Remove virtual _isBusy() function in favor of _read().

When writing to analogue outputs pins, the digital _read() function now returns the 'busy' status of the analogue pin.  Consequently, the _isBusy() function becomes superfluous and has been removed.  The static IODevice::isBusy() function now calls the object's _read() function instead.
Also, limit in DFPlayer of 3 pins has been removed.

IODevice.cpp

@@ -53,7 +53,9 @@ void IODevice::begin() {
   MCP23017::create(180, 16, 0x21);
 
   // Call the begin() methods of each configured device in turn
+  unsigned long currentMicros = micros();
   for (IODevice *dev=_firstDevice; dev!=NULL; dev = dev->_nextDevice) {
+    dev->_nextEntryTime = currentMicros;
     dev->_begin();
   }
   _initPhase = false;
@@ -69,8 +71,14 @@ void IODevice::loop() {
   unsigned long currentMicros = micros();
   // Call every device's loop function in turn, one per entry.
   if (!_nextLoopDevice) _nextLoopDevice = _firstDevice;
-  if (_nextLoopDevice) {
+  // Check if device exists, and is due to run
+  if (_nextLoopDevice /* && ((long)(currentMicros-_nextLoopDevice->_nextEntryTime) >= 0) */ ) {
+    // Move _nextEntryTime on, so that we can guarantee that the device will continue to
+    // be serviced if it doesn't update _nextEntryTime.
+    _nextLoopDevice->_nextEntryTime = currentMicros; 
+    // Invoke device's _loop function
     _nextLoopDevice->_loop(currentMicros);
+    // Move to next device.
     _nextLoopDevice = _nextLoopDevice->_nextDevice;
   }
   
@@ -157,12 +165,13 @@ void IODevice::writeAnalogue(VPIN vpin, int value, uint8_t profile, uint16_t dur
 #endif
 }
 
-// isBusy returns true if the device is currently in an animation of some sort, e.g. is changing
+// isBusy, when called for a device pin is always a digital output or analogue output,
-//  the output over a period of time.
+//  returns input feedback state of the pin, i.e. whether the pin is busy performing
+//  an animation or fade over a period of time.
 bool IODevice::isBusy(VPIN vpin) {
   IODevice *dev = findDevice(vpin);
   if (dev) 
-    return dev->_isBusy(vpin);
+    return dev->_read(vpin);
   else
     return false;
 }
@@ -248,7 +257,7 @@ int IODevice::readAnalogue(VPIN vpin) {
       return dev->_readAnalogue(vpin);
   }
 #ifdef DIAG_IO
-  //DIAG(F("IODevice::readAnalogue(): Vpin %d not found!"), (int)vpin);
+  DIAG(F("IODevice::readAnalogue(): Vpin %d not found!"), (int)vpin);
 #endif
   return false;
 }

IODevice.h

@@ -129,7 +129,7 @@ public:
   static void write(VPIN vpin, int value);
 
   // write invokes the IODevice instance's _writeAnalogue method (not applicable for digital outputs)
-  static void writeAnalogue(VPIN vpin, int value, uint8_t profile, uint16_t duration=0);
+  static void writeAnalogue(VPIN vpin, int value, uint8_t profile=0, uint16_t duration=0);
 
   // isBusy returns true if the device is currently in an animation of some sort, e.g. is changing
   //  the output over a period of time.
@@ -203,13 +203,6 @@ protected:
     return 0;
   };
 
-  // _isBusy returns true if the device is currently in an animation of some sort, e.g. is changing
-  //  the output over a period of time.  Returns false unless overridden in sub class.
-  virtual bool _isBusy(VPIN vpin) {
-      (void)vpin;
-      return false;
-  }
-
   // Method to perform updates on an ongoing basis (optionally implemented within device class)
   virtual void _loop(unsigned long currentMicros) {
     (void)currentMicros; // Suppress compiler warning.
@@ -221,6 +214,11 @@ protected:
   // Destructor
   virtual ~IODevice() {};
 
+  // Non-virtual function
+  void delayUntil(unsigned long futureMicrosCount) {
+    _nextEntryTime = futureMicrosCount;
+  }
+  
   // Common object fields.
   VPIN _firstVpin;
   int _nPins;
@@ -242,6 +240,7 @@ private:
   static IODevice *findDevice(VPIN vpin);
 
   IODevice *_nextDevice = 0;
+  unsigned long _nextEntryTime;
   static IODevice *_firstDevice;
 
   static IODevice *_nextLoopDevice;
@@ -276,7 +275,7 @@ private:
   // Device-specific write functions.
   void _write(VPIN vpin, int value) override;
   void _writeAnalogue(VPIN vpin, int value, uint8_t profile, uint16_t duration) override;
-  bool _isBusy(VPIN vpin) override;
+  int _read(VPIN vpin) override; // returns the busy status of the device
   void _loop(unsigned long currentMicros) override;
   void updatePosition(uint8_t pin);
   void writeDevice(uint8_t pin, int value);

IO_DFPlayer.h

@@ -65,7 +65,7 @@ private:
 public:
   DFPlayer(VPIN firstVpin, int nPins, HardwareSerial &serial) {
     _firstVpin = firstVpin;
-    _nPins = min(nPins, 3);
+    _nPins = nPins;
     _serial = &serial;
     addDevice(this);
   }
@@ -159,7 +159,8 @@ protected:
     }
   }
 
-  bool _isBusy(VPIN) override {
+  // A read on any pin indicates whether the player is still playing.
+  int _read(VPIN) override {
     return _playing;
   }
 

IO_PCA9685.cpp

@@ -169,9 +169,9 @@ void PCA9685::_writeAnalogue(VPIN vpin, int value, uint8_t profile, uint16_t dur
   s->fromPosition = s->currentPosition;
 }
 
-// _isBusy returns true if the device is currently in executing an animation, 
+// _read returns true if the device is currently in executing an animation, 
 //  changing the output over a period of time.
-bool PCA9685::_isBusy(VPIN vpin) {
+int PCA9685::_read(VPIN vpin) {
   int pin = vpin - _firstVpin;
   struct ServoData *s = _servoData[pin];
   if (s == NULL)