Revision 5:a83508250db7, committed 2012-09-12

Comitter:

gltest26

Date:

Wed Sep 12 22:53:13 2012 +0000

Parent:

4:7a90125eedcf

Child:

6:05aa3555fce6

Commit message:

Collected common codes into a function getWord() to reduce duplicate codes.; Also made byte ordering and sign extending portable.

Changed in this revision

--- a/HMC5843.cpp	Wed Sep 12 15:54:13 2012 +0000
+++ b/HMC5843.cpp	Wed Sep 12 22:53:13 2012 +0000
@@ -114,53 +114,20 @@
     // Burst read all registers to enhance bus speed.
     i2c_.read(HMC5843_I2C_READ, rx, 6);
 
-    // Readings are expressed in 16bit 2's complement, so we must first
-    // concatenate two bytes to make a word and sign extend it to obtain
-    // correct negative values.
-    readings[0] = int(int16_t(unsigned(rx[0]) << 8 | unsigned(rx[1])));
-    readings[1] = int(int16_t(unsigned(rx[2]) << 8 | unsigned(rx[3])));
-    readings[2] = int(int16_t(unsigned(rx[4]) << 8 | unsigned(rx[5])));
-
-}
-
-int HMC5843::getMx() {
-
-    char tx[1];
-    char rx[2];
-    
-    
-    tx[0]=HMC5843_X_MSB;
-    i2c_.write(HMC5843_I2C_READ,tx,1);
-    i2c_.read(HMC5843_I2C_READ,rx,2);
-    return ((int)rx[0]<<8|(int)rx[1]);
+    readings[0] = swapExtend(&rx[0]);
+    readings[1] = swapExtend(&rx[2]);
+    readings[2] = swapExtend(&rx[4]);
 
 }
 
-int HMC5843::getMy() {
+int HMC5843::getWord(int regi){
 
-    char tx[1];
+    char tx = regi;
     char rx[2];
     
+    i2c_.write(I2C_ADDRESS, &tx, 1);
     
-    tx[0]=HMC5843_Y_MSB;
-    i2c_.write(HMC5843_I2C_READ,tx,1);
-    i2c_.read(HMC5843_I2C_READ,rx,2);
-    return ((int)rx[0]<<8|(int)rx[1]);
- 
+    i2c_.read(I2C_ADDRESS, rx, 2);
+    
+    return swapExtend(rx);
 }
-
-
-int HMC5843::getMz(){
-
-    char tx[1];
-    char rx[2];
-    
-    
-    tx[0]=HMC5843_Z_MSB;
-    i2c_.write(HMC5843_I2C_READ,tx,1);
-    i2c_.read(HMC5843_I2C_READ,rx,2);
-    return ((int)rx[0]<<8|(int)rx[1]);
- 
-}
-
-     

--- a/HMC5843.h	Wed Sep 12 15:54:13 2012 +0000
+++ b/HMC5843.h	Wed Sep 12 22:53:13 2012 +0000
@@ -114,6 +114,14 @@
 public:
 
     /**
+     * The I2C address that can be passed directly to i2c object (it's already shifted 1 bit left).
+     *
+     * You don't need to manually set or clear the LSB when calling I2C::read() or I2C::write(),
+     * the library takes care of it.  We just always clear the LSB.
+     */
+    static const int I2C_ADDRESS = HMC5843_I2C_WRITE;
+
+    /**
      * Constructor.
      *
      * @param sda mbed pin to use for SDA line of I2C interface.
@@ -179,21 +187,21 @@
      *
      * @return x-axis magnetic value
      */
-    int getMx();
+    int getMx(){ return getWord(HMC5843_X_MSB); }
     
     /**
      * Get the output of Y axis.
      *
      * @return y-axis magnetic value
      */
-    int getMy();
+    int getMy(){ return getWord(HMC5843_Y_MSB); }
     
     /**
      * Get the output of Z axis.
      *
      * @return z-axis magnetic value
      */
-    int getMz();
+    int getMz(){ return getWord(HMC5843_Z_MSB); }
    
     
     /**
@@ -203,6 +211,20 @@
      */
     int getStatus(void);
 
+
+protected:
+
+    /**
+     * Reads a word (2 bytes) from the sensor via I2C bus.
+     *
+     * The queried value is assumed big-endian, 2's complement value.
+     *
+     * This protected function is added because we shouldn't write getMx(), getMy() and getMz()
+     * independently, but collect common codes.
+     *
+     * @param regi Register address to be read.
+     */
+    int getWord(int regi);
   
 private:
 
@@ -217,7 +239,22 @@
      */
     I2C i2c_;
 
-   
+    /**
+     * Converts big-endian 2's complement byte pair to native byte order of
+     * the CPU and then sign extend it to the CPU's register size.
+     *
+     * Implemented here to make the compiler inline expand it.
+     */
+    int swapExtend(const char rx[2]){
+        // Readings are expressed in 16bit 2's complement, so we must first
+        // concatenate two bytes to make a word and sign extend it to obtain
+        // correct negative values.
+        // ARMCC compiles char as unsigned, which means no sign extension is
+        // performed during bitwise operations to chars. But we should make sure
+        // that lower byte won't extend its sign past upper byte for other
+        // compilers if we want to keep it portable.
+        return int16_t(((unsigned char)rx[0] << 8) | (unsigned char)rx[1]);
+    }
 
 };