[vz-dev] DS18B20 - Prüfsummencheck "85 Grad Problem" -> Sensoren senden falsche Werte

Udo1 udo1 at gmx.net
Sat Apr 1 22:35:19 CEST 2017

Am 01.04.2017 um 22:25 schrieb Frank Richter:
> Die Datenleitung soll ja für die Zeit, die der Sensor zur Verarbeitung
> braucht, hart auf high gelegt werden (per MOSFET o.ä.).
Genau. Siehe Datenblatt:
> In parasite power mode, the 1-Wire bus and CPP can provide
> sufficient current to the DS18B20 for most operations
> as long as the specified timing and voltage requirements
> are met (see the DC Electrical Characteristics and AC
> Electrical Characteristics). However, when the DS18B20
> is performing temperature conversions or copying data
> from the scratchpad memory to EEPROM, the operating
> current can be as high as 1.5mA. This current can cause
> an unacceptable voltage drop across the weak 1-Wire
> pullup resistor and is more current than can be supplied
> by CPP. To assure that the DS18B20 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 6. The
> 1-Wire bus must be switched to the strong pullup within
> 10μs (max) after a Convert T [44h] or Copy Scratchpad
> [48h] command is issued, and the bus must be held high
> by the pullup for the duration of the conversion (tCONV)
> or data transfer (tWR = 10ms). No other activity can take
> place on the 1-Wire bus while the pullup is enabled.


More information about the volkszaehler-dev mailing list