Re: [misterhouse-users] DS2438 sensor Get_Temp_2438 fails when Temp < 0

[Chris B. <ch...@ba...>]

I don't have a DS2438 but the following works for all of the examples on 
page 4 of the specification document 
(http://pdfserv.maxim-ic.com/en/ds/DS2438.pdf)

$temp = ($data[4] * 256 + $data[3])/256;
$temp -= 256 if ($data[4] > 128);
$temp = sprintf ("%3.2f",$temp);
return $temp;

From what I can tell the code isn't taking into account the two-complement 
encoding system.

----- Original Message ----- 
From: "Jason I. Comander" <jcomand@MIT.EDU>
To: <mis...@li...>
Sent: Tuesday, December 07, 2004 2:23 PM
Subject: [misterhouse-users] DS2438 sensor Get_Temp_2438 fails when Temp < 0


>
> Now that the temperature in Boston has dropped down below zero C
> for the first time since I installed my DS2438-based temperature/humidity
> sensor from AAG Electronica, I see that temperatures just below 0 C get
> mangled.  Anyone seen/fixed this?  At the end of a logn day, I don't have
> the brain power left right now to fix it!  Anyone see any obvious errors
> in Get_Temp_2438{} in devices.pm.  A true temperature somewhere around 0
> degrees C is reported as -254.84.
>
> Thanks!
> Jason
>
> # Byte 3&4 - Voltage
> sub Get_Temp_2438{
> ...
> my @data = unpack("C*",$result);
> ...
> my $sign = $data[4] > 128 ? -1 : 1;
> my $temp = sprintf ("%3.2f",((($data[4] * 256) + $data[3]) * $sign * 
> 0.03125 / 8));
> #$temp = sprintf ("%3.2f",$temp);
> return $temp;
>
>
>
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