Revision 0:df3b9e41edf3, committed 2015-03-08

Comitter:

eencae

Date:

Sun Mar 08 15:31:02 2015 +0000

Commit message:

Library for Freescale MMA8452 accelerometer.

Changed in this revision

--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/MMA8452.cpp	Sun Mar 08 15:31:02 2015 +0000
@@ -0,0 +1,142 @@
+/**
+@file MMA8452.cpp
+
+@brief Member functions implementations
+
+*/
+#include "mbed.h"
+#include "MMA8452.h"
+
+MMA8452:: MMA8452(PinName sdaPin, PinName sclPin)
+{
+    i2c = new I2C(sdaPin,sclPin); // create new I2C instance and initialise
+    i2c->frequency(400000);       // I2C Fast Mode - 400kHz
+    leds = new BusOut(LED4,LED3,LED2,LED1);  // for debug
+}
+
+void MMA8452::init()
+{
+
+    i2c->frequency(400000); // set Fast Mode I2C frequency (5.10 datasheet)
+
+    char data = readByteFromRegister(WHO_AM_I);  // p18 datasheet
+    if (data != 0x2A) { // if correct ID not found, hand and flash error message
+        error();
+    }
+
+    // put into STANDBY while configuring
+    data = readByteFromRegister(CTRL_REG1); // get current value of register
+    data &= ~(1<<0); // clear bit 0 (p37 datasheet)
+    sendByteToRegister(data,CTRL_REG1);
+
+    // Set output data rate, default is 800 Hz, will set to 100 Hz (clear b5, set b4/b3 - p37 datasheet)
+    data = readByteFromRegister(CTRL_REG1);
+    data &= ~(1<<5);
+    data |=  (1<<4);
+    data |=  (1<<3);
+    sendByteToRegister(data,CTRL_REG1);
+
+    //// Can also change default 2g range to 4g or 8g (p22 datasheet)
+    data = readByteFromRegister(XYZ_DATA_CFG);
+    data |=  (1<<0); // set bit 0 - 4g range
+    sendByteToRegister(data,XYZ_DATA_CFG);
+
+    // set ACTIVE
+    data = readByteFromRegister(CTRL_REG1);
+    data |= (1<<0);   // set bit 0 in CTRL_REG1
+    sendByteToRegister(data,CTRL_REG1);
+
+}
+
+// read acceleration data from device
+Acceleration MMA8452::readValues()
+{
+    // acceleration data stored in 6 registers (0x01 to 0x06)
+    // device automatically increments register, so can read 6 bytes starting from OUT_X_MSB
+    char data[6];
+    readBytesFromRegister(OUT_X_MSB,6,data);
+
+    char x_MSB = data[0];  // extract MSB and LSBs for x,y,z values
+    char x_LSB = data[1];
+    char y_MSB = data[2];
+    char y_LSB = data[3];
+    char z_MSB = data[4];
+    char z_LSB = data[5];
+
+    // [0:7] of MSB are 8 MSB of 12-bit value , [7:4] of LSB are 4 LSB's of 12-bit value
+    // need to type-cast as numbers are in signed (2's complement) form (p20 datasheet)
+    int x = (int16_t) (x_MSB << 8) | x_LSB;  // combine bytes
+    x >>= 4;  // are left-aligned, so shift 4 places right to right-align
+    int y = (int16_t) (y_MSB << 8) | y_LSB;
+    y >>= 4;
+    int z = (int16_t) (z_MSB << 8) | z_LSB;
+    z >>= 4;
+
+    // sensitivity is 1024 counts/g in 2g mode (pg 9 datasheet)
+    //  "   "          512      "      4g     "
+    //  "   "          256      "      8g     "
+    Acceleration acc;
+    
+    acc.x = x/512.0;
+    acc.y = y/512.0;
+    acc.z = z/512.0;
+    
+    return acc;
+}
+
+// reads a byte from a specific register
+char MMA8452::readByteFromRegister(char reg)
+{
+    int nack = i2c->write(MMA8452_W_ADDRESS,&reg,1,true);  // send the register address to the slave
+    // true as need to send repeated start condition (5.10.1 datasheet)
+    // http://www.i2c-bus.org/repeated-start-condition/
+    if (nack)
+        error();  // if we don't receive acknowledgement, flash error message
+
+    char rx;
+    nack = i2c->read(MMA8452_R_ADDRESS,&rx,1);  // read a byte from the register and store in buffer
+    if (nack)
+        error();  // if we don't receive acknowledgement, flash error message
+
+    return rx;
+}
+
+// reads a series of bytes, starting from a specific register
+void MMA8452::readBytesFromRegister(char reg,int numberOfBytes,char bytes[])
+{
+
+    int nack = i2c->write(MMA8452_W_ADDRESS,&reg,1,true);  // send the slave write address and the configuration register address
+    // true as need to send repeated start condition (5.10.1 datasheet)
+    // http://www.i2c-bus.org/repeated-start-condition/
+
+    if (nack)
+        error();  // if we don't receive acknowledgement, flash error message
+
+    nack = i2c->read(MMA8452_R_ADDRESS,bytes,numberOfBytes);  // read bytes
+    if (nack)
+        error();  // if we don't receive acknowledgement, flash error message
+
+}
+
+// sends a byte to a specific register
+void MMA8452::sendByteToRegister(char byte,char reg)
+{
+    char data[2];
+    data[0] = reg;
+    data[1] = byte;
+    // send the register address, followed by the data
+    int nack = i2c->write(MMA8452_W_ADDRESS,data,2);
+    if (nack)
+        error();  // if we don't receive acknowledgement, flash error message
+
+}
+
+void MMA8452::error()
+{
+    while(1) {
+        leds->write(15);
+        wait(0.1);
+        leds->write(0);
+        wait(0.1);
+    }
+}
\ No newline at end of file

--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/MMA8452.h	Sun Mar 08 15:31:02 2015 +0000
@@ -0,0 +1,110 @@
+/**
+@file MMA8452.h
+
+@brief Header file containing member functions and variables
+
+*/
+
+#ifndef MMA8452_H
+#define MMA8452_H
+
+// MMA8452 address is 0x1D by default (see EAGLE schematic of breakout - SA = 1)
+#define MMA8452_W_ADDRESS   0x3A
+#define MMA8452_R_ADDRESS   0x3B
+// Register Descriptions - p18 datasheet
+#define OUT_X_MSB           0x01
+#define WHO_AM_I            0x0D
+#define CTRL_REG1           0x2A
+#define XYZ_DATA_CFG        0x0E
+
+typedef struct Acceleration Acceleration;
+struct Acceleration {
+    float x;
+    float y;
+    float z;
+};
+
+#include "mbed.h"
+
+/**
+@brief Library for interfacing with MMA8452 I2C Accelerometer
+
+@see http://www.freescale.com/files/sensors/doc/data_sheet/MMA8452Q.pdf
+@see https://www.sparkfun.com/products/12756
+
+@brief Revision 1.0
+
+@author Craig A. Evans
+@date   March 2015
+ *
+ * Example:
+ * @code
+
+#include "mbed.h"
+#include "MMA8452.h"
+
+MMA8452 mma8452(p28,p27);  // SDA, SCL
+Serial serial(USBTX,USBRX);
+
+int main() {
+
+    mma8452.init();  // 100 Hz update rate, ±4g scale
+
+    Acceleration acceleration;  // Accleration structure declared in MMA8452 class
+
+    while(1) {
+
+        acceleration = mma8452.readValues();   // read current values and print over serial port
+        serial.printf("x = %.2f g y = %.2f g z = %.2f g\n",acceleration.x,acceleration.y,acceleration.z);
+        wait(0.1);   // short delay until next reading
+
+    }
+
+}
+
+
+ * @endcode
+ */
+class MMA8452
+{
+
+public:
+    /** Create a MMA8452 object connected to the specified pins
+    *
+    * @param sdaPin - mbed SDA pin
+    * @param sclPin - mbed SCL pin
+    *
+    */
+    MMA8452(PinName sdaPin, PinName sclPin);
+
+    /** Initialise accelerometer
+    *
+    *   Powers up the accelerometer, sets 100 Hz update rate and ±4g scale
+    */
+    void init();
+
+    /** Get values of acceleration
+    *
+    *   Reads the x,y,z values in g's
+    *   @returns an Acceleration structure with x,y,z members (float)
+    */
+    Acceleration readValues();
+
+
+private:
+    void sendByteToRegister(char byte,char reg);
+    char readByteFromRegister(char reg);
+    void readBytesFromRegister(char reg,int numberOfBytes,char bytes[]);
+    void error();
+
+public:
+
+private:  // private variables
+    I2C*    i2c;
+    BusOut*  leds;
+
+};
+
+
+
+#endif
\ No newline at end of file