Convert temperature data from sensor to celsius degree

Question

I have a sensor named LSM303DLHC ,it have 2 temp register but I can't figure it out how to convert it to degrees Celsius. 2 Reg is:

    TEMP_OUT_H_M register  // high reg

TEMP11 | TEMP10 | TEMP9 | TEMP8 | TEMP7 | TEMP6 | TEMP5 | TEMP4

     TEMP_OUT_L_M register  //low reg

TEMP3 | TEMP2 | TEMP1 | TEMP0 | 0 | 0 | 0 | 0

In datasheet say: "TEMP[11:0] Temperature data (8 LSB/deg - 12-bit resolution)" My current code is

uint8_t hig_reg = read(TEMP_OUT_H_M)  // value = 0x03
uint8_t low_reg = read(TEMP_OUT_L_M)  // value = 0x40
int16_t temp = ((uint16_t)hig_reg  << 8) | (uint16_t)low_reg;  // temp = 0x0340 = 832

float mTemp = temp/256;  // = 3.25 
mTemp = mTemp +20  ;      // =23.25 (°C)  i add 20 more

But I don't understand where the 20 °C offset comes from? Datasheet never mentions it.

Answer 1

Thank for your answer. Turn out that temperature sensor just determine comparative temperature to calculate the variation. It not use for absolute temperature.They should add that information in datasheet. I just waste 2 day of my life for that.

Answer 2

My try... First, I have note that you are taking the whole 8 bit TEMP_OUT_L_M register and as you described is just the first 4 bits of it. Then try to make the 12 bit register first. I use python ans SMBus library,

temph = i2cbus.read_byte_data(i2caddress, TEMP_OUT_H_M) << 4 
templ = i2cbus.read_byte_data(i2caddress, TEMP_OUT_L_M) >> 4
tempread = temph + templ   # it is all ready converted to Decimal

Then you can go ahead with the transformation: see page 11 title 2.2 "Temperature sensor characteristics: 8 LSB/ºC , 12 bit resolution and 2.5 Vdd."

Then it is clear that:

ºC = (read_value * VDD * 10^(log 2 (LSB/ºC)) / ((resolution - 1) * (10*(ºC/LSB))

In the LSM303 then following the python code:

# temperature = (tempread * 2.5 * 1000)/(2^12-1) * (10/8))  better to write:
temperature = (tempread *2500)/(4095 * 1.25)

In your case: you have read: 0x0340, in 12 bits 0x34 in decimal: 54

temperature = (54 * 2500) / (4095 * 1.25) = 23.443223

I also noticed that:

• The maximum secure register to read is 85ºC = 0x55, so we better make the register with the 4 bit LSB of TEMP_OUT_H_M and 4 bit MSB of TEMP_OUT_L_M.
• In further test the LSM303 can resist near 125 ºC for a while, with out permanent damage, but it a good practice to use this temperature to put the Magnetometer and Accelerometer in a sleep mode. When the temperature reaches 80.

Answer 3

My opinion is that TEMP is on 10 bits and one for the sign (value max you can read : 0x3FF), so:

0x03FF - 0x0340 = 0x0BF

0x0BF / 8 = 0x17 (23.875 in decimal).

As said, don't forget the two's complement in your computation.