Using the device step-by-step, from basic to advanced

AN4450

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DocID025978 Rev 1

4.3

One-shot mode measurement sequence

1.

Power down the device (clean start)

–

WriteByte(CTRL_REG1_ADDR = 0x00); // @0x20=0x00

2.

Turn on the pressure sensor analog front end in single shot mode

–

WriteByte(CTRL_REG1_ADDR = 0x84); // @0x20=0x84

3.

Run one-shot measurement (temperature and pressure), the set bit will be reset by the

sensor itself after execution (self-clearing bit)

–

WriteByte(CTRL_REG2_ADDR = 0x01); // @0x21=0x01

4.

Wait until the measurement is completed

–

ReadByte(CTRL_REG2_ADDR = 0x00); // @0x21=0x00

5.

Read the temperature measurement (2 bytes to read)

–

Read((u8*)pu8, TEMP_OUT_ADDR, 2); // @0x2B(OUT_L)~0x2C(OUT_H)

–

Temp_Reg_s16 = ((u16) pu8[1]<<8) | pu8[0]; // make a SIGNED 16 bit variable

–

Temperature_DegC = 42.5 + Temp_Reg_s16 / 480; // offset and scale

6.

Read the pressure measurement

–

Read((u8*)pu8, PRESS_OUT_ADDR, 3); //

@0x28(OUT_XL)~0x29(OUT_L)~0x2A(OUT_H)

–

Pressure_Reg_s32 = ((u32)pu8[2]<<16)|((u32)pu8[1]<<8)|pu8[0]; // make a

SIGNED 32 bit

–

Pressure_mb = Pressure_Reg_s32 / 4096; // scale

7.

Check the temperature and pressure values make sense

–

Reading fixed 760 hPa, means the sensing element is damaged.

Example of register measurements and conversion:

P = 0x3FF58D means 4191629 / 4096 = 1023.347 hPa

T = 0xE07C means 42.5 + (-8068/480) = 25.7 °C

{ as reference: 0x6BD0 = 100°C; 0xB050=0 °C }

The value read from the PRESS_OUT registers is always the difference of the sensor

measured pressure (after averaging) and a base value, that can be either the value copied

in the RPDS register (reference pressure after soldering, to correct the pressure offset due

to mechanical stress, 16-bit value corresponding to the MSB part of the pressure value) by

default after power-up, or the value written in the REF_P registers (24-bit value).

To select REF_P as base value, and hence enable differential mode, bit AUTOZERO (bit[1]

of CTRL_REG2, (0x21), which is a self-clearing bit) must be set: the current PRESS_OUT

value is copied into REF_P registers, and from the next available measurement, the

PRESS_OUT value is the difference between the current measure and the “frozen”

reference value. In case the application requires to switch back to default mode, the bit

RESET_AZ (bit[1] of CTRL_REG1 (0x20), which is a self-clearing bit) must be set.

PRESS_OUT is then computed using RPDS registers value as reference.

RPDS_L and RPDS_H calibration values can be written on the volatile memory by the

microcontroller after sensor power-on: the value shall be permanently stored in the

application memory during manufacturing (see Section 6.7).

In special cases, there’s also the possibility to store the RPDS calibration value inside the

LPS25H non-volatile memory.