How would you interpret this? - Page 1
Forum section: electronics / general

author: KE5FX
author_rank: Super Contributor
timestamp: December 02, 2018, 10:43:11 pm
content: FromMCP23018 / MCP23S18data sheet page 6:

author: IanB
author_rank: Super Contributor
timestamp: December 02, 2018, 10:48:27 pm
content: Is there any way to misinterpret it?OK, yes. Either it says "do not connect to VSS", or it says "you can connect it to VSS"I would go with "do not connect to anything" personally.

author: wraper
author_rank: Unknown
timestamp: December 02, 2018, 10:53:27 pm
content: I would connect to Vss/GND just because this way you can provide better cooling without breaking ground plane. Thermal pad is generally connected to GND, sometimes obligatory, sometimes not. What's not clear? Don't connect at all, or do connect to Vss.

author: grumpydoc
author_rank: Super Contributor
timestamp: December 02, 2018, 11:03:02 pm
content: Quote from: wraper on December 02, 2018, 10:53:27 pmThe language is ambiguous - the "do not" could bind to the "electrically connect" alone or it could bind to "electrically connect, or connect to VSS".Given the comma I would say "do not" applies only to electrically connect" and that it is OK to optionally connect to VSS- there is also a logical redundancy if you are supposed to not connect to anything. Given that connecting to VSSis electrically connecting it there isn't really a need to specify both - just "do not electrically connect" would suffice.I'm with Ian B though - if in doubt not connecting it to anything seems safe, the data sheet definitely says that it OK.

author: wraper
author_rank: Unknown
timestamp: December 02, 2018, 11:10:44 pm
content: Most likely thermal pad it's already connected to GND within the chip anyway.QuoteWhere there is logical redundancy? You must not connect it to anything other than Vss, if you decide to connect at all.QuoteHow would it suffice?It would explicitly prohibit connecting to Vss when it's completely OK to do so.

author: CatalinaWOW
author_rank: Super Contributor
timestamp: December 02, 2018, 11:27:44 pm
content: This kind of thing is what applications engineers are for. Call the vendor, or email them. Personally I am sure that connection to Vss is optional, and all other connections are not allowed.

author: Benta
author_rank: Super Contributor
timestamp: December 02, 2018, 11:38:22 pm
content: I see no problem, the comma says it all.Either don't connect the pad, or connect it to Vss.

author: amyk
author_rank: Super Contributor
timestamp: December 02, 2018, 11:38:52 pm
content: It doesn't sound ambiguous to me.This is a substrate, and thus it can be connected to Vss if so desired.If it couldn't be connected to anything, they would just use "Do not electrically connect."

author: KE5FX
author_rank: Super Contributor
timestamp: December 03, 2018, 12:21:06 am
content: Quote from: wraper on December 02, 2018, 11:10:44 pmIt isn't; I checked. The pad exhibits >> 1M resistance with respect to all other pins.It looks like this inside, for what it's worth:Quote from: amyk on December 02, 2018, 11:38:52 pmSo what bad things can I expect to happen if I connect it to, say, Vdd? No sign of any parasitic diode behavior, just very high resistance.

author: wraper
author_rank: Unknown
timestamp: December 03, 2018, 12:47:32 am
content: Quote from: KE5FX on December 03, 2018, 12:21:06 amFailure, increased current consumption, weird behavior.

author: KE5FX
author_rank: Super Contributor
timestamp: December 03, 2018, 12:52:16 am
content: Quote from: wraper on December 03, 2018, 12:47:32 amThrough several megs of bulk resistance? How?I'm trying to put myself inside the head of whoever wrote that part of the data sheet. Mostly out of idle curiosity, not out of any compelling need to connect the pad to ground or anything else. (In fact, it's helpful that it doesn't require a ground connection.)It's unclear why they wouldn't just mark the pad as NC and call it good. I suppose they have to worry about someone connecting 100 volts to it just because the data sheet didn't say not to.

author: MK14
author_rank: Super Contributor
timestamp: December 03, 2018, 01:05:09 am
content: Quote from: KE5FX on December 03, 2018, 12:52:16 amThis app note or datasheet, seems to explain about the EP and why it can be connected to Ground/Vss, but not other signals/voltages.tl;drBecause of EMC.N.B. It is from an entirely different manufacturer, and completely different parts. So take it with a tiny pinch of salt!http://cache.freescale.com/files/microcontrollers/doc/eng_bulletin/EB806.pdf

author: KE5FX
author_rank: Super Contributor
timestamp: December 03, 2018, 01:08:13 am
content: Quote from: MK14 on December 03, 2018, 01:05:09 amThat makes good sense, thanks. Given how much the X-ray cross section resembles figure 4, it seems safe to say the same advice is applicable.

author: Brumby
author_rank: Unknown
timestamp: December 03, 2018, 02:01:01 am
content: Quote from: KE5FX on December 03, 2018, 12:52:16 amI was thinking Field Effects as well.

author: KE5FX
author_rank: Super Contributor
timestamp: December 03, 2018, 02:05:34 am
content: Quote from: Brumby on December 03, 2018, 02:01:01 amPossibly, but that substrate would make an awfully thick gate region.

author: Brumby
author_rank: Unknown
timestamp: December 03, 2018, 02:45:37 am
content: I was thinking a bit wider than that - to include capacitive coupling to the wrong plane.Anyway, the manufacturer will have had reason and they'd be the best to answer. I'm just spitballing.

author: grumpydoc
author_rank: Super Contributor
timestamp: December 03, 2018, 07:24:22 am
content: Quote from: wraper on December 02, 2018, 11:10:44 pmOh FFS - the redundancy is in the language used, Amyk saw this.IF you wanted to instruct the user not to connect the pad at all it suffices to say "do not electrically connect", you do not need to tack on "or connect to VSS" because that is also "electrically connecting" it.Benta also made the point re: the comma (which I made in my post) - it provides some disambiguation but, English being an imprecise language, not 100%So, broadly I would read it the way most people are doing - either a) do not connect the pad or b) connect the pad to ground.But, as I said, Ian B has the safe option covered - if not sure just leave it unconnected. Whatever the ambiguity over VSSthe datasheet clearly says leaving it disconnected is OK.

author: wraper
author_rank: Unknown
timestamp: December 03, 2018, 12:27:42 pm
content: Quote from: grumpydoc on December 03, 2018, 07:24:22 amThere is no instruction or intention to say to not connect itat all. But if you do connect, there is only one right option to do so, hence there is written what's written. "Connect to Vss" is only a tiny subset of "electrically connect". What's written is clear as day for me. Dunno, maybe you are adept of electronic totalitarianism who becomes confused when given 2 choices what to do.

author: grumpydoc
author_rank: Super Contributor
timestamp: December 03, 2018, 03:34:38 pm
content: Quote from: wraper on December 03, 2018, 12:27:42 pmNo, I generally manage when presented with two choices. In this case I think the intent of instructionisclear enough but the language is slightly ambiguous. I still don't think that you are "getting"whysome of us agree that the language is a bit less than "as clear as day" - but I'm not going to lose sleep over it

author: Wan Huang Luo
author_rank: Regular Contributor
timestamp: December 03, 2018, 03:53:08 pm
content: Quote from: grumpydoc on December 02, 2018, 11:03:02 pm

author: ogden
author_rank: Super Contributor
timestamp: December 03, 2018, 03:53:20 pm
content: Quote from: grumpydoc on December 03, 2018, 03:34:38 pmIt's result of globalization. Often design/documentation engineersdo not think in English, thus may construct ambiguous sentences. AFAIK Microchip have design facilities in India and China as well.

author: IanB
author_rank: Super Contributor
timestamp: December 03, 2018, 05:11:30 pm
content: For the avoidance of doubt, the natural and unambiguous way to write the instruction in English would be this:"Leave unconnected, or connect to Vss."Alternatively:"Connect to Vss, or leave unconnected."The difference between the two is that whatever comes first is likely to be the preferred option. So the first sentence above says "Leaving unconnected is best, but you can connected to Vss if you like."

author: macboy
author_rank: Super Contributor
timestamp: December 05, 2018, 04:39:47 pm
content: Quote from: Benta on December 02, 2018, 11:38:22 pmQuite right. In addition, if the meaning was to specify that it not be connected to Vss, the correct English wording is "Do not electrically connect, nor connect to Vss" or similar. Thenoris very important, and very different thanor.

author: gamalot
author_rank: Super Contributor
timestamp: December 05, 2018, 05:04:36 pm
content: My English is not good enough, I understand that this sentence means you can choose "floating" or "grounding".

author: vk6zgo
author_rank: Super Contributor
timestamp: December 06, 2018, 11:48:47 pm
content: Sorry, as I read it, the "do not" is the operative part of the sentence, so it is the same as saying:-"Do not electrically connect",&"Do not connect to Vss", as two sentences.My assumption is that the latter is because mounting screws to "chassis" may not always be recognised as"electrical connections" by the user.

author: MK14
author_rank: Super Contributor
timestamp: December 07, 2018, 12:32:06 am
content: Quote from: IanB on December 03, 2018, 05:11:30 pmUnfortunately, I suspect, that saying "Leave unconnected, or connect to Vss.", may actually cause confusion, of a different kind.Namely, that the EP (ignoring certain exceptions), does still need to connect up thermally, to the copper pad, cooling solution.So, saying "UNCONNECTED", may itself, also cause issues/confusion.So it would need to say, something like ..."As well as thermally interfacing to the lower pad, there are two electrical connection choices. Either leave it completely electrically isolated (but still thermally connected to the copper pad) or you can connect it to VSS. E.g. Because of EMC requirements"The problem would be that I'd turn a half page, easy to quickly print/read datasheet, into a 550 page, heavy book volume.

author: KE5FX
author_rank: Super Contributor
timestamp: February 09, 2019, 02:29:12 am
content: For posterity's benefit: it does not, in fact, appear to do any harm to ground the pad.