Fix Wire register

Move TWBR register from Wire.cpp to twi.c file.

libraries/Wire/src/Wire.cpp

@@ -80,9 +80,9 @@ void TwoWire::end(void)
   twi_disable();
 }
 
-void TwoWire::setClock(uint32_t frequency)
+void TwoWire::setClock(uint32_t clock)
 {
-  TWBR = ((F_CPU / frequency) - 16) / 2;
+  twi_setFrequency(clock);
 }
 
 uint8_t TwoWire::requestFrom(uint8_t address, uint8_t quantity, uint32_t iaddress, uint8_t isize, uint8_t sendStop)

libraries/Wire/src/utility/twi.c

@@ -118,6 +118,22 @@ void twi_setAddress(uint8_t address)
   TWAR = address << 1;
 }
 
+/* 
+ * Function twi_setClock
+ * Desc     sets twi bit rate
+ * Input    Clock Frequency
+ * Output   none
+ */
+void twi_setFrequency(uint32_t frequency)
+{
+  TWBR = ((F_CPU / frequency) - 16) / 2;
+  
+  /* twi bit rate formula from atmega128 manual pg 204
+  SCL Frequency = CPU Clock Frequency / (16 + (2 * TWBR))
+  note: TWBR should be 10 or higher for master mode
+  It is 72 for a 16mhz Wiring board with 100kHz TWI */
+}
+
 /* 
  * Function twi_readFrom
  * Desc     attempts to become twi bus master and read a

libraries/Wire/src/utility/twi.h

@@ -41,6 +41,7 @@
   void twi_init(void);
   void twi_disable(void);
   void twi_setAddress(uint8_t);
+  void twi_setFrequency(uint32_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);
   uint8_t twi_transmit(const uint8_t*, uint8_t);