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Jose Marcelino
Visitor II
June 19, 2017
Solved

Temperature sensor on LIS2HH12 accelerometer

  • June 19, 2017
  • 2 replies
  • 3450 views
Posted on June 19, 2017 at 20:00

We're using the LIS2HH12 accelerometer and there's a temperature sensor listed in the datasheet accessible via the temp_L and temp_H registers (16 bits).

However the numbers I get don't make any sense.

The datasheet mentions 'Temperature sensor output change vs. temperature' of 8 digit/�C

11-bit resolution, -40 to 80C range and that the it uses two's complement, but I can't find a way to make the readings return any meaningful value.

What do these 

temp_L and temp_H numbers really mean? Thank you.

#lis2hh12 #temperature-sensor
    This topic has been closed for replies.
    Best answer by
    Posted on June 21, 2017 at 08:52

    I think Jose is asking for the data interpretation. I would also appreciate a more detailed description in the datasheet.

    TEMP_H:TEMP_L is the raw sensor output - a signed 16-bit number, stored as two's complement.

    At 25 centigrades, the output is 0. For every centigrade, the sensor output value changes for 8 LSBs, meaning 1 LSB is a 0,125 centigrades change (2 LSBs 0,25 centigrades, 4 LSBs 0,5 centigrades etc.).

    So the basic formula is: TEMP_H:TEMP_L + temp_offset_25_centigrades = measured_temperature

    (Please note that negative TEMP_H:TEMP_L value doesn't necessarily mean a negative temperature.)

    I personally display 2 decimal digits in my applications, but use integer variables to keep the code fast and clean, so I read the sensor output data like this:

    int16_t tempHex = (temp_h << 8) | temp_l;

    and then do the following computation: multiply the sensor output value by

    TEMP_FRACTION_MULTIPLIER = 

    125 and divide it with

    TEMP_FRACTION_DIVIDER = 

    10. This way the value stored in the temperature register contains temperature value multiplied by a hunded (12.34 centigrades is stored as 1234 etc.). 

    TEMP_OFFSET = 2500 (for 25.00 centigrades).

    int32_t temperature = ((tempHex * TEMP_FRACTION_MULTIPLIER) / TEMP_FRACTION_DIVIDER) + TEMP_OFFSET;

    Or you can just divide TEMP_H:TEMP_L by 8 (shift by 3 positions to the right), discard the decimal points and use only the round value of centigrades. (You still need to add the offset of 25 centigrades.)

    Hope this helps a bit.

    2 replies

    M
    Miroslav BATEK
    ST Employee
    June 20, 2017
    Posted on June 20, 2017 at 11:07

    The temperature sensor can be used to measure temperature variations. 

    It isn't suitable to return absolute temperatures measures. The value represents difference respect to a reference not specified value.
    A
    Anonymous
    Answer
    June 21, 2017
    Posted on June 21, 2017 at 08:52

    I think Jose is asking for the data interpretation. I would also appreciate a more detailed description in the datasheet.

    TEMP_H:TEMP_L is the raw sensor output - a signed 16-bit number, stored as two's complement.

    At 25 centigrades, the output is 0. For every centigrade, the sensor output value changes for 8 LSBs, meaning 1 LSB is a 0,125 centigrades change (2 LSBs 0,25 centigrades, 4 LSBs 0,5 centigrades etc.).

    So the basic formula is: TEMP_H:TEMP_L + temp_offset_25_centigrades = measured_temperature

    (Please note that negative TEMP_H:TEMP_L value doesn't necessarily mean a negative temperature.)

    I personally display 2 decimal digits in my applications, but use integer variables to keep the code fast and clean, so I read the sensor output data like this:

    int16_t tempHex = (temp_h << 8) | temp_l;

    and then do the following computation: multiply the sensor output value by

    TEMP_FRACTION_MULTIPLIER = 

    125 and divide it with

    TEMP_FRACTION_DIVIDER = 

    10. This way the value stored in the temperature register contains temperature value multiplied by a hunded (12.34 centigrades is stored as 1234 etc.). 

    TEMP_OFFSET = 2500 (for 25.00 centigrades).

    int32_t temperature = ((tempHex * TEMP_FRACTION_MULTIPLIER) / TEMP_FRACTION_DIVIDER) + TEMP_OFFSET;

    Or you can just divide TEMP_H:TEMP_L by 8 (shift by 3 positions to the right), discard the decimal points and use only the round value of centigrades. (You still need to add the offset of 25 centigrades.)

    Hope this helps a bit.

    M
    Miroslav BATEK
    ST Employee
    June 21, 2017
    Posted on June 21, 2017 at 09:10

    I would like to add that the TEMP_OFFSET is not guaranteed to be 25 

    centigrades. There is certain level of uncertainty so please don't expect high precision of the temperature measurement.

    A
    Anonymous
    June 21, 2017
    Posted on June 21, 2017 at 17:00

    On the other hand, assuming that the temperature value output is linear in sensor's working conditions, the TEMP_OFFSET can be measured using a known calibrated thermometer and adjusted as a constant in a firmware later. This approach creates quite a reliable output (I personally do this with prototypes). However, needs to be done with every sensor, which is always not possible or meaningful.

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