Am 17.06.19 um 22:35 schrieb Mick Sulley:
>
> So the question is - is it possible to have a fault on a DS18S20 which
> can cause it to heat up and pull the voltage down?
>
Certainly. But the fault isn't the DS18S20 but the 1W host.
The 1W host has to supply current to the 1W line to power all the
parasitic powered slave devices. It does so through the 1.5–4.7kΩ pullup
resistor in normal operation.
That current isn't sufficient during a temperature conversion however.
During that time, the power supply is connected to the 1W through a
considerably lower resistance. That's called the "string pullup" That's
okay as long the slaves idle.
If you have a short 1W dip in the bus to to a short or interferences,
the slaves may think it's a bus reset from the host and answer with a
presence pulse, which means they pull the bus low. If the host supplies
power through the strong pullup, that means there is a short through the
pulldown transistor of the slave.
Nothing bad happens if that occurs once. It's bad however if it occurs
again and again. Then, those shorts will heat up the slaves.
Check your onewire cabling and sources of interference, e.g. power
cables nearby.
Kind regards
Jan
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