implement Wire.end() for AVR core

hardware/arduino/avr/libraries/Wire/Wire.cpp

@@ -75,6 +75,11 @@ void TwoWire::begin(int address)
   begin((uint8_t)address);
 }
 
+void TwoWire::end(void)
+{
+  twi_disable();
+}
+
 void TwoWire::setClock(uint32_t frequency)
 {
   TWBR = ((F_CPU / frequency) - 16) / 2;

hardware/arduino/avr/libraries/Wire/Wire.h

@@ -27,6 +27,9 @@
 
 #define BUFFER_LENGTH 32
 
+// WIRE_HAS_END means Wire has end()
+#define WIRE_HAS_END 1
+
 class TwoWire : public Stream
 {
   private:
@@ -49,6 +52,7 @@ class TwoWire : public Stream
     void begin();
     void begin(uint8_t);
     void begin(int);
+    void end();
     void setClock(uint32_t);
     void beginTransmission(uint8_t);
     void beginTransmission(int);

hardware/arduino/avr/libraries/Wire/utility/twi.c

@@ -90,6 +90,22 @@ void twi_init(void)
   TWCR = _BV(TWEN) | _BV(TWIE) | _BV(TWEA);
 }
 
+/* 
+ * Function twi_disable
+ * Desc     disables twi pins
+ * Input    none
+ * Output   none
+ */
+void twi_disable(void)
+{
+  // disable twi module, acks, and twi interrupt
+  TWCR &= ~(_BV(TWEN) | _BV(TWIE) | _BV(TWEA));
+
+  // deactivate internal pullups for twi.
+  digitalWrite(SDA, 0);
+  digitalWrite(SCL, 0);
+}
+
 /* 
  * Function twi_slaveInit
  * Desc     sets slave address and enables interrupt

hardware/arduino/avr/libraries/Wire/utility/twi.h

@@ -39,6 +39,7 @@
   #define TWI_STX   4
   
   void twi_init(void);
+  void twi_disable(void);
   void twi_setAddress(uint8_t);
   uint8_t twi_readFrom(uint8_t, uint8_t*, uint8_t, uint8_t);
   uint8_t twi_writeTo(uint8_t, uint8_t*, uint8_t, uint8_t, uint8_t);