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DS18B20 problem — Parallax Forums

DS18B20 problem

jagrifenjagrifen Posts: 36
edited 2011-10-21 13:52 in General Discussion
I bought a new ds18b20 temperature sensor and I am having problems interfacing with an old Prop circuit I built a while ago. I can retrieve the serial number of the device but when I try to do a temperature reading the chip returns 85 degrees C which is the default scratchpad value. Whenever I hook up any of my other Ds18b20 chips they operate just fine. Is there a setting within the chip that needs to be addressed? It seems like the chip just isn't doing any new conversions.

Comments

  • ElectricAyeElectricAye Posts: 4,561
    edited 2011-10-21 07:37
    Any chance you might have bought the parasitic version? There is a version of this chip that can be powered by the data line and it doesn't work the same as the regular DS18B20. I don't know if the parasitic version will cause the kind of behavior you are describing, but it might be worth double checking. The parasitic version often has PAR somewhere in its catalog number.

    http://search.digikey.com/us/en/products/DS18B20%2BPAR/DS18B20%2BPAR-ND/1197285
  • jagrifenjagrifen Posts: 36
    edited 2011-10-21 07:53
    I tried powering it parasitically with the V+ pin tied to ground and it behaved the same.
  • ElectricAyeElectricAye Posts: 4,561
    edited 2011-10-21 08:57
    jagrifen wrote: »
    I tried powering it parasitically with the V+ pin tied to ground and it behaved the same.

    I'm not sure it's as simple as that when operating the parasitic version. Did you look at the data sheet for the parasitic version?

    "...To assure that the DS18B20-PAR has sufficient supply current, it is necessary to provide a strong pullup on the 1-Wire bus whenever temperature conversions are taking place or data is being copied from the scratchpad to EEPROM. This can be accomplished by using a MOSFET to pull the bus directly to the rail as shown in Figure 2.... "

    Did you check to see if the chip you have is a normal version or parasitic? I think the chip label should have some sort of indicator on it. But again, I don't know if that's really your problem - just a suggestion.
  • jagrifenjagrifen Posts: 36
    edited 2011-10-21 09:22
    I see what you're saying. Unfortunately I can't say for sure if the part number is correct. I bought the sensor encased in a stainless probe. The company sells these all the time and says they have never heard of this problem. They are very sure that the chip inside is indeed a DS18b20. It seems like if the chip was bad it wouldn't communicate at all.
  • FranklinFranklin Posts: 4,747
    edited 2011-10-21 13:52
    If you are getting the ID the first part of that is the chip type. I think it should be 10 or 27 (check the data from Maxim) If one works and the other does not in the same circuit and same software that would lead me to think the chip is bad.
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