I installed a DS18B20 temp sensor using a 10K pullup resistor. It seems to be functioning correctly but I noticed the datasheet indicated the resistor should be 4.7K. Will that affect the temp indication?
None, the pull up is used to get a better signal (more square wave) and depending on the distance you need a smaller resistor.
It is explained for I2C on this page - Gammon Forum : Electronics : Microprocessors : I2C - Two-Wire Peripheral Interface - for Arduino -
Search for: Pull-up resistors
It works the same for every digital bus.
It works the same for every digital bus.
Well that's perhaps a bit of a simplification - some busses use push-pull drivers, some use matched termination resistors.
If power drain isn't an issue the pull-up resistor would be matched to the signal cable characteristic impedance (typically 150 to 250ohms), but at 5V that's a lot of current, so for short-range busses the compromise of a higher valued resistor is used.
The temperature value coming back from the DS18B20 cannot be affected(*) by the signal path (that's one of its great plus points) as the measurement is made on-chip and digitised before communication takes place. All you need is to provide stable power supply during conversion.
(There is a mode called "parasite power" in which power is provided over the signal path - this is different and requires an additional "strong pullup" as well as the resistor - this is best avoided unless you really have to save on wiring)
(* well it could be completely lost/garbled in the extreme case, but until then it is communicated flawlessly)
Well that's perhaps a bit of a simplification
definitely, thanks for this better explanation.
Good explanations!
For some basic how-to, sample Arduino Software etc: http://arduino-info.wikispaces.com/Brick-Temperature-DS18B20
Hello,
I used the same circuit, as I only had a 10k ohm resistor. I used terryking228's link as well. Is there any link to a website with a circuit diagram for normal (non-parasitic) mode? Thanks everyone for explaining the whole thing, I was dreading the thought of ordering the entire resistor kit (which i have done now) and having to delay my project for a week.
Cheers,
Mark
Hi Mark,
See:
At short distances, 10K usually works. And two 10K in parallel := 5K... Non-critical.
What that diagram doesn't show is that the decoupling capacitor should be
mounted right at the sensor and the pull-up resistor is best at the Arduino
end of the cable.
Mark,
You're right! I'll update that and try to organize the "Longer Distance and Multiple Sensors" stuff.