MMA8452.cpp

00001 /**
00002 @file MMA8452.cpp
00003 
00004 @brief Member functions implementations
00005 
00006 */
00007 #include "mbed.h"
00008 #include "MMA8452.h"
00009 
00010 MMA8452:: MMA8452(PinName sdaPin, PinName sclPin)
00011 {
00012     i2c = new I2C(sdaPin,sclPin); // create new I2C instance and initialise
00013     i2c->frequency(400000);       // I2C Fast Mode - 400kHz
00014     leds = new BusOut(LED4,LED3,LED2,LED1);  // for debug
00015 }
00016 
00017 void MMA8452::init()
00018 {
00019 
00020     i2c->frequency(400000); // set Fast Mode I2C frequency (5.10 datasheet)
00021 
00022     char data = readByteFromRegister(WHO_AM_I);  // p18 datasheet
00023     if (data != 0x2A) { // if correct ID not found, hand and flash error message
00024         error();
00025     }
00026 
00027     // put into STANDBY while configuring
00028     data = readByteFromRegister(CTRL_REG1); // get current value of register
00029     data &= ~(1<<0); // clear bit 0 (p37 datasheet)
00030     sendByteToRegister(data,CTRL_REG1);
00031 
00032     // Set output data rate, default is 800 Hz, will set to 100 Hz (clear b5, set b4/b3 - p37 datasheet)
00033     data = readByteFromRegister(CTRL_REG1);
00034     data &= ~(1<<5);
00035     data |=  (1<<4);
00036     data |=  (1<<3);
00037     sendByteToRegister(data,CTRL_REG1);
00038 
00039     //// Can also change default 2g range to 4g or 8g (p22 datasheet)
00040     data = readByteFromRegister(XYZ_DATA_CFG);
00041     data |=  (1<<0); // set bit 0 - 4g range
00042     sendByteToRegister(data,XYZ_DATA_CFG);
00043 
00044     // set ACTIVE
00045     data = readByteFromRegister(CTRL_REG1);
00046     data |= (1<<0);   // set bit 0 in CTRL_REG1
00047     sendByteToRegister(data,CTRL_REG1);
00048 
00049 }
00050 
00051 // read acceleration data from device
00052 Acceleration MMA8452::readValues()
00053 {
00054     // acceleration data stored in 6 registers (0x01 to 0x06)
00055     // device automatically increments register, so can read 6 bytes starting from OUT_X_MSB
00056     char data[6];
00057     readBytesFromRegister(OUT_X_MSB,6,data);
00058 
00059     char x_MSB = data[0];  // extract MSB and LSBs for x,y,z values
00060     char x_LSB = data[1];
00061     char y_MSB = data[2];
00062     char y_LSB = data[3];
00063     char z_MSB = data[4];
00064     char z_LSB = data[5];
00065 
00066     // [0:7] of MSB are 8 MSB of 12-bit value , [7:4] of LSB are 4 LSB's of 12-bit value
00067     // need to type-cast as numbers are in signed (2's complement) form (p20 datasheet)
00068     int x = (int16_t) (x_MSB << 8) | x_LSB;  // combine bytes
00069     x >>= 4;  // are left-aligned, so shift 4 places right to right-align
00070     int y = (int16_t) (y_MSB << 8) | y_LSB;
00071     y >>= 4;
00072     int z = (int16_t) (z_MSB << 8) | z_LSB;
00073     z >>= 4;
00074 
00075     // sensitivity is 1024 counts/g in 2g mode (pg 9 datasheet)
00076     //  "   "          512      "      4g     "
00077     //  "   "          256      "      8g     "
00078     Acceleration acc;
00079     
00080     acc.x = x/512.0;
00081     acc.y = y/512.0;
00082     acc.z = z/512.0;
00083     
00084     return acc;
00085 }
00086 
00087 // reads a byte from a specific register
00088 char MMA8452::readByteFromRegister(char reg)
00089 {
00090     int nack = i2c->write(MMA8452_W_ADDRESS,&reg,1,true);  // send the register address to the slave
00091     // true as need to send repeated start condition (5.10.1 datasheet)
00092     // http://www.i2c-bus.org/repeated-start-condition/
00093     if (nack)
00094         error();  // if we don't receive acknowledgement, flash error message
00095 
00096     char rx;
00097     nack = i2c->read(MMA8452_R_ADDRESS,&rx,1);  // read a byte from the register and store in buffer
00098     if (nack)
00099         error();  // if we don't receive acknowledgement, flash error message
00100 
00101     return rx;
00102 }
00103 
00104 // reads a series of bytes, starting from a specific register
00105 void MMA8452::readBytesFromRegister(char reg,int numberOfBytes,char bytes[])
00106 {
00107 
00108     int nack = i2c->write(MMA8452_W_ADDRESS,&reg,1,true);  // send the slave write address and the configuration register address
00109     // true as need to send repeated start condition (5.10.1 datasheet)
00110     // http://www.i2c-bus.org/repeated-start-condition/
00111 
00112     if (nack)
00113         error();  // if we don't receive acknowledgement, flash error message
00114 
00115     nack = i2c->read(MMA8452_R_ADDRESS,bytes,numberOfBytes);  // read bytes
00116     if (nack)
00117         error();  // if we don't receive acknowledgement, flash error message
00118 
00119 }
00120 
00121 // sends a byte to a specific register
00122 void MMA8452::sendByteToRegister(char byte,char reg)
00123 {
00124     char data[2];
00125     data[0] = reg;
00126     data[1] = byte;
00127     // send the register address, followed by the data
00128     int nack = i2c->write(MMA8452_W_ADDRESS,data,2);
00129     if (nack)
00130         error();  // if we don't receive acknowledgement, flash error message
00131 
00132 }
00133 
00134 void MMA8452::error()
00135 {
00136     while(1) {
00137         leds->write(15);
00138         wait(0.1);
00139         leds->write(0);
00140         wait(0.1);
00141     }
00142 }