Diff: HMC5843.h

Revision:

5:a83508250db7

Parent:

4:7a90125eedcf

Child:

6:05aa3555fce6

--- 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]);
+    }
 
 };