From b5cbba7ed8e0823719c8b8ce86b6e866ac0c3871 Mon Sep 17 00:00:00 2001
From: Robert Konklewski <nythil@gmail.com>
Date: Thu, 21 Apr 2016 19:32:23 +0200
Subject: [PATCH] Temp: read ADC on-demand, instead of constantly running.

On AVRs analog conversions were running in essentially
free-running mode, i.e.  next conversion was being started
immediately after the previous one ended. This caused many
unnecessary interrupts.

Now, conversions are initiated manually by start_adc(). This
causes a single read on each of the registered channels. Once
all channels are read, conversions stop until the next call of
start_adc().
---
 analog-avr.c | 17 +++++++++++++----
 analog.h     | 16 ++++++++++++++++
 temp.c       | 19 +++++++++++++------
 3 files changed, 42 insertions(+), 10 deletions(-)

diff --git a/analog-avr.c b/analog-avr.c
index 2c34a22..bb49135 100644
--- a/analog-avr.c
+++ b/analog-avr.c
@@ -46,11 +46,18 @@ void analog_init() {
 			DIDR2 = (analog_mask >> 8) & 0xFF;
 		#endif
 
-		// now we start the first conversion and leave the rest to the interrupt
-		ADCSRA |= MASK(ADIE) | MASK(ADSC);
+    // Enable the ADC.
+    ADCSRA |= MASK(ADIE);
   } /* analog_mask */
 }
 
+/**
+  Start a new ADC conversion.
+*/
+void start_adc() {
+  ADCSRA |= MASK(ADSC);
+}
+
 /*! Analog Interrupt
 
 	This is where we read our analog value and store it in an array for later retrieval
@@ -77,8 +84,10 @@ ISR(ADC_vect, ISR_NOBLOCK) {
 				ADCSRB &= ~MASK(MUX5);
 		#endif
 
-		// After the mux has been set, start a new conversion
-		ADCSRA |= MASK(ADSC);
+    // If there is another channel to read, start a new conversion.
+    if (adc_counter != 0) {
+      ADCSRA |= MASK(ADSC);
+    }
 	}
 }
 
diff --git a/analog.h b/analog.h
index 2f72191..a2f7681 100644
--- a/analog.h
+++ b/analog.h
@@ -49,10 +49,26 @@
   #error REFERENCE undefined. See analog.h on how to choose it.
 #endif
 
+/** \def NEEDS_START_ADC
+
+  Whether the board needs and implements start_adc(), i.e. analog reads
+  on-demand (as opposed to free-running ADC conversions).
+
+  Currently only implemented on AVR.
+*/
+#define NEEDS_START_ADC
+
 #endif /* __AVR__ */
 
 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 */
diff --git a/temp.c b/temp.c
index 1686a45..e06700d 100644
--- a/temp.c
+++ b/temp.c
@@ -110,15 +110,19 @@ void temp_init() {
       #endif
 
 		#ifdef	TEMP_THERMISTOR
-			// handled by analog_init()
-/*			case TT_THERMISTOR:
-				break;*/
+      // Mostly handled by analog_init().
+      case TT_THERMISTOR:
+        // Start a conversion so it's ready for the first temp_sensor_tick().
+        DO_START_ADC();
+        break;
 		#endif
 
 		#ifdef	TEMP_AD595
-			// handled by analog_init()
-/*			case TT_AD595:
-				break;*/
+      // Mostly handled by analog_init().
+      case TT_AD595:
+        // Start a conversion so it's ready for the first temp_sensor_tick().
+        DO_START_ADC();
+        break;
 		#endif
 
       #ifdef TEMP_INTERCOM
@@ -309,6 +313,8 @@ void temp_sensor_tick() {
             temp = temp_table_lookup(analog_read(i), i);
 
             temp_sensors_runtime[i].next_read_time = 0;
+            DO_START_ADC();
+
             break;
 				#endif	/* TEMP_THERMISTOR */
 
@@ -332,6 +338,7 @@ void temp_sensor_tick() {
 					temp = (temp * 500L) >> 8;
 
 					temp_sensors_runtime[i].next_read_time = 0;
+          DO_START_ADC();
 
 					break;
 				#endif	/* TEMP_AD595 */