Firstly, ds18s20, have a look here: http://bjornsenfamily.com/blog/?p=69
, there's some great info on removing the resistors, if that's what you wish to do.
Secondly, thanks so much to xcplanet and jremington. On first reading this about a month ago, I was mightily confused. But yesterday I got it working like a dream. I'd been muddling along with fairly arbitrary numbers converting the reading, which would get me OK results, but not as accurate as it should be. Thinking it through, it's pretty straightforward, but the situation you find yourself in with only the manual from the manufacturer
to guide you is very frustrating!
I've posted my solution using PHP here: http://www.tinkerfailure.com/2013/01/wifly-sensor-pins/
I too have read this, as well as Chris's postings ( Great stuff btw )... However, after reading these postings, I'm
still perplexed by the inaccuracy in the measurements I get using the sensor pins ( I've tried pin 20 - sensor 2
and pin 17 - sensor 5 ) and I'm not seeing a way to get accurate measurements.
I understand there is a voltage divider on the pins. I can see and measure 100K to gnd and 470K to the sensor
inputs. It's easy to see that this makes the 3.3 V compliant, since 3.3V to a sensor pin would get divided down
by 10/57 ( ~0.175 ) to supply 579mV to the RN-XV chip A/D input. This is within it's 0-600mV range before
saturation. Likewise, when I read the sensor using "show q <n>", convert from hex to dec and multiply by the
inverse 5.7 to get the converted value I can see that it is close
to the measured value.
What I'm seeing is non-linearity in the conversion. Forgive me, as I'm a bit new to this. I understand the concepts but this is my first time using the RN-XV A/D.
I set up a test rig. 3.3V lab regulated supply to the RN-XV. I attached my HP DVM to the sensor 2 input pin and
applied fixed voltages. On the other end, I had the RN-XV read the sensors every 15 seconds and post them to
my PHP page, similar to what Chris had done. Here is my data.
All values are in milli Volts ( 1000mv = 1V ) As you can see, the error goes from 88 mv @ 400mv , 21mv at 800mv yet at larger input voltage 1.322V ( 1320 mv ) the A/D is off 65mv but now under voltage rather than over. Similar results are seen using sensor 2 or 5 or by reading the voltages by what is reported using "show q". My application sensor will output voltages from about 193mv to 3200 mv. so I need accuracy over the entire range.
READINGS IN mV