i have some problems with pic16f877a reading 85 degree, what i do? read again, if it's 85 degree ok (maybe ds1820 are burning) if i'm sure that it isn't at 85 degree i change the ds1820 and every thing go ok
Thomas Berg escreveu: > 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. 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