Baro: average over all pressure/temperature samples on MS5611

this allows us to use the MS5611 barometer at its full 100Hz sample
rate (80Hz for pressure, 20Hz for temperature). The pressure and
temperature values are averaged between reads without adding any
latency. Previously the driver would throw away values between
readings

This also fixes a race condition in reading from the SPI bus that
could lead to bad values from the barometer

libraries/AP_Baro/AP_Baro_MS5611.cpp

@@ -32,6 +32,7 @@
 
 */
 
+#include <FastSerial.h>
 #include <SPI.h>
 #include "AP_Baro_MS5611.h"
 
@@ -51,12 +52,14 @@
 #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;
+uint32_t volatile AP_Baro_MS5611::_s_D1;
+uint32_t volatile AP_Baro_MS5611::_s_D2;
+uint8_t  volatile AP_Baro_MS5611::_d1_count;
+uint8_t  volatile AP_Baro_MS5611::_d2_count;
 uint8_t  AP_Baro_MS5611::_state;
 uint32_t AP_Baro_MS5611::_timer;
 bool     AP_Baro_MS5611::_sync_access;
-bool     AP_Baro_MS5611::_updated;
+bool     volatile AP_Baro_MS5611::_updated;
 
 uint8_t AP_Baro_MS5611::_spi_read(uint8_t reg)
 {
@@ -132,13 +135,21 @@ bool AP_Baro_MS5611::init( AP_PeriodicProcess *scheduler )
 	//Send a command to read Temp first
 	_spi_write(CMD_CONVERT_D2_OSR4096);
 	_timer = micros();
-	_state = 1;
+	_state = 0;
 	Temp=0;
 	Press=0;
 
+    _s_D1 = 0;
+    _s_D2 = 0;
+    _d1_count = 0;
+    _d2_count = 0;
+
     scheduler->resume_timer();
 	scheduler->register_process( AP_Baro_MS5611::_update );
 
+    // wait for at least one value to be read
+    while (!_updated) ;
+
 	healthy = true;
     return true;
 }
@@ -160,20 +171,36 @@ void AP_Baro_MS5611::_update(uint32_t tnow)
 
     _timer = tnow;
 
-    if (_state == 1) {
-	    _s_D2 = _spi_read_adc();  				 // On state 1 we read temp
+    if (_state == 0) {
+	    _s_D2 += _spi_read_adc();  				 // On state 0 we read temp
+        _d2_count++;
+        if (_d2_count == 32) {
+            // we have summed 128 values. This only happens
+            // when we stop reading the barometer for a long time
+            // (more than 1.2 seconds)
+            _s_D2 >>= 1;
+            _d2_count = 16;
+        }
 	    _state++;
 	    _spi_write(CMD_CONVERT_D1_OSR4096);  // Command to read pressure
-    } else if (_state == 5) {
-	    _s_D1 = _spi_read_adc();
-	    _state = 1;			                // Start again from state = 1
-	    _spi_write(CMD_CONVERT_D2_OSR4096);	// Command to read temperature
-	    _updated = true;					                // New pressure reading
     } else {
-	    _s_D1 = _spi_read_adc();
+	    _s_D1 += _spi_read_adc();
+        _d1_count++;
+        if (_d1_count == 128) {
+            // we have summed 128 values. This only happens
+            // when we stop reading the barometer for a long time
+            // (more than 1.2 seconds)
+            _s_D1 >>= 1;
+            _d1_count = 64;
+        }
 	    _state++;
-	    _spi_write(CMD_CONVERT_D1_OSR4096);  // Command to read pressure
 	    _updated = true;					                // New pressure reading
+        if (_state == 5) {
+            _spi_write(CMD_CONVERT_D2_OSR4096);	// Command to read temperature
+            _state = 0;
+        } else {
+            _spi_write(CMD_CONVERT_D1_OSR4096);	// Command to read pressure
+        }
     }
 }
 
@@ -181,10 +208,24 @@ uint8_t AP_Baro_MS5611::read()
 {
     _sync_access = true;
     bool updated = _updated;
-    _updated = 0;
-    if (updated > 0) {
-        D1 = _s_D1;
-        D2 = _s_D2;
+    if (updated) {
+        uint32_t sD1, sD2;
+        uint8_t d1count, d2count;
+        // we need to disable interrupts to access
+        // _s_D1 and _s_D2 as they are not atomic
+        cli();
+        sD1 = _s_D1; _s_D1 = 0;
+        sD2 = _s_D2; _s_D2 = 0;
+        d1count = _d1_count; _d1_count = 0;
+        d2count = _d2_count; _d2_count = 0;
+        _updated = false;
+        sei();
+        if (d1count != 0) {
+            D1 = sD1 / d1count;
+        }
+        if (d2count != 0) {
+            D2 = sD2 / d2count;
+        }
         _raw_press = D1;
         _raw_temp = D2;
     }
@@ -209,7 +250,8 @@ void AP_Baro_MS5611::_calculate()
 	// 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);
+    // Serial.printf("D1=%lu D2=%lu\n", (unsigned long)D1, (unsigned long)D2);
+	dT = D2-((int32_t)C5*256);
 	TEMP = 2000 + ((int64_t)dT * C6)/8388608;
 	OFF = (int64_t)C2 * 65536 + ((int64_t)C4 * dT ) / 128;
 	SENS = (int64_t)C1 * 32768 + ((int64_t)C3 * dT) / 256;

libraries/AP_Baro/AP_Baro_MS5611.h

@@ -23,8 +23,10 @@ class AP_Baro_MS5611 : public AP_Baro
   /* Asynchronous handler functions: */ 
   static void _update(uint32_t );
   /* Asynchronous state: */
-  static bool _updated;
-  static uint32_t _s_D1, _s_D2;
+  static volatile bool _updated;
+  static volatile uint8_t _d1_count;
+  static volatile uint8_t _d2_count;
+  static volatile uint32_t _s_D1, _s_D2;
   static uint8_t _state;
   static uint32_t _timer;
   /* Gates access to asynchronous state: */

libraries/AP_Baro/examples/AP_Baro_MS5611_test/AP_Baro_MS5611_test.pde

@@ -34,7 +34,7 @@ void setup()
 	SPI.setClockDivider(SPI_CLOCK_DIV32); // 500khz for debugging, increase later
 
 	baro.init(&scheduler);
-	timer = micros();
+	timer = millis();
 }
 
 void loop()
@@ -42,8 +42,8 @@ void loop()
 	float tmp_float;
 	float Altitude;
 
-	if((micros()- timer) > 50000L){
-		timer = micros();
+	if((millis() - timer) > 100){
+		timer = millis();
 		baro.read();
 		unsigned long read_time = micros() - timer;
 		if (!baro.healthy) {