From 31eb465ebfbe14edfbe07d48d850c66b3efeb28f Mon Sep 17 00:00:00 2001
From: Stephen Olesen <slepp@slepp.ca>
Date: Wed, 1 Jul 2015 14:27:10 -0600
Subject: [PATCH] Allow for high idle duty cycle when output is 'off',
 amplitude adjustments.

---
 DDS.cpp | 14 +++++++++++---
 DDS.h   | 30 ++++++++++++++++++++----------
 2 files changed, 31 insertions(+), 13 deletions(-)

diff --git a/DDS.cpp b/DDS.cpp
index b88b556..5b2336b 100644
--- a/DDS.cpp
+++ b/DDS.cpp
@@ -61,22 +61,30 @@ void DDS::clockTick() {
   accumulator += stepRate;
   if(running) {
     if(tickDuration == 0) {
+#ifdef DDS_IDLE_HIGH
+      // Set the duty cycle to 50%
+      OCR2B = pow(2,COMPARATOR_BITS)/2;
+#else
+      // Set duty cycle to 0, effectively off
       OCR2B = 0;
+#endif
       running = false;
     } else {
-      OCR2B = getPhaseAngle();
+      OCR2B = getDutyCycle();
     }
     // Reduce our playback duration by one tick
     tickDuration--;
   }
 }
 
-uint8_t DDS::getPhaseAngle() {
+uint8_t DDS::getDutyCycle() {
 #if ACCUMULATOR_BIT_SHIFT >= 24
   uint16_t phAng;
 #else
   uint8_t phAng;
 #endif
   phAng = (accumulator >> ACCUMULATOR_BIT_SHIFT);
-  return pgm_read_byte_near(ddsSineTable + phAng)>>(8-COMPARATOR_BITS);
+  uint8_t position = pgm_read_byte_near(ddsSineTable + phAng)>>(8-COMPARATOR_BITS);
+  // Apply scaling and return
+  return position >> amplitude;
 }
diff --git a/DDS.h b/DDS.h
index edb8dcf..8c17432 100644
--- a/DDS.h
+++ b/DDS.h
@@ -11,6 +11,10 @@
 #define ACCUMULATOR_BITS      32
 #endif
 
+// If defined, the timer will idle at 50% duty cycle
+// This leaves it floating in the centre of the PWM/DAC voltage range
+#define DDS_IDLE_HIGH
+
 #define COMPARATOR_BITS       6
 
 #define DDS_REFCLK_DEAULT     38400
@@ -123,7 +127,7 @@ public:
   void on(unsigned short duration) {
     // Duration in ticks from milliseconds is:
     // t = (1/refclk)
-    tickDuration = duration / (1000.0/refclk);
+    tickDuration = (duration * refclk) / 1000;
     running = true;
   }
   void off() {
@@ -137,8 +141,7 @@ public:
   }
   // Blocking version
   void playWait(unsigned short freq, unsigned short duration) {
-    setFrequency(freq);
-    on(duration);
+    play(freq, duration);
     delay(duration * 1000);
   }
   
@@ -149,20 +152,27 @@ public:
   // Adjustable reference clock
   void setReferenceClock(unsigned long ref);
 
-  uint8_t getPhaseAngle();
+  uint8_t getDutyCycle();
 
+  // Set a scaling factor. To keep things quick, this is a power of 2 value.
+  // Set it with 0 for lowest (which will be off), 8 is highest.
+  void setAmplitude(unsigned char amp) {
+    amplitude = 8 - amp;
+  }
+  
   void clockTick();
   
 private:
-  bool running;
-  unsigned long tickDuration;
+  volatile bool running;
+  volatile unsigned long tickDuration;
+  volatile unsigned char amplitude;
 #ifdef SHORT_ACCUMULATOR
-  unsigned short accumulator;
-  unsigned short stepRate;
+  volatile unsigned short accumulator;
+  volatile unsigned short stepRate;
   unsigned short refclk;
 #else
-  unsigned long accumulator;
-  unsigned long stepRate;
+  volatile unsigned long accumulator;
+  volatile unsigned long stepRate;
   unsigned long refclk;
 #endif
   static DDS *sDDS;