At a guess, I'd say that you have it set to higher than 9-bit resolution?
AFAIK, the DS1822 has user configurable 9 to 12-bit resolution.

-marc

On Mon, Aug 10, 2009 at 9:23 AM, jim <jim@jpes.com> wrote:

> All,
>
> I worked yesterday getting my one wire routines working, which I
> accomplished.
> The 1822 One Wire Temp Sensor is a 9 bit sensor.  It has a range of -55C to
> +125C.
> It uses 2's complement math for the negative temps.  It's temperature
> measurement is
> contained in 2 bytes.  The LSB is the temp reading in .5C increments.  The
> MSB is the
> sign bit.  All 8 positions in the MSB are sign bits.  So, I gather that if
> the temp is negative,
> all 8 bits of the MSB would be set.   However, when the PIC reads the  Temp
> sensor, I get
> the correct (approximately) reading in the LSB, but the MSB has 1 or 2 bits
> set.  It's not
> clear to me from the datasheet whether all 8 bits should be set for
> negative temps, or if it can
> be any number of bits set for negative temps.
>
> Does anyone have any experience with one wire comms, especially with this
> temp sensor?
> If so, do you have any idea about the MSB and how the sign bit is supposed
> to work?
> Or any other insight that may give me an idea if I'm doing anything wrong.
>
> BTW, I've tried powering this sensor from Vdd and using parasite power, and
> it acts the same either way.
> Also, I opowered it from 5 volts and from 3 volts, and that doesn't change
> anything either.
>
> Please advise.
>
>
>                                                                    Thanks
> and Regards,
>
>
>   Jim
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