linux-kernel - Re: W1: w1_slave units, standardize 1C or .001C? Break API

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Date:	Mon, 21 Jan 2008 17:08:05 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	David Fries <david@...es.net>
CC:	Evgeniy Polyakov <johnpol@....mipt.ru>,
	linux-kernel@...r.kernel.org
Subject: Re: W1: w1_slave units, standardize 1C or .001C?  Break API

David Fries wrote:
> The ds18b20 one wire temperature sensor conversion routine is
> returning the units in degrees C while the ds1820 (ds18s20) is
> returning it in .001 degrees C.  20C vs 20312C.  Once you know the
> units I'm liking the latter as it gives a higher precision.  Time to
> break user applications so the driver can give the temperature in the
> same units for both sensors.
> 
> I only have the ds18b20 sensor model.  Here is the current output from
> the sys file for this sensor.
> /sys/devices/w1_bus_master1/28-0000000e84a2/w1_slave
> 45 01 4b 46 7f ff 0b 10 84 : crc=84 YES
> 45 01 4b 46 7f ff 0b 10 84 t=20
> 
> I ran the example data from the specification for the ds1820 through
> it's conversion routine and found that t= was 1000 times the value.  
> What should the displayed units be?  
> 
> This is the same ds18b20 conversion *1000.  Is everyone ok or is any
> objecting to .001 degrees C for the units?  Patch will follow.  The
> .001 C does truncate one bit of precision from the ds18b20 by the way.
> 

Millikelvins would have the nice property of never being negative.  :)

	-hpa
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