TN1180

How to obtain temperature values in C

3

How to obtain temperature values in °C

The LPS331AP also contains a temperature sensor which is used for compensation of the

pressure sensor. The temperature data is also available for outside applications. The

temperature raw count is stored in registers Temp_Out_H (2Ch) and Temp_Out_L (2Bh).

The temperature in °C can be determined by calculating the two’s complement, if

necessary, and perform the scaling. An example of a temperature calculation when the

sign bit is 0 is shown below.

Figure 5: Temperature example 1: sign bit is 0, this is a positive value

Likewise, a negative value is indicated when the MSB in the temperature word is equal to

1. To obtain the temperature, we first take the two’s complement of the complete word and

then perform the scaling and offset operation. An example of a temperature calculation

when the sign bit is 1 is shown below.

Figure 6: Temperature example 2: sign bit is 1, two's complement

As shown in the figure above, a negative value in the raw count register does not always

result in a negative a temperature value in degrees Celsius. This is because the offset and

scaling factor must be applied to determine the final value in °C units.

Temperature Counts = 2Ch & 2Bh = 6BD0h = 27600(dec)

6

B

D

0

Bit

15

Bit

14

Bit

13

Bit

12

Bit

11

Bit

10

Bit

9

Bit

8

Temp_Out_H (0x2C)

1

1

0

1

0

0

0

0

Temp_Out_L (0x2B)

1

0

1

0

1

1

Bit

7

Bit

6

Bit

5

Bit

4

Bit

3

Bit

2

Bit

1

Bit

0

= 42.5 +

scale

# counts

480 counts / °C

27600 counts

1110 0000 0111 1100

0001 1111 1000 0011

+

1

; invert

; add 1

; need to take the 2's complement

0001 1111 1000 0100

=

=

42.5 +

scale

# counts

E

0

7

C

Bit

15

Bit

14

Bit

13

Bit

12

Bit

11

Bit

10

Bit

9

Bit

8

Temp_Out_H (0x2C)

0

1

1

1

1

1

0

0

Temp_Out_L (0x2B)

1

0

0

0

0

0

Bit

7

Bit

6

Bit

5

Bit

4

Bit

3

Bit

2

Bit

1

Bit

0

1

1

480 counts / °C

- 8068 counts

DocID026298 Rev 1

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