Temp: request ADC reads only when necessary.

Issues ADC reads (via start_adc()) only when actually needed.
When EWMA is enabled, then reads are performed at every
temp_sensor_tick() call, i.e. every 10ms. That's to ensure EWMA
gets updated frequently enough.

When EWMA is disabled, then the only other use for analog reads
is heater control, i.e. heater_tick(). It's run on a 250ms clock,
so that's how often analog reads need to be performed in that
case.

Additionally, ADC conversions are being started 10ms in advance,
so that they can finish before their results are read.

analog.h

@@ -65,10 +65,7 @@ void 			analog_init(void);
 uint16_t	analog_read(uint8_t index);
 
 #ifdef NEEDS_START_ADC
-  #define DO_START_ADC() do { start_adc(); } while (0)
   void start_adc(void);
-#else
-  #define DO_START_ADC()
 #endif
 
 #endif	/* _ANALOG_H */

temp.c

@@ -89,6 +89,35 @@ struct {
 	uint16_t					next_read_time; ///< how long until we can read this sensor again?
 } temp_sensors_runtime[NUM_TEMP_SENSORS];
 
+#ifdef NEEDS_START_ADC
+  /**
+    Timer/counter for initiating ADC reads via start_adc(). Runs on the same
+    interval as analog_read(), but is shifted in time, which is why this
+    additional counter is needed.
+  */
+  uint16_t next_start_adc_time;
+#endif
+
+/** \def TEMP_EWMA
+
+  Default alpha constant for the Exponentially Weighted Moving Average (EWMA)
+  for smoothing noisy sensors. Instrument Engineer's Handbook, 4th ed,
+  Vol 2 p126 says values of 0.05 to 0.1 are typical.
+
+  Setting it to 1.0 turns EWMA off.
+*/
+#ifndef TEMP_EWMA
+  #define TEMP_EWMA 1.0
+#endif
+
+/**
+  Interval between analog_read() calls. 10ms if EWMA is enabled, because EWMA
+  needs frequent updates, otherwise 250ms so that we don't waste time on
+  unnecessary reads.
+*/
+#define ANALOG_READ_INTERVAL ((TEMP_EWMA == 1.0) ? 25 : 0)
+
+
 /// Set up temp sensors.
 void temp_init() {
 	temp_sensor_t i;
@@ -112,16 +141,16 @@ void temp_init() {
 		#ifdef	TEMP_THERMISTOR
       // Mostly handled by analog_init().
       case TT_THERMISTOR:
-        // Start a conversion so it's ready for the first temp_sensor_tick().
-        DO_START_ADC();
+        // Schedule first read.
+        temp_sensors_runtime[i].next_read_time = ANALOG_READ_INTERVAL;
         break;
 		#endif
 
 		#ifdef	TEMP_AD595
       // Mostly handled by analog_init().
       case TT_AD595:
-        // Start a conversion so it's ready for the first temp_sensor_tick().
-        DO_START_ADC();
+        // Schedule first read.
+        temp_sensors_runtime[i].next_read_time = ANALOG_READ_INTERVAL;
         break;
 		#endif
 
@@ -142,6 +171,16 @@ void temp_init() {
 				break;
 		}
 	}
+
+  #ifdef NEEDS_START_ADC
+    /**
+      Schedule the first ADC conversion so that it starts one 10ms cycle
+      before the sensors are actually read. This way analog_read() returns a
+      value that's only 10ms old.
+    */
+    next_start_adc_time = (ANALOG_READ_INTERVAL > 0) ?
+                          (ANALOG_READ_INTERVAL - 1) : 0;
+  #endif
 }
 
 /**
@@ -312,8 +351,7 @@ void temp_sensor_tick() {
             // Read current temperature.
             temp = temp_table_lookup(analog_read(i), i);
 
-            temp_sensors_runtime[i].next_read_time = 0;
-            DO_START_ADC();
+            temp_sensors_runtime[i].next_read_time = ANALOG_READ_INTERVAL;
 
             break;
 				#endif	/* TEMP_THERMISTOR */
@@ -337,8 +375,7 @@ void temp_sensor_tick() {
 					// >>8 instead of >>10 because internal temp is stored as 14.2 fixed point
 					temp = (temp * 500L) >> 8;
 
-					temp_sensors_runtime[i].next_read_time = 0;
-          DO_START_ADC();
+          temp_sensors_runtime[i].next_read_time = ANALOG_READ_INTERVAL;
 
 					break;
 				#endif	/* TEMP_AD595 */
@@ -378,9 +415,6 @@ void temp_sensor_tick() {
 			/* Exponentially Weighted Moving Average alpha constant for smoothing
 			   noisy sensors. Instrument Engineer's Handbook, 4th ed, Vol 2 p126
 			   says values of 0.05 to 0.1 for TEMP_EWMA are typical. */
-			#ifndef TEMP_EWMA
-				#define TEMP_EWMA 1.0
-			#endif
 			#define EWMA_SCALE  1024L
 			#define EWMA_ALPHA  ((long) (TEMP_EWMA * EWMA_SCALE))
 			temp_sensors_runtime[i].last_read_temp = (uint16_t) ((EWMA_ALPHA * temp +
@@ -388,6 +422,21 @@ void temp_sensor_tick() {
 			                                         ) / EWMA_SCALE);
 		}
   }
+
+  #ifdef NEEDS_START_ADC
+    // Start ADC conversion when the time comes.
+    if (next_start_adc_time > 1) {
+      next_start_adc_time--;
+    } else {
+      start_adc();
+      /**
+        start_adc() runs on the same interval as analog_read(), but it's
+        shifted in time so that it happens 10ms earlier. This way the ADC
+        conversion can finish before its results are read.
+      */
+      next_start_adc_time = ANALOG_READ_INTERVAL;
+    }
+  #endif
 }
 
 /**