How to convert the value read by LIS3DH

Question

Hallo everyone, 
 I have interfaced LIS3DH with nRF52832 and reading a value as below 
 X 230 
 Y 249 
 Z 54

How to convert this value to standard acceleration unit? 
 (im using 100Hz sample rate and 2g scale.)

awneil · Answer

The LIS3DH is an independent 3rd-party product - nothing to do with Nordic. 
 You need to study the LIS3DH documentation to answer this. 
 https://www.st.com/en/mems-and-sensors/lis3dh.html 
 For support with the LIS3DH, you need to contact the manufacturer - ST : 
 https://community.st.com/s/

hmolesworth · Answer

Actually sometimes confusing, so maybe this would be of help. The values on the LIS2DH are signed but held in 2 x 8-bit registers which have to be handled with correct shift and sign-extension. Have a look at this (working) code to see if that helps: 
 typedef struct {
 uint8_t OUT_d_L; // LIS2DH12_OUT_XYX_L
 uint8_t OUT_d_H; // LIS2DH12_OUT_XYZ_H
}__attribute__((packed)) axis_t;

typedef union {
 axis_t Bytes; // LIS2DH12_OUT_X_L, H 0x28, 0x29
 int16_t sWord;
}__attribute__((packed)) axisUn_t;

typedef struct {
 axisUn_t rOUT_X; // LIS2DH12_OUT_X_L,H 0x28,29
 axisUn_t rOUT_Y; // LIS2DH12_OUT_Y_L,H 0x2A,2B
 axisUn_t rOUT_Z; // LIS2DH12_OUT_Z_L,H 0x2C,2D
}__attribute__((packed)) reg2_t;
STATIC_ASSERT((offsetof(reg2_t, rOUT_X) & 0x01) == 0);

reg2_t Sea2; // Sea of byte registers
int16_t x, y, z;

/**
 * @brief Collect signed xyz registers and correctly align for signed 16-bit
 * @param
 */
void ReadXYZ(void)
{
 ReadAccRegisters(); // <== this is your code to read the 6 consecutive 8-bit registers forming X, Y and Z
 // Ensure we have correct sign extension, also assuming even byte alignment
 x = Sea2.rOUT_X.sWord / 16;
 y = Sea2.rOUT_Y.sWord / 16;
 z = Sea2.rOUT_Z.sWord / 16;
} 
 Some care is required on alignment, since 8-bit values (packed, or cuddled up) must be sent to even-byte aligned 16-bit signed result registers. You can see from the byte addresses on the LIS2DH12 that they are sequential (cuddled up). The LIS3DH is similar. 
 Edit: Just noticed this is 5 months old, <sigh>