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

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Message-ID: <47955ECB.3050100@zytor.com>
Date:	Mon, 21 Jan 2008 19:11:07 -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

H. Peter Anvin wrote:
> 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.  :)
> 

Alternatively, centikelvins would fit nicely in 16 bits if anyone cares...

655.35 K = 382.20 °C = 719.96 °F

	-hpa
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