tmp102 for the Raspberry Pi

I`ve been using the tmp102 for a couple , but never really got to grips with how to deal with minus temperatures.
If you want a good night sleep try reading and fully understanding it`s pdf document .
I can now understand just about any spi or i2c device document, But this a is most harder to decipher.

http://www.ti.com/lit/ds/symlink/tmp102.pdf

How to read from the chip :

It`s I2C address depend on where you connect the ADD0 too.

Ground = 0x48 , V+ = 0x49 , SDA = 0x4A , SCL = 0x4B


Read two Bytes  ( High byte is first followed by the Low byte )

High byte ( Full degrees )
Low byte ( fraction degrees )


To get the temperature
Multiply the Hi_byte by 16 ( <<  4 )  +  Divide Low_byte by 16 ( >> 4 )
Now multiply the result by 0.0625 and you have the temperature.

Temperature = (( Hi_byte << 4) | ( Low_byte >> 4)) * 0.0625  

This works if the temperature is above zero.

Minis temperature problem
The most significant bit of High byte says if the temperature is a negative or positive value.
The documents explanation on how to convert the negative value is:-

For negative temperatures (for example, –25°C):
Generate the twos complement of a negative number by complementing the absolute value binary number
and adding 1. Denote a negative number with MSB = 1.
Example: (|–25°C|)/(0.0625°C/count) = 400 = 190h = 0001 1001 0000
Twos complement format: 1110 0110 1111 + 1 = 1110 0111 0000

My way I converting this into a usable form , is to test if the most significant bit is set  :-

if Hi_byte & 0x80 == 0x80:

If this is True we need to add 1 to the Hi_byte and remove any overflow and keep the value byte size  :-

Hi_byte = (Hi_byte + 1 & 0xff) 

This don`t work in fact , after waiting for it to freeze to test out my code.

I`ve found the way to do it , is 

0 - ( 256 - Temperature )  

To download the complete working code for python 3 using Quick2wire my link is below.
This can be converted to any i2c library , all you need is the High and Low byte.

 https://github.com/peter247/Pi_Weather_monitor/blob/master/examples/tmp102.py