> excuse-me to disturb you, but I am working on a system to manage the
> temperature of  a CCD camera (Audine), but we have a problem with the
> DS1820 sensor.
> We read the temperature (in fact 8 bytes), next the CRC byte. But 10% o=
f
> the time the CRC is bad. It's seem that a bit is read as 1, but it must
> be read as 0. We have a pull-up resistor of 4.7k between the +5v and th=
e
> data line. We have made test with a PIC controller and we have used two
> different compiler (picc and pic-basic). I have also tried to weld the
> sensor just on the pins of the pic to avoid distorsion (but the problem
> is the same).
> So this problem is not the most important, because when we have a bad
> CRC, we read again. But after a while (between 5min and 1 hour),  the
> temperature send is allways the same, the CRC is OK, but we know that
> it's not the good temperature, for example 40C with ice on the CCD chip=
!!!
> If you have an idea, tell it to me.

If the temperature is always the same it could be that, for=20
some reason, the temperature sampling is never done, that is -=20
the sensor doesn't execute the CONVERT_T command. When the=20
sensor is powered with +V5 (not parasitic power) you can check=20
that the sensor is indeed doing a conversion (by issuing read-
time slots and verify that it is transmitting 0 while=20
conversion is in progress). If it is powered with parasitic=20
power You should be able to see on the data line that the=20
voltage is lowered some 10ths of mV due to the increase of=20
current.

Have you verified that the read and write timing is OK? I have=20
equipment reading this type of sensor all the time but I very=20
seldom (never) get any CRC errors while reading the scratchpad.


>=20
> Thank you
>=20
> Best regards
>=20
> Michel Meunier

Regards / Ruben=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

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