There was a thread on this group regarding DS18B20 and 3.3V logic.
I cannot add comment there (the thread is closed) however google still points there anyway. It might be good to reopen it or to redirect further discussion here. That's why I am opening a new thread about it.
I have a few comments and one question:
- did anybody achieve large DS18B20 network to work on 3V3 logic? If yes then how? With any extra circuit? to allow sensor network to operate on higher voltage than cpu? Or keeping 3V3 bug giving ultra small pullup resistor?
My experiences with DS18B20 on 5V logic:
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if you are using many sensors (12+) and long wires (10m+) you need to scale down the pullup resistor significantly (really drastically). I managed to make 30 thermometers work on a topology which has weight over 30-40 meters. I did not use any decupling because the problem was with slow signal rise, not with echo (as measured using oscilloscope).
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to achieve this I had to reduce the pullup resistor to 1k ohm. Yes. 1000ohm. I was checking on oscilloscope how the signal rise gets degraded when attaching extra sensors and extra wire length. Now I have several installations with 20+ sensors each, with cables weight up to 50m and they are working.
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one comment: cable weight I am counting as the total length of all cables, plus 2m for each crossroad (star split), plus 2m for each sensor. It is not fully compliant with literature but it is working for me. In literature the cable weight calculations are sligthly different.
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please note, that this is without using parasite power. in literature there is a schema for large networks using parasite power - search for a term 'strong pullup'. This is an extra transistor which is strongly pulling up power/data line to 5V when not transmitting/receiving data. You have to dedicate one extra microcontroller output for this, and disable strong pullup during transmission, re-enable it after transmission. I did not test it much because my networks are large (20+ thermometers) and I wanted to be on a safe side.
However now I am preparing to replace ATMEGA 5V cpus with 3.3V logic 32bit cpus. And now there is a question if anybody has any experience with 3.3V logic for DS18B20. Did anybody make it work, and if yes than what was the weight, and how the circuit did look like.
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