what the best method to use parasite? put +5v pin to ground? it will not float :) that´s the best method?
2011/3/10 Eloy Paris <[email protected]>: > On 03/10/2011 12:27 PM, Farmer iButtonLink wrote: > >> Argh! I hate it when the correction needs a correction. >> >> Should have read: >> >> IF, however, you are using a master (in our case the Link45 family) that >> does NOT have a connection to +5 (Pin 2 ... orange) on the cable, the >> net result is that the Vdd pin of the DS18B20 is left floating. > > Ahh, totally clear now. Thanks! > > Cheers, > > Eloy Paris.- > >> >> -----Original Message----- >> From: Eloy Paris [mailto:[email protected]] >> Sent: Thursday, March 10, 2011 9:57 AM >> To: [email protected] >> Subject: Re: [Owfs-developers] [SOLVED] Dreaded 85 degrees C read >> fromDS18B20 located outside >> >> Bill, >> >> On 03/10/2011 09:52 AM, Farmer iButtonLink wrote: >> >>> The statement in our manual is misleading. The "floating" Vdd pin is >>> not really floating. It is connected to +5 on the cable. IF, >> however, >>> you are using a master (in our case the Link45 family) that does have >> a >>> connection to +5 on the cable, the net result is that the Vdd pin of >> the >>> DS18B20 is left floating. >> >> Thanks for the clarification. Agreed that the manual is misleading given >> >> what you've explained. One more question, though: is there a typo in >> your last sentence, i.e. should it be "the net result is that the Vdd >> pin of the DS18B20 is *not* left floating"? (Note added "not".) >> Otherwise I don't totally follow and would have to wait for the reworked >> >> statement. >> >> Cheers, >> >> Eloy Paris.- >> >>> >>> The LinkUSB family and the LinkHub family all put +5V on the cable so >>> the T-Sense is never left in parasitic mode. >>> >>> All Link masters have a switched +5V connection to the cable (Aux). >>> This connection is either grounded or powered but never floating. >>> >>> I will have Mike rework the statement. >>> >>> Bill Farmer >>> >>> >>>> -----Original Message----- >>>> From: Eloy Paris [mailto:[email protected]] >>>> Sent: Wednesday, March 09, 2011 11:33 PM >>>> To: [email protected] >>>> Subject: Re: [Owfs-developers] [SOLVED] Dreaded 85 degrees C read >>>> fromDS18B20 located outside >>>> >>>> On 03/09/2011 06:40 PM, Doug Collinge wrote: >>>> >>>> > So, I had a look at the datasheet to see what would happen if >> the >>> Vdd >>>> > lead on a DS18B20 was left floating. All there is to work from >> is a >>>> > block diagram, which doesn't contain enough information. So I >> asked >>>> tech >>>> > support at Maxim the following question: >>>> > >>>> > The block diagram of the DS18B20 shows a block labeled "Power >>> Supply >>>> > Sense" that determines whether the chip has Vdd power or not. >> The >>>> > datasheet says to ground Vdd for parasitic operation and it's >> clear >>>> that >>>> > the input to Power Supply Sense will see 0V in that case. My >>> question >>>> is >>>> > what will it see if the Vdd input is allowed to float? If the >>> capacitor >>>> > is charged the diode will be a high impedance. What is the >>> impedance of >>>> > the Power Supply Sense input? If it is also a high impedance >> that >>> might >>>> > explain why someone is seeing inconsistent behaviour with Vdd >>> floating. >>>> > >>>> > They promptly replied as follows: >>>> > >>>> > The Vdd 'sensing circuit' is a high-impedance node, but the Vdd >> pad >>> is >>>> > not. Simply disconnecting the Vdd supply will leave the >> capacitor >>>> > charged and the component would therefore not be powered >>> parasitically. >>>> > The capacitor cannot provide enough current to power the >> component, >>> but >>>> > defeats the 0V detection required. >>>> > >>>> > Floating Vdd is an invalid/undefined condition for the DS18B20. >>>> > >>>> > We also sell a DS18B20-PAR+ (TO-92 only), which precludes the >>> necessity >>>> > for the Vdd pin to be grounded on a board. >>>> > >>>> > So I think that's a pretty clear explanation: the component >> expects >>> Vdd >>>> > to be wired either to +5 or ground. If it isn't it may think it >> is >>>> > powered when it is not and not function correctly. >>>> > >>>> > In the specific situation described, outdoors but sheltered from >>> rain, >>>> > but high humidity my guess is that the humidity was supplying >>> enough >>>> > conductivity to a positive voltage source to keep the Vdd >> terminal >>> high >>>> > enough for the internal sensor to conclude that the chip was >>> powered. >>>> It >>>> > would therefore not be able to do a conversion correctly on the >>> power >>>> > stored in the capacitor and report an error. >>>> > >>>> > The obvious conclusion is that people should follow the clear >>>> > instructions of the manufacturer and ground Vdd for parasitic >>>> operation. >>>> >>>> Thanks Doug, great information. I guess I read bits and pieces of >> the >>>> DS18B20 datasheet a long time ago and either forgot this particular >>>> detail (after things worked for me without following the >>> requirement), >>>> or missed it altogether: >>>> >>>> "When the DS18B20 is used in parasite power mode, the VDD pin must >> be >>>> connected to ground." >>>> >>>> As you say, it does not get any clearer than that. >>>> >>>> Their (Maxim tech. support's) response seems to imply that things >>> would >>>> not work by leaving Vdd floating. However, it has worked for me for >>>> months, and even iButtonLink seem to be shipping sensors with Vdd >>>> floatin (the following comes from the manual for iButtonLink's >>> T-Sense >>>> sensor >>>> >>> >> [http://www.ibuttonlink.com/pdf/manuals%20for%20t-sense%2005-22-08.pdf]) >>> : >>>> >>>> "NOTE: Important information >>>> >>>> There is a small chance that the bus could become unstable when >> using >>>> T-Sense units as the only sensor on the bus. This condition comes >>> about >>>> as a result of the new design of the T-Sense, using the DS18B20 >>> sensor. >>>> The third lead on the DS18B20, (Vdd) is left floating, (to allow >>> design >>>> flexibility for sophisticated networks) which leaves the >> possibility >>> of >>>> unpredictable performance. Our engineering department has not been >>> able >>>> to create this condition. Experiments with> 30 T-Sensors on the >> bus, >>>> and a variety of interfaces, have shown all devices working >> properly. >>>> >>>> In the unlikely event of unstable behavior on the bus, resolution >> can >>> be >>>> accomplished by simply connecting pin 2 (+5 volts) to pin 6 (Aux). >>>> Alternately, any MS-xx product on the bus will provide the jumper >>>> internally, or the use of a LinkHub as the interface." >>>> >>>> So I either don't understand what Maxim tech. support said, or I do >>>> understand it but then it's mistery why so many DS18B20 seem to >> work >>>> fine with a floating Vdd ;-) >>>> >>>> In any case, thanks again for the information, and I'll be sure to >>>> ground Vdd little by little (but no rush since things are working >>> great >>>> for me ;-) ) >>>> >>>> Your explanation on why high humidity could have caused the 85 >> degree >>>> problem makes sense to me. >>>> >>>> Cheers, >>>> >>>> Eloy Paris.- >>>> >>>> P.S. Regarding iButtonLink's statement that they have not been able >>> to >>>> recreate network instability with more than 30 sensors on a bus all >>> with >>>> floating Vdd, perhaps they haven't tried in a high humidity >> scenario >>>> like was the case for the one sensor that I had outside? >>>> > > ------------------------------------------------------------------------------ > Colocation vs. Managed Hosting > A question and answer guide to determining the best fit > for your organization - today and in the future. > http://p.sf.net/sfu/internap-sfd2d > _______________________________________________ > Owfs-developers mailing list > [email protected] > https://lists.sourceforge.net/lists/listinfo/owfs-developers > > -- Roberto Spadim Spadim Technology / SPAEmpresarial ------------------------------------------------------------------------------ Colocation vs. Managed Hosting A question and answer guide to determining the best fit for your organization - today and in the future. http://p.sf.net/sfu/internap-sfd2d _______________________________________________ Owfs-developers mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/owfs-developers
