Raw Data of MMA8452

[sriramv]

I am using a MMA8452 accelerometer. I did not quite understand the following piece of code

        data->x = ((tmp_data[0] << 8) & 0xff00) | tmp_data[1];

        data->y = ((tmp_data[2] << 8) & 0xff00) | tmp_data[3];

        data->z = ((tmp_data[4] << 8) & 0xff00) | tmp_data[5];

But, MMA8452 has 12 bit samples. Should these again be shifted by 4 bits to the right as tmp_data[1], tmp_data[3], tmp_data[4] have only 4 bits of data?

When the Device is Flat. What should be the values of x, y, z.

[sriramv]

Hello Tomas,

I found the linux driver here. it is written by nxp. Let me know if this driver can be

used.

MMA8x5x 3Axis Accelerometer Sensor Linux Driver for Smart Android Devices

My accelerometer is mounted like this:

       --------------------------------------------------- LCD

TOP LAYER OF PCB

===================================== PCB

bottom            ======

                      MMA8452

I am first verifying if the values provided by accelerometer are correct or not

Then i want to convert the values so that Orientation on the LCD is proper

when we rotate the device.

[TomasVaverka]

Hello Sriram,

Yes, you can use this driver, just shift the raw 16-bit values by 4 bits to the right to get 12-bit acceleration data.

If the accelerometer is mounted on the bottom side of your PCB, then the Z-axis reads approximately -1g which corresponds to Zout_12_bit = 0xC00. Invert the board and check that the Z-axis now reads approximately 1g (Zout_12_bit = 0x400). Repeat with the X- and Y-axes pointing downward and then upward and check that the X- and Y-axis accelerometer readings are near 1g and –1g.

Hope it helps.

Best regards,

Tomas

[TomasVaverka]

Hi Sriram,

I am not sure where you found this piece of code, but you are right that these are 16-bit values which need to be shifted by 4 bits to the right to get 12-bit acceleration data.

Have a look at my example code where I use:

I2C_ReadMultiRegisters(MMA8652FC_I2C_ADDRESS, OUT_X_MSB_REG, 6, AccelData);   // Read data output registers 0x01-0x06

// 12-bit accelerometer data

Xout_12_bit = ((short) (AccelData[0]<<8 | AccelData[1])) >> 4;   // Compute 12-bit X-axis acceleration output value

Yout_12_bit = ((short) (AccelData[2]<<8 | AccelData[3])) >> 4;   // Compute 12-bit Y-axis acceleration output value

Zout_12_bit = ((short) (AccelData[4]<<8 | AccelData[5])) >> 4;   // Compute 12-bit Z-axis acceleration output value

// Accelerometer data converted to g's

Xout_g = ((float) Xout_12_bit) / SENSITIVITY_2G;   // Compute X-axis output value in g's

Yout_g = ((float) Yout_12_bit) / SENSITIVITY_2G;   // Compute Y-axis output value in g's

Zout_g = ((float) Zout_12_bit) / SENSITIVITY_2G;   // Compute Z-axis output value in g's              

With the device placed horizontally on a flat surface (X-axis = 0g, Y-axis = 0g and Z-axis = 1g), the output data for both X and Y axes should vary between 0x0001, 0x0000, 0x0FFF, 0x0FFE etc. as they are expressed as 2’s complement number (see the attached file for more information). There may be some offset so the output values can be a bit different, but still close to 0g.

I hope it helps.

Best regards,

Tomas

PS: If my answer helps to solve your question, please mark it as "Correct". Thank you.