topic Re: MMA8452 freefall detection in Sensors

MMA8452 freefall detection

jusantony89 — Tue, 03 Feb 2026 21:57:24 GMT

I am trying to detect free fall detection as per AN4070 application note, below are the register settings i have configured

MMA8452_CTRL1_REG = 0x20; //Set the device in 50 Hz ODR, Standby

MMA8452_FF_MT_CFG= 0xA0; //Configuration Register set for Freefall Detection enabling “AND” condition, OAE = 0, Enabling X, Y, Z and the Latch

MMA8452_FF_MT_THS = 0x03; //0.2g/0.063g = 3.17 counts //Round to 3 counts
MMA8452_FF_MT_COUNT = 0x06; //120 ms/20 ms (steps) = 6 counts

MMA8452_CTRL4_REG = 0x00; //Enable Motion/Freefall Interrupt Function in the System
MMA8452_CTRL5_REG= 0x00; //Route the Motion/Freefall Interrupt Function to INT2 hardware pin
IMMA8452_CTRL1_REG = MMA8452_CTRL1_REG | 0x01; //Put the device in Active Mode, 50 Hz

This configuration is followed by reading MMA8452_INT_SOURCE register, and the result is 0xFF;

and followed by reading MMA8452_FF_MT_SRC register, and the result of this register is also 0xFF;

Reading of register MMA8452_FF_MT_SRC should have cleared the freefall interuupt, and interrupt is always active and not clearing at all. Is there anything i am missing out? (I am not using hardware pin INT2 now, as there is hardware limitation)

Re: MMA8452 freefall detection

TomasVaverka — Wed, 23 Jan 2019 11:55:33 GMT

Hi Justin,

First off, could you please double check that you are able to read the WHO_AM_I register (0x0D) and other previously initialized registers?

Then please try to change MMA8452_CTRL4_REG = 0x00 to MMA8452_CTRL4_REG = 0x04.

Best regards,

Tomas

Re: MMA8452 freefall detection

jusantony89 — Thu, 24 Jan 2019 12:12:18 GMT

Hi Tomas,

Thank you for your help.

I have corrected control Register 4 value to 0x04. Still not successful.

I tried to read WHO_AM_I register (0x0D) and the result is also 0xFF, It is supposed to be 0x2A,right?

I have verified the presence of 0x1C device in my I2C line and it is showing up correctly.

I also tried reading 6 bytes of data continuously from 0x01 to 0x06 , I am able to read the data and is acknowledged properly.

Here also the first data byte received is always 0xFF, remaining 5 data bytes are changing as per the acceleration too.

Meanwhile the same loop have another I2C based 1 byte memory write and read, and that works perfectly fine.

Any comments!

Re: MMA8452 freefall detection

jusantony89 — Fri, 25 Jan 2019 06:24:11 GMT

Hi Tomas,

MMA8452 is working fine, free fall also detecting.

I have changed my I2C communication mode for write operation (when write is used to set register for next read)

to Complete the transfer without a Stop.

Earlier i was using Full transfer with Start and Stop.

Thanks.

Justin

Re: MMA8452 freefall detection

TomasVaverka — Fri, 25 Jan 2019 08:36:32 GMT

Hi Justin,

Glad to hear that, such troubles with I2C communication are quite common, that is why I always recommend to read the WHO_AM_I register at first as well as previously written registers. Using a logic analyzer or an oscilloscope helps a lot.

Best regards,

Tomas