CODEWITHSUNDEEP
cembeddedbit-shift
craig-nerd
craig-nerd

Reputation: 748

Reading MSB and LSB if data is left-justified

While going through my coursework, I came across a function that reads temperature from the TMP102 sensor (only required to measure positive temperatures).
The function first reads the MSB and LSB using I2C. Since the temperature data is 12-bit and left-justified, the function proceeds as follows: 

temp = ( (MSB << 8) | LSB) >> 4

I do not understand why this is done. Could someone please help me explain how the above line of code is related to the data being 12-bit and left-justified?

Upvotes: 2

Views: 5863

Answers (1)

user2371524
user2371524

Reputation:

Let v be a bit of the temperature value and p be a padding bit on the right, then you have

MSB = vvvvvvvv
LSB = vvvvpppp

---

  MSB << 8               = vvvvvvvv 00000000
 (MSB << 8) | LSB        = vvvvvvvv vvvvpppp
((MSB << 8) | LSB) >> 4  = 0000vvvv vvvvvvvv

In the last line, you see the correct representation as a 16bit value (with the upper 4 bits always 0).

Upvotes: 7

Related Questions