85 deg C is the default value after power up (or reset) before a temperat=
ure=20
conversion has been done. I have an application where the sensor can be r=
emoved=20
during conversion so I have to know if it has been removed and power cycl=
ed=20
after a temperature conversion command has been sent. I also use 2 wire=20
interface (parasite power). This is how I have solved it and it seems to =
be=20
working OK.

1. Do a recall EE to read the alarm trigger values into the scratchpad.=20

2. Read the scratchpad.

3. Invert the alarm trigger values and remember the values.

4. Write back the scratchpad (without copying it to EEPROM)

5. Do a temperature conversion and wait the time required by the selected=
=20
resolution.

6. Read the scratchpad.

7. If the alarm trigger values still are the same as the values written t=
o the=20
scratchpad (which is the inverted value of the EEPROM) the temperature is=
 good.

Of course also check the CRC after every read.

This is based on the fact that the EEPROM is copied to the scratchpad on =
power=20
up and if the scratchpad has changed back to the data in EEPROM after a=20
temperature conversion, power is most likely cycled to the sensor and the=
=20
temperature is not valid.

Regards / Ruben


> I've implemented some 1-Wire code to read a DS18B20 sensor using a PIC,=
 using=20
> parasite power.  Still in testing on a breadboard, it sits there all da=
y with=20
> 2 sensors on the bus reading ~28 deg C.  But periodically, perhaps once=
 every=20
> few hours, I get an odd value of "85", or "-21" once interpreted from t=
he=20
> DS18B20's format.  Seemingly important, this works out to B'10101010', =
and=20
> even more important is that it passes the checksum test.
>=20
> I can now reproduce this by removing the positive power from the sensor=
.  And=20
> it's always that magic "85".  Is there some significance to this, so th=
at I=20
> can figure out how to determine it's bogus and not display 85?  I can't=
 find=20
> any reference to this value or bit pattern being generated when power t=
o the=20
> chip is lost or otherwise.  That it's passing the checksum test is real=
ly=20
> bothering me.
>=20
> Cheers,
> -Neil.
>=20
>=20
>=20
> --=20
> http://www.piclist.com PIC/SX FAQ & list archive
> View/change your membership options at
> http://mailman.mit.edu/mailman/listinfo/piclist
>=20

=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D
Ruben J=F6nsson
AB Liros Electronic
Box 9124, 200 39 Malm=F6, Sweden
TEL INT +46 40142078
FAX INT +46 40947388
ruben@pp.sbbs.se
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D

--=20
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