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
