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
