So, I had a look at the datasheet to see what would happen if the Vdd lead
on a DS18B20 was left floating. All there is to work from is a block
diagram, which doesn't contain enough information. So I asked tech support
at Maxim the following question:
The block diagram of the DS18B20 shows a block labeled "Power Supply Sense"
that determines whether the chip has Vdd power or not. The datasheet says to
ground Vdd for parasitic operation and it's clear that the input to Power
Supply Sense will see 0V in that case. My question is what will it see if
the Vdd input is allowed to float? If the capacitor is charged the diode
will be a high impedance. What is the impedance of the Power Supply Sense
input? If it is also a high impedance that might explain why someone is
seeing inconsistent behaviour with Vdd floating.
They promptly replied as follows:
The Vdd 'sensing circuit' is a high-impedance node, but the Vdd pad is not.
Simply disconnecting the Vdd supply will leave the capacitor charged and the
component would therefore not be powered parasitically. The capacitor cannot
provide enough current to power the component, but defeats the 0V detection
required.
Floating Vdd is an invalid/undefined condition for the DS18B20.
We also sell a DS18B20-PAR+ (TO-92 only), which precludes the necessity for
the Vdd pin to be grounded on a board.
So I think that's a pretty clear explanation: the component expects Vdd to
be wired either to +5 or ground. If it isn't it may think it is powered when
it is not and not function correctly.
In the specific situation described, outdoors but sheltered from rain, but
high humidity my guess is that the humidity was supplying enough
conductivity to a positive voltage source to keep the Vdd terminal high
enough for the internal sensor to conclude that the chip was powered. It
would therefore not be able to do a conversion correctly on the power stored
in the capacitor and report an error.
The obvious conclusion is that people should follow the clear instructions
of the manufacturer and ground Vdd for parasitic operation.
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