The arithmetic used in lesson6 exercise1 puzzled me.

[Chaoyue Ying]

In exercise1 lesson6,the step 10 add a picture to show what the number in two registers mean.

And in step 11, the temp is calculate by the way as follow:

int temp = ((int)temp_reading[1] * 256 + ((int)temp_reading[0] & 0xF0)) / 16;
if(temp > 2047)
{
	temp -= 4096;
}

But,ad the follow picture,When the temperature is -1℃,the high byte is 0x81 and the low byte is 0x00.So that the final temp should equal to -2032.

However,in order to get -1℃,the temp should equal to 16.It puzzled me.

[Naeem Maroof]

Hi,

The code could have been written in different ways to implement as per the datasheet.

However, the calculations performed here gives correct result.

As you have said that [0x00 , 0x81] should give -2032;

Yes, it is giving -2032

in the first line of code temp becomes 2064

and then the if-condition corrects the result and temp becomes -2032

[Chaoyue Ying]

Hi,

After the if-condition,it will calculate the cTemp.

According to the formula,the result of cTemp is -127.Based on the previous assumptions,the result  of cTemp should equal to -1.It puzzled me.

[Naeem Maroof]

Hi

The reason is that the two bytes that you have referred to have 2's complement representation:

So, the two's complement of -1 is 0xFF, and if you store this byte instead of 0x81, you would get -1C.