DS18S20 reading negative temperature

[presario1425]

Actually, the solution to your problem is easier than I thought. DS18S20's resolution is 0.5 C. So, you can use the following algorithm for negative temps -

Lets say RAWTEMP is where you stored the raw data out of the DS18S20. If bit#8 of RAWTEMP is 1, then you know that the temp is negative. In that case, invert RAWTEMP and add 1 to it (RAWTEMP = ~RAWTEMP + 1). If you multiply this number by 5 and store it in a variable, say TempC, you'll see that TempC will be the actual temperature, only multiplied by 10. For example, when TempC reads 450, then your actual temperature is -45 C. Let's try to do this with a real raw binary reading out of the DS18S20 -

Let's say that the raw binary reading out of your DS18S20 is

1111 1111 0101 1101

Since bit#8 is 1, we know that this is some negative temperature reading. Now we invert and then add 1 to the reading-

 RAWTEMP = 1111 1111 0101 1101
~RAWTEMP = 0000 0000 1010 0010
      +1 = 0000 0000 1010 0011

"0000 0000 1010 0011" represents a decimal value of 163. Multiply this number by 5 and we get 815. So, this really is -81.5C (of course this is out of the range of your sensor, but you get the idea). For example, take one of the readings from DS18S20's datasheet. -55C is represented by the raw binary value of "1111 1111 1001 0010". Inverting and then adding 1 reselts in "0000 0000 0110 1110" whis is 110 in decimal. Multiply this number by 5 and you get 550, which is -55C.

Hope this helps.

[srob]

This is true presario, and is perfectly adequate for my needs - the circuit needs to trigger an alarm when the ambient temperature falls below zero, and another alarm if the temperature reaches -7 degrees C. So I was going to use a spare indoor thermostat that I make previously, and reprogramme the PIC to suit (saving time and build cost). However, my pride would like me to display the temperature to the same resoultion of 0.1 degree, both in the negative and possitive, the formula used in onewire.bas, gives a resoultion of 0.01 degrees.

I think the best way forward is to try it and see what happens, and look at the contents of the DS18S20 COUNT_REMAIN and COUNT_PER_C registers, with the sensor left in my freezer for an hour or so, with a known accurate thermometer and adjust algorithm accordingly.

many thanks