Last shipment"http://www.onsemi.com/PowerSolutions/obsolete.do?searchStr=nuf2030NUF2230Digikey NUF2230XV6T1GOSCT-NDMouser 863-NUF2230XV6T1GRS 802-4326 (50 pcs)NUF2042Digikey NUF2042XV6T1GOSCT-NDMouser 863-NUF2042XV6T1GRS 805-1879 (50 pcs)

author: Siwastaja
author_rank: Super Contributor
timestamp: February 10, 2015, 08:17:01 am
content: I also went through a phase when I thought I was so great as a HOBBYIST that I could dismiss everything they doin the industry. Then I grew up; my stance on hobbyism is that the hobby is something I reallylove, and something I want to reallyunderstand. Bypass caps are a great example. I started to put more of those, and nearer to the pins, but only after I gained understanding how they really work. Or ground planes; I had this crazy phase where I believed that the ground plane is an industry scam and the star ground approach is the right one, on a digital / mixed signal PCB. (I'm the brilliant one and they all do it wrong because they are stupid, but I'm the hobbyist and I can be an intelligent rebel!) After fixing these issues, I don't have random microcontroller resets or "weird behavior" anymore. Haven't had in ten years now.Another thing is the misconception of simplicity. Leaving random things out does not have anything to do with simplicity whatsoever. Sorry, but electronics always isn't as simple as you would expect, but that's because laws of physics. For example, many things work at all only because parasitic "components" such as ESR, ESL, capacitance... Sometimes the parasitics are enough, sometimes you want to supplement them. Series resistors are a great example. So, usually adding components is taking control of the situation, and THAT IS simplicity. Random is never simple. Controlled is.You can run an LED from an ultimately stable voltage source, in a stable thermal environment just fine. But it is not SIMPLE. Adding a single resistor makes everything simpler, even though it's an extra component.So, I might leave those resistors out, but not because I want to cut corners or "make things simpler"; I would do that deliberately to get it fail, to gain understanding why it fails and why the resistors are needed (or are they? then I would see). On the other hand, if I didn't want to spend my time on that right now, I would use the resistors. As a hobbyist, I have time to solder in two resistors (10 seconds each). A low-cost mass market product, on the other hand, may benefit from getting rid of everything "excess", but the volume has to be high so that it can be analyzed whether it's safe for the business to do so or not. And, it may happen that removing those two components puts more burden somewhere else; resistors, pick&place time and 0402/0201 footprints are extremely cheap, after all. But, this is far from the world of a hobbyist.TLDR; components are used in electronics to control things. Industry bean counters are there to remove components randomly, but that's the opposite of simplicity, and in my opinion, does not belong to a "hobby".

author: tggzzz
author_rank: Super Contributor
timestamp: February 10, 2015, 10:01:38 am
content: Quote from: ralphd on February 10, 2015, 01:27:53 amStupid is also asserting that parts can be dropped without knowing why they are there. Of course the person making such an assertion would probably not accept their stupidity due to the Dunning-Kruger effect!QuoteStupid is also testing a circuit under asubsetof the operating conditions and declaring that it works - without understanding and testing the edge-of-the-envelope operating conditions.

author: tggzzz
author_rank: Super Contributor
timestamp: February 10, 2015, 10:03:30 am
content: Quote from: wraper on February 10, 2015, 01:58:21 amPrecisely.There's nothing wrong with taking an amateur approach, provided that is stated and the limitations are stated.

author: tggzzz
author_rank: Super Contributor
timestamp: February 10, 2015, 10:07:26 am
content: Quote from: Siwastaja on February 10, 2015, 08:17:01 amJust so.Simplicity also means working predictably and reliably across the whole range of operating conditions. And people are prepared to pay more, much more, for predictability and reliability. For examples, look atHP AgilentKeysight and Tektronix equipment, prices, and businesses plans.

author: ralphd
author_rank: Frequent Contributor
timestamp: February 10, 2015, 12:45:06 pm
content: Quote from: wraper on February 10, 2015, 01:58:21 amIf you re-read my responses, you'll see I suggested a RTL-SDR dongle to test EMI. There were other suggestions to look at the signal on a scope - that would verify if slew rate and overshoot are in spec. For compatibility, the OP would get a fair idea from testing on a desktop PC, a cheap netbook, and a phone or tablet with USB OTG.

author: ralphd
author_rank: Frequent Contributor
timestamp: February 10, 2015, 12:49:19 pm
content: Quote from: tggzzz on February 10, 2015, 10:01:38 amClassic straw man argument. The OP never asserted that the series resistors can be dropped. That is obvious from the subject, "Are these USB series resistors important?"

author: zapta
author_rank: Super Contributor
timestamp: February 10, 2015, 04:35:38 pm
content: Quote from: Niklas on February 10, 2015, 06:47:43 amThe NUF2230 is specified as 100 ohm (I presume it is each of the two resistors, the datasheet is not clear what Ra means), the lpc11u35 MCU I am using calls for 33 ohm. Anything else with closer value and no hard wired pullup resistor?

author: tggzzz
author_rank: Super Contributor
timestamp: February 10, 2015, 05:26:48 pm
content: Quote from: ralphd on February 10, 2015, 12:49:19 pmCan you point to where I made that assertion? If not,yoursis the classic straw man argument!

author: zapta
author_rank: Super Contributor
timestamp: February 10, 2015, 06:19:36 pm
content: Quote from: tggzzz on February 10, 2015, 05:26:48 pmtggzz, any suggestion for a small USB protection filter that has ~33 ohm resistors and no hard wired 1.5K pullup resistor?

author: tggzzz
author_rank: Super Contributor
timestamp: February 10, 2015, 06:37:02 pm
content: Quote from: zapta on February 10, 2015, 06:19:36 pmQue?That is yoursecondstrawman argument!Can I suggest reading a little more slowly and waiting 5 minutes before clicking the "Post" button.

author: zapta
author_rank: Super Contributor
timestamp: February 10, 2015, 07:48:46 pm
content: Quote from: tggzzz on February 10, 2015, 06:37:02 pmQue?That is yoursecondstrawman argument!Can I suggest reading a little more slowly and waiting 5 minutes before clicking the "Post" button.

author: ralphd
author_rank: Frequent Contributor
timestamp: February 11, 2015, 05:48:07 pm
content: Quote from: tggzzz on February 10, 2015, 05:26:48 pmIt's right there above, "Stupid is also asserting that parts can be dropped without knowing why they are there." Unless the forum software is screwed up, those are your words.You misrepresent the OP by implying he's asserting parts can be dropped by without knowing why they are there, then you knock it down as being stupid.A straw man argument is when you build a straw man to represent your opponent, and then knock it down. The typical motivation is that you can't criticize the opponents actual position, so create a proxy for your opponent which you criticize instead.

author: tggzzz
author_rank: Super Contributor
timestamp: February 11, 2015, 06:04:18 pm
content: Quote from: ralphd on February 11, 2015, 05:48:07 pmPlease re-read the statements and point out where I asserted thatthe OPsaid the resistors could be dropped. The OP didn't say that, and I didn't assert they had. Hence your statement "The OP never asserted that the series resistors can be dropped" was strawman statement.

author: wraper
author_rank: Unknown
timestamp: February 11, 2015, 06:59:04 pm
content: Quote from: ralphd on February 11, 2015, 05:48:07 pmAren't you a politician by the way? You seem to have a good talent to misrepresent the words other people have said and slap your straw argument on top of that. I don't seetggzzzmentioning OP and particular situation with resistors at all.Sorry for the long quotations but they are needed to show what you call straw arguments.Quote from: ralphd on February 10, 2015, 01:27:53 amShow where I said that dropping resistors is stupid? I made a comment about pride doing things well in elegant way, not about dropping resistors. I only see that you try to insult others in your comments, particularly:QuoteShow me please where I was "equating" something with something.

author: tggzzz
author_rank: Super Contributor
timestamp: February 11, 2015, 07:26:06 pm
content: I'm going to pigeonhole ralphd as a troll, and cease trading points with him. Others are free to take a different view, of course.

author: paulie
author_rank: Frequent Contributor
timestamp: February 11, 2015, 07:53:38 pm
content: "Pride" seems to be the main theme here for several of the opposing team. As though it's an essential ingredient of good engineering. There are other words for it and BTW consider that in western (christian) and other religions it's one of The Seven Deadly Sins. I'm not religious but IMO economy and efficiency play a far more important role.I see 2 warring factions: the "professionals" who try to throw out as many buzz words and irrelevant technical issues to cloud the issues and generally show how smart they are. Then there's the "git 'er done (and git 'er done cheap)" hobby dudes who just want to build something and have fun. The former seem to harbor great resentment over this and tend to jump up and down and harass at every opportunity.Fortunately this is the internet and there's not much they can actually do about it. For example if I were designing Worlds Smallest Arm for personal amusement those 33R resistors would be the first things to go. No caps on the crystal either. And guess what... I'M LEAVING OUT BYPASS CAPS TOO !!! And the board still works. I do know what these things are for but gonna yank 'em anyway.So nya... nya... nya... how about that windbags? LOL.(ps. It is kinda funny to see them turning on each other when they run out of noobs to pummel)

author: luky315
author_rank: Regular Contributor
timestamp: February 11, 2015, 09:46:10 pm
content: When you're done with your "discussion":The STF202 from On Semi is a all in one USB input protection with integrated resistors.It works fine and is big enough (0,95mm pitch) for amateur soldering.

author: zapta
author_rank: Super Contributor
timestamp: February 11, 2015, 09:55:14 pm
content: Quote from: luky315 on February 11, 2015, 09:46:10 pmThanks for the pointer. I need one that doesn't have a hard wired pullup resistor. This is because the board needs to be re-enumerated when it changes USB profile (e.g. from serial CDC to virtual disk).BTW, the pullup signaling is done on the D+ signal. Will it work if I will use the STF202 with the fixed pullup on the D- signal and adding my own switched pullup on the D+?

author: retrolefty
author_rank: Super Contributor
timestamp: February 11, 2015, 11:08:34 pm
content: Quote from: paulie on February 11, 2015, 07:53:38 pmBut it's those various flavors that make this forum so entertaining for many of us. It's not just about education you know. The various xxx-nuts threads can entertain for months and years.

author: Pjotr
author_rank: Frequent Contributor
timestamp: February 11, 2015, 11:32:17 pm
content: Yeah, but lengthy quibbling about just 2 resistors costing a fraction of a cent? By hell, what are that pro EE's supposed to design? The hobbyists do it and find their own way anyway

author: free_electron
author_rank: Super Contributor
timestamp: February 12, 2015, 12:40:11 am
content: Quote from: paulie on February 11, 2015, 07:53:38 pmand the board may very well work apart from sometimes random resetting , locking up , doing strange things, not working on some computers , disconnecting itself, reconnecting random re-enumerating.but we can blame that on windows and crappy software right ...while we are at it. why don't you just remove the locks from your doors , they are not necessary. anyone who wants to enter your house will ram the door, drill the lock or smash the window anyway.

author: zapta
author_rank: Super Contributor
timestamp: February 12, 2015, 01:01:24 am
content: Quote from: free_electron on February 12, 2015, 12:40:11 amThat's a very good example. There is a wide spectrum of home safety equipment and each household pick whatever fits them. I would expect that this would be the same with securing (need a better word) electronic designs. That is, it's not necessarily one size fits all. Same goes to design processes, assembly environment (e.g. ESD), EMI, USB impedance matching and so on.I would like to hear from the pros here, do you always design with the same level of protection or does it 'depend'?

author: tggzzz
author_rank: Super Contributor
timestamp: February 12, 2015, 01:54:07 am
content: Quote from: zapta on February 12, 2015, 01:01:24 amYou assess the threats and relative threats. You assess methods of mitigating the threats, and their costs. You make a cost-benefit tradeoff.Sometimes you get it wrong; the Ford Pinto being the classic example.I have several times realised during my journey to work that I hadn't shut the front door. I didn't bother to go back and close it. There were no problems.I have occasionally deliberately left the front door open, when I judged the risk from having it locked was worse than the risk of leaving it open.

author: luky315
author_rank: Regular Contributor
timestamp: February 12, 2015, 07:36:56 am
content: @zapta:I don't know a protection device without the pullup resistor in a "solder freindly" package. There is the NUF2042 from On Semi, but it's in a 1.2mm x 1.6mm SOT?563 with 0.5mm pitch... Not a real problem if you go into production, but a challenge for hand soldering.

author: zapta
author_rank: Super Contributor
timestamp: February 12, 2015, 07:42:55 am
content: Quote from: luky315 on February 12, 2015, 07:36:56 amLuky315, I just finalized the design with nuf2042 and sent to oshpark. It's about the same pitch as the MCU so I should be able to solder it. The only issue is that the resistors are 22 ohm and not 33 as the MCU's datasheet recommend but should be better than zero.

author: paulie
author_rank: Frequent Contributor
timestamp: February 13, 2015, 12:33:47 pm
content: Quote from: retrolefty on February 11, 2015, 11:08:34 pmSometimes entertaining, other times plain sad. At least from my viewpoint.Admittedly I learned a lot in those LM399 and LT2000 threads. Not just electronics but physics and science in general. But then the blood fest started with that Jesus freak Ebay seller. Ironically several (not just one or two) initial reviews showed his product performed more than 10x better than it was supposed to. Very disappointing to see "upper echelon" individuals I had great respect for take part in the lynching. Maybe the same personality defect responsible for this behavioral quirk goes hand in hand with ability to innovate and think outside the box so.... I dunno.