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'[PIC:]DS18S20 Accuracy'
2004\08\17@195747 by Steve Mercer

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I am a complete newbie with PIC Microcontrollers but I have a long
history with electronics and programming.

I bought myself a development/learning board as well as a DS18S20 (My
ultimate goal is a battery operated slave temperature/humidity/light
sensor that can be polled from a master via RF).

A number of examples came with the board and they all work as advertised.

The problem I have is with the DS18S20. It keeps telling me that the
temperature in my house is around 6 degrees C.

There are a lot of example programs on the net for the Dallas 1-wire
DS18S20 temperature sensor and subsequent display on LCD. I have
downloaded a few of these, compiled and programmed the 16F877A in my
dev board. They all produce the same result. I have even tried moving
the output of the DS18S20 to another pin. It appears that the DS18S20
is working as if I hold onto the package with my fingers, the
temperature display rises accordingly (And falls once I let go).

I have to assume that many others have experimented with these
sensors. Have any of you seen this before? If so, what did you do? I
am tempted to just add a constant to the number read but how will
that affect the accuracy?

Any insights most appreciated.




Best Regards




Steve

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2004\08\17@212501 by Olin Lathrop

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Steve Mercer wrote:
> The problem I have is with the DS18S20. It keeps telling me that the
> temperature in my house is around 6 degrees C.

Give your air conditioner a rest.

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(978) 742-9014, http://www.embedinc.com

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2004\08\17@231514 by Bruce Partridge

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You should take a look at AN247 "DS18x20 EEPROM Corruption Issue" on the
maxim website.

Bruce Partridge
http://www.rebreather.ca

> {Original Message removed}

2004\08\17@234042 by Steve Mercer

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>Steve Mercer wrote:
>>  The problem I have is with the DS18S20. It keeps telling me that the
>>  temperature in my house is around 6 degrees C.
>
>Give your air conditioner a rest.

Actually, I am down under in Australia. The temperature outside is
around 10C but with the heater on inside it is closer to 20C.

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2004\08\18@001207 by Steve Mercer

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Bruce

Do you have a copy of this App Note you could send to me?

I have found out that DS18X20 chips built with the B7 die can return erroneous temperatures.

When the temperature read by the DS18X20 is near or below 20°C, the raw temperature value obtained using a READTEMP command is often '129', which converts to either 129°C, or with 'negative temperature' correction applied, -1°C. Other incorrect values can also be returned.

This sounds like exactly my problem - the temperature in my house sits around 20°C. I will need to check my ROM code to see if it is a B7 die type.

The AN247 App Note is meant to detail workarounds for this problem whilst hardware dies are fixed (Its now estimated that a new revision for the troubled chip will be available mid August, early September time frame) but Dallas has decided this should have been a "internal confidential document" and would rather not have it available on the web.




Best Regards




Steve

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2004\08\18@013005 by Bruce Partridge

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part 1 3873 bytes content-type:text/plain; charset=iso-8859-1 (decoded quoted-printable)

Here it is.

Bruce Partridge
http://www.rebreather.ca

> {Original Message removed}
part 2 24251 bytes content-type:application/pdf; name=ds1820bug1.pdf (decode)

2004\08\18@014211 by Dave VanHorn

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At 12:12 AM 8/18/2004, Bruce Partridge wrote:

>Here it is.

Oog. That is a bit ugly.

They probably have a quadrillion or so on hand.

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2004\08\18@031430 by Jason S

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I've been waiting for Randy at Glitchbuster.com to get these in stock since
March.  His website says that in April his supplier offered to ship him
"non-conforming product".  Presumably the chips are defective in some way.
He decided to wait for the chips that will work properly, and he's still
waiting.

As far as I know, it's almost impossible to get non-defective DS18S20's.

Jason


From: "Steve Mercer" <stevespamspam_OUTJASPER.NET.AU>
Sent: Tuesday, August 17, 2004 4:55 PM

> The problem I have is with the DS18S20. It keeps telling me that the
> temperature in my house is around 6 degrees C.

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2004\08\18@101238 by Bruce Partridge

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The fixed silicon is done, but I suspect that at this point all of the
production is being shipped to people who buy these by the thousands
(millions).  I have these sensors in two of my products, but it looks like
only a few weeks delivery, and I have enough on hand to get me through.

Bruce Partridge
http://www.rebreather.ca

> {Original Message removed}

2004\08\18@103138 by Philip Pemberton

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In message <KILLspam6.1.2.0.2.20040818004256.041a22d8KILLspamspammail.dvanhorn.org>>          Dave VanHorn <RemoveMEdvanhornTakeThisOuTspamDVANHORN.ORG> wrote:

> At 12:12 AM 8/18/2004, Bruce Partridge wrote:
>
> >Here it is.
>
> Oog. That is a bit ugly.
>
> They probably have a quadrillion or so on hand.

What I'd like to know is why they couldn't use the same laser-coding
technology they use for the 64-bit 1-Wire serial numbers to store the
calibration factors. Or even use OTP cells (EPROM, fuserom, something like
that) to store the calibration factors, instead of using an EEPROM cell.

Later.
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2004\08\19@094552 by Howard Winter

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Phil,

On Wed, 18 Aug 2004 15:27:03 +0100, Philip Pemberton wrote:

>...<
> What I'd like to know is why they couldn't use the same laser-coding
> technology they use for the 64-bit 1-Wire serial numbers to store the
> calibration factors.

You can only do that before encapsulating - they may calculate and program the trim values at the very end of
the production process - in fact it may be the only way, since encapsulation might affect the calibration.

>Or even use OTP cells (EPROM, fuserom, something like that) to store the calibration factors, instead of
using an EEPROM cell.

Their production process may be such that EEPROM is easier to do.  Or it could be that they wanted to allow
end users to tweak the calibration for their own ends.

Cheers,



Howard Winter
St.Albans, England

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