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

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Message-ID: <4796C0EF.6010600@zytor.com>
Date:	Tue, 22 Jan 2008 20:22:07 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	David Fries <david@...es.net>
CC:	linux-kernel@...r.kernel.org
Subject: Re: W1: w1_slave units, standardize 1C or .001C?  Break API

David Fries wrote:
> On Mon, Jan 21, 2008 at 07:11:07PM -0800, H. Peter Anvin wrote:
>> H. Peter Anvin wrote:
>>> Millikelvins would have the nice property of never being negative.  :)
> 
> True, but the sensor returns the value as a signed integer in C.  That
> is where the earlier negative number problem was, it would have to do
> yet another conversion to go to Kelvin, and it would be just one more
> potential for error.  Everyone knows that a bad conversion doomed at
> least one space craft, let's stick to Centigrade.
> 

Uhm... the conversion is exact as long as you have at least centikelvin 
precision (0 °C = 273.15 K by definition, and the multiplier is 1.)

>> Alternatively, centikelvins would fit nicely in 16 bits if anyone cares...
>>
>> 655.35 K = 382.20 °C = 719.96 °F
> 
> The range for the sensor is -55 to 125 C, if an application didn't
> care about precision they could store it in a signed 8 bit value just
> fine.

This was more a comment as to it possibly being a convenient format for 
more than this particular sensor.

The nice thing with kelvins is no need to worry about negative numbers 
and something misparsing them, that's all.

I certainly did not imply that we should even consider use °F.  That's 
obviously ridiculous.

	-hpa

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