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MMA8452Q accelerometer

Question asked by Bryan Bennett on May 15, 2015

I am using the the MMA8452Q Breakout board from Spark Fun (SparkFun Triple Axis Accelerometer Breakout - MMA8452Q - SEN-12756 - SparkFun Electronics)

I have followed the steps exactly on page 11 of "Motion and Freefall Detection Using the MMA8451, 2, 3Q" (, but my MMA8452Q accelerometer continues to indicate motion detection even when it is completely still.


My C code is:



#define MMA8452Q_ADDR  0x3A  //0b 0011 1010 (accelerometer device I2C address (0x1D) plus 0 LSb for R/W bit)


#define INT_SOURCE 0x0C  // system interrupt status register

#define FF_MT_CFG  0x15  // motion configuration register address

#define FF_MT_SRC 0x16   // motion source register

#define FF_MT_THS  0x17  // motion threshold register address

#define FF_MT_COUNT 0x18 // motion debounce counter register address

#define CTRL_REG1  0x2A  // system control 1 register address

#define CTRL_REG4  0x2D  // system control 4 register address

#define CTRL_REG5  0x2E  // system control 5 register address



volatile int FFMT_IntSourceReg_data; // holds data from FF_MT_SRC register in accelerometer MMA8452Q


volatile int IntSourceSystemRegData; // holds data from INT_SOURCE register in accelerometer MMA8452Q


int main (void)




  consecutive = 0;


  n = 0;

  configI2C1(400); // configure I2C baud rate for 400 KHz

  write2I2C1(MMA8452Q_ADDR, CTRL_REG1, 0x18); // ODR=100Hz, LNOISE=1, F_READ=0, ACTIVE=0

  write2I2C1(MMA8452Q_ADDR, FF_MT_CFG, 0xD8); // ELE=1, OAE=1, ZEFE=0, YEFE=1, XEFE=1

  write2I2C1(MMA8452Q_ADDR, FF_MT_THS, 0x30); // 3g/0.063g=47.6 (round up to 48), Threshold > 3g

  write2I2C1(MMA8452Q_ADDR, FF_MT_COUNT, 0x0A); // set debounce counter to eliminate false readings for 100Hz sample rate with requirement of 100 ms timer

  write2I2C1(MMA8452Q_ADDR, CTRL_REG4, 0x04); // disable auto SLEEP/WAKE interrupt, enable FF_MT interrupt

  write2I2C1(MMA8452Q_ADDR, CTRL_REG5, 0x04); // FF_MT interrupt output signal assigned to INT1 pin of MMA8452Q

  write2I2C1(MMA8452Q_ADDR, CTRL_REG1, 0x19); // put MMA8452Q in ACTIVE mode



if (TimerCount % 1000 == 0)



    write1I2C1(MMA8452Q_ADDR, INT_SOURCE);

    read1I2C1(MMA8452Q_ADDR, &IntSourceSystemRegData);  

    if ((IntSourceSystemRegData & 0x04)==0x04)



        consecutive = -3;

        readRTCC(); // read real time clock and calendar

        printf("MOTION DETECTION n=%d consecutive=%3d %02d:%02d:%02d\n",n,consecutive,HOUR,MIN,SEC);

        write1I2C1(MMA8452Q_ADDR, FF_MT_SRC);

        read1I2C1(MMA8452Q_ADDR, &FFMT_IntSourceReg_data); // read the FF_MT_SRC register to clear the interrupt





} // end of main


My "TimerCount" variable just assures that the INT_SOURCE register will be read and checked for an indication of motion every second.


I have actually run the above program with 2 different MMA8452Q accelerometers, and I get the same results with both accelerometers: Every second the accelerometer claims that motion has occurred (i.e., "MOTION DETECTION" is printed) even though the accelerometer is perfectly still. I am only trying to detect when motion occurs in a sleeping subject; I am not interested in the specifics of the motion; however, I also ran a program that looked at the values of the OUT_X_MSB, OUT_Y_MSB, and OUT_Z_MSB registers, and they displayed a constant value of 255 even when no motion was occurring.


Can someone please tell me what I am doing wrong?




Bryan Bennett