MS5540C

Miniature Barometer Module

09/2015

Page 10

FUNCTION

GENERAL

The MS5540C consists of a piezo-resistive sensor and a sensor interface IC. The main function of the MS5540C is

to convert the uncompensated analogue output voltage from the piezo-resistive pressure sensor to a 16-bit digital

value, as well as providing a 16-bit digital value for the temperature of the sensor.

Measured pressure (16-bit)

“D1”

Measured temperature (16-bit)

“D2”

As the output voltage of a pressure sensor is strongly dependent on temperature and process tolerances, it is

necessary to compensate for these effects. This compensation procedure must be performed by software using an

external microcontroller.

For both pressure and temperature measurement the same ADC is used (sigma delta converter):

•

for the pressure measurement, the differential output voltage from the pressure sensor is converted

•

for the temperature measurement, the sensor bridge resistor is sensed and converted

During both measurements the sensor will only be switched on for a very short time in order to reduce power

data is independent of the supply voltage.

FACTORY CALIBRATION

Every module is individually factory calibrated at two temperatures and two pressures. As a result, 6 coefficients

necessary to compensate for process variations and temperature variations are calculated and stored in the 64-bit

PROM of each module. These 64-bit (partitioned into four words of 16-bit) must be read by the microcontroller

software and used in the program converting D1 and D2 into compensated pressure and temperature values.

PRESSURE AND TEMPERATURE MEASUREMENT

The sequence of reading pressure and temperature as well as of performing the software compensation is depicted

in Fig. 3 and Fig. 5.

First Word1 to Word4 have to be read through the serial interface. This can be done once after reset of the

microcontroller that interfaces to the MS5540C. Next, the compensation coefficients C1 to C6 are extracted using

bit-wise logical- and shift-operations (refer to Fig. 4 for the bit-pattern of Word1 to Word4).

For the pressure measurement, the microcontroller has to read the 16-bit values for pressure (D1) and

temperature (D2) via the serial interface in a loop (for instance every second). Then, the compensated pressure is

calculated out of D1, D2 and C1 to C6 according to the algorithm in Fig. 3 (possibly using quadratic temperature

compensation according to Fig. 5). All calculations can be performed with signed 16-bit variables. Results of

multiplications may be up to 32-bit long (+sign). In the flow according to Fig. 3 a division follows each

multiplication. This division can be performed by bit-wise shifting (divisors are to the power of 2). It is ensured that

the results of these divisions are less than 65536 (16 bit).

For the timing of signals

to read out Word1 to Word4, D1, and D2 please refer to the paragraph “Serial Interface".

Sensor

D1

D2

Word 1..4

Calculation

in external

micro-

controller

Pressure

Temperature