Uncommented cold temp compensation after testing. Fixed read throttle so read rate ends up nailing 100hz, not 90hz. minor commenting improvements.

libraries/AP_Baro/AP_Baro_MS5611.cpp

@@ -21,7 +21,7 @@
 		read() : Read sensor data and _calculate Temperature, Pressure and Altitude
 		         This function is optimized so the main host don´t need to wait
 				 You can call this function in your main loop
-				 Maximun data output frequency 100Hz
+				 Maximum data output frequency 100Hz - this allows maximum oversampling in the chip ADC
 				 It returns a 1 if there are new data.
 		get_pressure() : return pressure in mbar*100 units
 		get_temperature() : return temperature in celsius degrees*100 units
@@ -49,8 +49,8 @@
 #define CMD_MS5611_PROM_C5 0xAA
 #define CMD_MS5611_PROM_C6 0xAC
 #define CMD_MS5611_PROM_CRC 0xAE
-#define CMD_CONVERT_D1_OSR4096 0x48   // Maximun resolution
-#define CMD_CONVERT_D2_OSR4096 0x58   // Maximun resolution
+#define CMD_CONVERT_D1_OSR4096 0x48   // Maximum resolution (oversampling)
+#define CMD_CONVERT_D2_OSR4096 0x58   // Maximum resolution (oversampling)
 
 uint32_t AP_Baro_MS5611::_s_D1;
 uint32_t AP_Baro_MS5611::_s_D2;
@@ -121,6 +121,7 @@ bool AP_Baro_MS5611::init( AP_PeriodicProcess *scheduler )
 	delay(4);
 
 	// We read the factory calibration
+  // The on-chip CRC is not used
 	C1 = _spi_read_16bits(CMD_MS5611_PROM_C1);
 	C2 = _spi_read_16bits(CMD_MS5611_PROM_C2);
 	C3 = _spi_read_16bits(CMD_MS5611_PROM_C3);
@@ -150,8 +151,11 @@ void AP_Baro_MS5611::_update(uint32_t tnow)
 {
     if (_sync_access) return;
 
-    if (tnow - _timer < 10000) {
-	    return; // wait for more than 10ms
+    // Throttle read rate to 100hz maximum.
+    // note we use 9500us here not 10000us
+    // the read rate will end up at exactly 100hz because the Periodic Timer fires at 1khz
+    if (tnow - _timer < 9500) {
+	    return;
     }
 
     _timer = tnow;
@@ -199,14 +203,14 @@ void AP_Baro_MS5611::_calculate()
 	long long P;
 
 	// Formulas from manufacturer datasheet
-	// TODO: optimization with shift operations... (shift operations works well on 64 bits variables?)
-	// We define parameters as 64 bits to prevent overflow on operations
+	// as per data sheet some intermediate results require over 32 bits, therefore
+  // we define parameters as 64 bits to prevent overflow on operations
+  // sub -20c temperature compensation is not included
 	dT = D2-((long)C5*256);
 	TEMP = 2000 + ((long long)dT * C6)/8388608;
 	OFF = (long long)C2 * 65536 + ((long long)C4 * dT ) / 128; 
 	SENS = (long long)C1 * 32768 + ((long long)C3 * dT) / 256;
 
-	/*
 	if (TEMP < 2000){   // second order temperature compensation
 		long long T2 = (long long)dT*dT / 2147483648;
 		long long Aux_64 = (TEMP-2000)*(TEMP-2000);
@@ -216,7 +220,7 @@ void AP_Baro_MS5611::_calculate()
 		OFF = OFF - OFF2;
 		SENS = SENS - SENS2;
 	}
-	*/
+ 
 	P = (D1*SENS/2097152 - OFF)/32768;
 	Temp = TEMP;
 	Press = P;