Mostle hardware related tips, as other software works without any prblem redaing temperatures from the same hardware-setup I strongly beleave that this 85C problem is related to owfs somehow.
Well, here is the schematics of the 1-wire master hub http://www.hobby-boards.com/catalog/links/6cmh1-r1/6%20Channel%20Master%20Hub%20v1.1%20Schematic.pdf, if you se any consturction errors it would be nice to know. /Thomas David Lissiuk skrev: > Hi all, > > As I have done fairly extensive testing of various 1-wire issues I can > maybe help shed some light on possible hardware causes of seeing 85.0C > errors for those new to these issues. > > In general a 85.0C reading indicates that a POR has occurred in the > chip: > > The following issues are common, note that not all are things that > software alone can correct. > > 1) Poor 1-wire line levels > A). If the chip is wired for parasitic operation, the VDD line must be > grounded. Otherwise unknown parasitic results may occur. (This is not > required with -PAR devices, which are internally grounded inside the > chip). > > B). While operating under powered mode of operation, the power supply > has to have a reasonably fast rise time, else the chip may enter an > internal test mode. The voltage should ideally be 5 volts to insure > maximum signal headroom for the 1-wire signal. Line losses through the > cable need to be considered. > > C). The slave device (the DS18B20) may not have enough power/current to > complete the temperature conversion and this may cause a POR to occur in > the chip instead (Generating the 85.0C reading). This may be caused by > the following: > > 2). Insufficient weak-pull-up current on simple 1-wire bus master > designs > I've seen issues when the weak-pull up current is insufficient, causing > the chip to do a POR during the temp conversion time. Generally a value > between 1.1K and 2.5K for the weak pull-up resistor is what I recommend. > I've often seen this problem on bus master designs using a 4.7K or > greater weaker pull-up resistors. > > 3) Insufficient conversion time/current. > Parasitic driven devices take considerably longer to do their conversion > than a powered device does (for this reason I generally recommend > powered sensors (See 1WRJ45 for one way how to supply power through a > cable). You will get a 85.0 result if the conversion has not completed > and a POR had occurred previously. > > A powered device temperature conversion is typically around 650ms at > 12bits (750ms max). The DS18B20 uses a relatively crude internal timing > oscillator. And variations from both, temperature and internal chip > composition, etc. may effect the duration needed for a successful > temperature conversion cycle to complete, and the chip may draw as much > as 1.5ma during the conversion processes. This may be why one chip > works and the other doesn't if you're just marginal with the timings. > (generally a good time to redesign your network to fix the fundamental > problems). I have heard of one batch of DS18X20's working in a circuit > and another not due to these slight internal differences on marginal > designed networks. This is not a failure of the parts, just of poor > network design and is generally corrected by insuring sufficient > conversion current is received by the temperature device for the entire > temperature conversion cycle. Powered temperature sensors may also be > polled for when the temperature conversion is complete (thus saving > time). See the datasheet for more information on this. > > Another source of errors is too long delays in providing a strong > pull-up current for parasitic device. The sensor is issued a conversion > command, but a strong pull-up current is not applied for one reason or > another and the device loses power. This is only an issue with parasitic > operated devices obviously. It is generally a poor bus master design or > improper programming of the strong pull-up of the bus master. > > 4), Current supply limitations: > Since during a temperature conversion current use in the device can be > fairly high, (1.5ma max by spec) limitation of the network design may > affect this supplied current. > > A). Limitations of the Bus master to supply conversion current. > A LINK based design bus master can supply more current than a DS2480B > design (DS9097U-S09). A DS2490 (USB) can supply slightly more current > than a DS2480B unit. The Impedance matching line filter on the bus > master must also be considered if added to a DS9097U-S) or USB unit > (Generally a highly recommended practice to add the filter for signal > reflection issues). If the bus master is also parasiticlly powered this > may also be an issue or limitation. > > B). Any additional bus line resistance. > The use of a DS2409 adds additional current limiting to what can be > supplied to downstream devices. Each channel of a DS2409 has a different > resistance. So channel use may also effect operation. In addition the > DS2409 chip package sets the maximum pass-gate current to 20ma max. > according to Dallas (info not in data sheet) This can be a limitation in > any attempt to do bulk temperature conversions of parasitic downstream > devices. > > C) Time delays generated by inline devices > A DS2409 can also cause a POR to occur on a temperature sensor after > switching channels using a smart-on command. (There are several errors > in the DS2409 data sheet that I've notified Dallas about, including the > flow diagram. Dallas has verified them and is correcting them in a new > revision of the data sheet. Though I am not sure if all of them will be > corrected by then). > > Much of this information has been developed in my efforts on basic > research for a hardware book on 1-wire I am developing in my free time > (Still in early stages of writing, as I have very little of that. I do > however welcome suggestions and reports of problems that need to be > researched (contact me off list)). > > I hope this information can be of help by the software writers in > understanding some of the possible conditions that can cause the > reporting of a 85.0C error in the software that should be considered. As > you can see when reporting an 85.0C error it might also be useful to > others to report the bus master used, any line filter, the number and > channels used in any hubs (and the hub design being used), along with > the temperature sensor wiring (powered or parasitic), the delivered > power voltage and the chip revision if known. > > Hope this helps the software efforts, > Cheers > > David Lissiuk > Sr. Computer Scientist > Springbok Digitronics > > > ------------------------------------------------------------------------- > Take Surveys. Earn Cash. Influence the Future of IT > Join SourceForge.net's Techsay panel and you'll get the chance to share your > opinions on IT & business topics through brief surveys-and earn cash > http://www.techsay.com/default.php?page=join.php&p=sourceforge&CID=DEVDEV > _______________________________________________ > Owfs-developers mailing list > Owfs-developers@lists.sourceforge.net > https://lists.sourceforge.net/lists/listinfo/owfs-developers > ------------------------------------------------------------------------- Take Surveys. Earn Cash. Influence the Future of IT Join SourceForge.net's Techsay panel and you'll get the chance to share your opinions on IT & business topics through brief surveys-and earn cash http://www.techsay.com/default.php?page=join.php&p=sourceforge&CID=DEVDEV _______________________________________________ Owfs-developers mailing list Owfs-developers@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/owfs-developers
