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 > > ------------------------------------------------------------------------------ > 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
