AN4159 datasheet(24/27 Pages) STMICROELECTRONICS

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Software

AN4159

24/27

Doc ID 023639 Rev 3

Case 12:

“How is the pressure variation threshold interrupt activated?”

●

We assume in the explanation that the registers in RIF_PAS (@3C, 3D, 3E) are not

used in the application.

Mechanism:

First, differential mode must be activated (DIFF_EN=1)

Second, program the reference pressure in REF_P register (for example, copy the

current P_OUT value to REF_P registers).

Caution: P_OUT is the measured ADC pressure minus the REF_P, so at this point all

the future P_OUT measurements should be near 0 mb. To keep track of the absolute

pressure, the SW in the host should remember REF_P and add it back to P_OUT.

Third, program the threshold THS register (1 LSB = 1/16 mb step). This defines the

pressure band: When P_OUT absolute difference exceed THS value, and interrupt will

be generated.

Fourth: Configure the INT pin and the type of interrupt needed.

Source code example:

u8 LPS331AP_ActivatePressureBand(u16 Threshold_mb_x16)

{

u8 reg;

u8 regs[4];

// Warning: this can only be done if the 1 point calibration HW feature is

unused

reg = LPS331AP_ReadByte(LPS331AP_CTRL_REG1_ADDR);

if(reg & 0x02)

return FALSE;

// if DIFF_EN bit is set, to clear it, and increase

LPS331AP_Adjust_Value_Pressure_Reg by the DELTA_PRESS value.

LPS331AP_WriteByte(LPS331AP_CTRL_REG1_ADDR, reg | 0x08); // activate the

delta pressure logic

LPS331AP_Read(regs,LPS331AP_PRESS_OUT_ADDR,3);// no reading back of REF_P

required.

LPS331AP_Write(regs,LPS331AP_REF_P_ADDR,3);

LPS331AP_Adjust_Value_Pressure_Reg = ((u32)regs[2])<<16;

LPS331AP_Adjust_Value_Pressure_Reg |= ((u32)regs[1])<<8;

LPS331AP_Adjust_Value_Pressure_Reg |= regs[0];

LPS331AP_Adjust_Value_Pressure_Reg =~LPS331AP_Adjust_Value_Pressure_Reg;

// now activate the delta pressure logic

LPS331AP_WriteByte(LPS331AP_THS_P_ADDR,Threshold_mb_x16); // LSB Threshold

LPS331AP_WriteByte(LPS331AP_THS_P_ADDR+1,Threshold_mb_x16>>8); // MSB

Threshold

LPS331AP_WriteByte(LPS331AP_INTERRUPT_CONFIG_ADDR,0x03); // write 0x07 if

you want the interrupt output to stop by reading INT_SOURCE Reg

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