RE: Dallas temp sensors

["Jon Whiten" <jon@xxxxxxx>]

Li,

I have checked back a little more this morning.

>From the Dallas Weather mailing list a couple of years ago...

"Some users are seeing an offset from a reference that  current
software
does not allow calibration for. Others have seen a 'jump' of about 1.8
degrees F around freezing. Both of these are a result of the type
conversion
architecture used in the original DS1820. The new version uses a
different
architecture which is not prone to these issues but has less
resolution."

Note: the New version is the DS18S20 and a full report on this may be
downloaded from  http://www.ibutton.com/weather/report.pdf

Also, I remember now what happens with not connecting Vdd to ground. 
When I
replaced the sensor in my Weather Station I just did a pin for pin
replacement that did not connect the DS18S20 vdd to ground.  The
device
worked very well with the exception of these +85C readings every few
minutes.  After connecting Vdd to ground these pretty much
disappeared.


Also (I am pretty sure) that in the case of the DS18S20 detecting an
error
it sends +85C as a valid temp reading (CRC Correct).


Regards,

Jon
http://www.whiten.co.uk/



> -----Original Message-----
> From: K. C. Li [mailto:li@xxxxxxx]
> Sent: 19 July 2002 12:28
> To: ukha_d@xxxxxxx
> Subject: RE: [ukha_d] Dallas temp sensors
>
>
> On Fri, 19 Jul 2002, Jon Whiten wrote:
>
> > The DS1820 does not have the error checking.  I have
somewhere
> a report from
> > Dallas regarding the problems with the DS1820 I can let you have
if you
> > like.
>
> The DS18S20 (or DS1820) outputs can be checked by calculating the
CRC
> using the scratchpad values return. If the resultant CRC value is
zero,
> the readings are assumed to be reliable. The algorithm is given in
the
> Dallas technical reference for the temperature chip.
>
> I have attached a small PIC C program that reads the DS1820 and
displays
> the various temperature readings on a LCD display. The PIC C compiler
was
> developed by us for internal use. If anyone is interested, I can post
the
> CRC routine in PIC16F84 assembler. We are also happy to provide a copy
of
> our PIC C compiler to UKHA FOC but unfortunately it is not packaged
for
> distribution at the moment.
>
> Regards,
>
> Kwong Li
> li@xxxxxxx
> Laser Business Systems Ltd.
> http://www.laser.com
>
>
>

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