Am 06.12.2016 um 07:17 schrieb Robin Gilks:
> After years of using all sorts of 1-wire devices I've just read in the
> spec sheet that if the DS18B20 is to be parasite powered then the VDD pin
> should be connected to ground.
>
> Does anyone know of the consequence of not doing so?
>
The Vdd pin also works as an input pin.
Before starting a "Convert T" command, the host issues a "Read Power
Supply" command on the selected bus. That command samples the Vdd pin.
When the result is low, it means Vdd is grounded and we have parasite
power. When it is held high *or floating*, the bus pullup will have the
host sample high - no parasite power.
That's why it's important to tie Vdd to GND on parasite-powered DS18x20.
The host mistakes them for powered ones and does not activate the strong
pullup. In combination with a too weak pullup selection (the
ever-repeated 4.7kΩ@3.3V), this gives you random power failures,
power-on resets during conversion and the known 85°C "error" readout.
> Also, does anyone have an opinion either way on whether the MAX31826 is
> better than the DS18B20 in a 3.3 volt environment since it seems to be
> tailored to 3.0-3.7 volts rather than 3.0 to 5.5 volts ?
>
The MAX31826 has a typical active current of 0.65mA while the DS18B20
needs 1mA. That's the main difference in electrical characteristics.
It also means the MAX31826 *may* actually work with the too weak
4.7kΩ@3.3V pullup and without the strong pullup. I won't count on it,
the datasheet says it may draw up to 1.2mA (DS18B20: 1.5mA.). The
4.7kΩ@3.3V give you 0.7mA.
So: Use a pullup of 1.5kΩ@3.3V with the DS18B20. Tie Vdd to GND. You are
double-fine.
Kind regards
Jan
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