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MMA8453Q accelerometer routines

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MMA8453Q accelerometer routines
PostPosted: Sat Jul 04, 2015 2:58 am     Reply with quote

Here is code to set up and get readings from the MMA8453Q 3-axis accelerometer from Freescale (nice chip!)
It should give you a push in the right direction if you plan to use this part.
I also set up two interrupt lines from the chip to the MCU. Ack errors are not handled.


// MMA8453Q acceletormeter routines & constants:

// hardware-specific declarations:
#define   ACCEL_SA0   PIN_B6   // accelerometer address bit (output)
#define   ACCEL_IRQ1   PIN_A2   // Input from MMA8453Q IRQ1
#define   TILT_IRQ      PIN_A2   // Input from MMA8453Q IRQ1

// global vars (send results to main program)
signed int16 acc_xyz[3];         // new samples from accelerometer

#define   ACCEL_I2C_ADDR   0x1C

// accel. internal registers:
#define   TRANS_CFG   0x1D
#define   TRANS_THS   0x1F
#define   TRANS_COUNT   0x20
#define   TRANS_SRC   0x1E   // read-only. Check interrupt source & clear int.
#define   CTRL_REG1   0x2A   // setup, standby
#define   CTRL_REG4   0x2D   // interrupt enable
#define   CTRL_REG5   0x2E   // interrupt routing etc.


void AccelWrite(byte a,d) {
   // send register address & data over I2C
   short ack;   // 0=ACK 1=NAK

   ack=I2C_Write(ACCEL_I2C_ADDR<<1);   // address+Write


byte AccelRead(byte a) {
   byte i;
   short ack;   // 0=ACK 1=NAK
   ack=I2C_Write((ACCEL_I2C_ADDR<<1));   // address+Write
   I2C_Start();                  // I2C restart
   ack|=I2C_Write((ACCEL_I2C_ADDR<<1)|1);// address+Read


void init_accel() {
//   byte i;
   output_low(ACCEL_SA0);      // bit0 of slave address = 0
//Step 1: Put the device in Standby Mode: Register 0x2A CTRL_REG1
   AccelWrite(CTRL_REG1,0x10);   //Set device in 200 Hz ODR, Standby, Slow read for 10-bit XYZ data
// test:   i=AccelRead(0x0D);
//Step 2: Enable X,Y,Z Axes and enable the latch: Register 0x1D Configuration Register
   AccelWrite(TRANS_CFG,0x1E);   //transient detect on X,Y,Z with latch
//Step 3: Set the Threshold: Register 0x1F
//Note: Step count is 0.063g per count
   AccelWrite(TRANS_THS,HIT_SENS);   //transient detect threshold set to 96 counts (6.048g)
//Step 4: Set the Debounce Counter: Register 0x20
   AccelWrite(TRANS_COUNT,4);   //debounce 20ms with ODR=200Hz,Normal->time step=5ms,20ms minimum detection time
//Step 5: Enable Transient Detection Interrupt in the System (CTRL_REG4)
   AccelWrite(CTRL_REG4,0x20);   //transient interrupt enable
//Step 6: Route the Transient Interrupt to INT 1 hardware pin (CTRL_REG5)
   AccelWrite(CTRL_REG5,0x20);   //transient interrupt on IRQ1
//Step 7: Put the device in Active Mode: Register 0x2A CTRL_REG1
//!!   i=AccelRead(CTRL_REG1);      //Read out the contents of the register
//!!   i|= 0x01;                //Change the value in the register to Active Mode.
//!!   AccelWrite(CTRL_REG1,i);   //Write in the updated value to put the device in Active Mode
   AccelWrite(CTRL_REG1,0x11);   //Write in the updated value to put the device in Active Mode



void updateXYZ() {
   // read XYZ data from accelerometer and update <acc_xyz> array
   byte i,h,l;
   int16 t;
   short ack;   // 0=ACK 1=NAK
   // use multiple byte read:
   ack=I2C_Write((ACCEL_I2C_ADDR<<1));   // address+Write
   ack|=I2C_Write(0x01);            // OUT_X_MSB, first read address
   I2C_Start();                  // I2C restart
   ack|=I2C_Write((ACCEL_I2C_ADDR<<1)|1);// address+Read
   // 10-bit data :
   for(i=0;i<3;i++) {
      if(h&0x80) t|=0xFC00;         // if negative pad with 1's
   h=I2C_Read(0);               // last read
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