Elroy, 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.
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 ------------------------------------------------------------------------------ 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
