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January 13, 2013
about 2 years ago
I am using the MMA8452Q Triple Axis Accelerometer Breakout on a breadboard circuit that is connected to a mini-Bully PIC24HJ64GP502 (no longer sold by Spark Fun) to detect motion of the board. As long as my MMA8452Q is situated in a horizontal plane when I download its code from the PIC24 microcontroller, it seems to work fine (i.e., the I1 pin outputs a 3.3 volt signal when the breadboard is moved, and it outputs zero volts when motionless). However, if the MMA8452Q is not situated in a horizontal plane during downloading of its code, it does not function appropriately (i.e., the I1 pin constantly outputs 3.3 volts whether there is motion of the breadboard or not). This is important because my application circuit will eventually be connected to my wrist when first activated, and then worn throughout the night while I sleep (I just need to know when motion occurs, not any of the characteristics of the motion).
I have set the MMA8452Q’s registers thus:
write2I2C1(MMA8452Q_ADDR, FF_MT_CFG, 0x78); // ELE=0, OAE=1, ZEFE=1, YEFE=1, XEFE=1
write2I2C1(MMA8452Q_ADDR, FF_MT_THS, 0x8F); // DBCNTM=1, THS[6:0]=15 threshold=(15)x(0.063g)=0.945g
write2I2C1(MMA8452Q_ADDR, FF_MT_COUNT, 0xFF); // 0xFF=255(decimal), ODR=800Hz, debounce time=(1.25/1000)x(255)=0.319 second
write2I2C1(MMA8452Q_ADDR, CTRL_REG1, 0x04); // ODR=800HZ, LNOISE=1, F_READ=0, ACTIVE=0 (STANDBY mode)
write2I2C1(MMA8452Q_ADDR, CTRL_REG2, 0x00); // disable SELF TEST, disable RESET, OVERSAMPLING MODE (Normal), disable auto-SLEEP
write2I2C1(MMA8452Q_ADDR, CTRL_REG3, 0x08); // enable FF_MT wake up, IPOL=0(interrupt polarity=active low), enable PUSH PULL
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, 0x05); // now place MMA8452Q (accelerometer) in ACTIVE mode to detect motion
Can anyone advise me as to how to get the MMA8452Q to work properly no matter its spatial orientation when initially programmed by the microcontroller?
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