Why won't internal pullup work with DS18B20?

Using Arduino Sensors
#1

Hi

I got a DS18B20 OneWire Tempsensor for my Mega ADK board today.

I saw that it needs a 4k7 resistor pull-up on the data line. However, since there's internal pull-up resistors that can be used, I initialized the IO-pin 2 as INPUT_PULLUP, rather than just INPUT.

It didn't work though.
When I added the 4k7 resistor it worked fine.

How come the internal pull-up didn't work instead of the 4k7 external pull-up. The IO-pin is said to have a very high open impedance, so even if the pullup-resistor is of a higher value, it should still work fine, right?

Thanks

#2

Have you looked up the value of the internal pullup resistor?

Comparing that value with the data sheet recommendations for the DS18B20 pullup might be informative.

#3

How come the internal pull-up didn't work instead of the 4k7 external pull-up. The IO-pin is said to have a very high open impedance, so even if the pullup-resistor is of a higher value, it should still work fine, right?

That depends on how you feed the sensor. In parasite mode it won't work with the internal pull-up as the current delivered is to low. If you bring three wires to the sensor (so not using parasite mode) it probably works.

#4