I2C Slave Lib API

The I2C_Slave_lib is an interrupt driven I2C Slave. The state machine like code is designed to automatically receive an incoming message into its i2c_buffer_in and record the number of byte received.
The interrupt service then disables itself to prevent the buffer from being overwritten while the application program reads the buffer.
After reading the buffer, the application program re-enables the interrupt service.

Prep: In i2c_slave.asm, set SLAVE_ADRS to a valid 7 bit I2C address.

i2c_error				Cleared on Slave address match.  
	I2C_ERROR_NONE		No errors.  
	I2C_ERROR_OV		Overflow on input buffer. Last byte in buffer is  
                        overwritten.  

i2c_status  
	I2C_STATUS_IDLE        Awaiting to be addressed.  
	I2C_STATUS_RECEIVING   Incoming message being received.  
	I2C_STATUS_MSG_RCVD    Rec'd message in buffer. Blocks until read out.  
                           Call i2c_slave_init after reading buffer.  
	I2C_STATUS_SENDING     Outgoing message being sent.  

i2c_buffer_in  
	Receiving buffer.  
i2c_buffer_in_cnt  
	Number of bytes in the i2c_buffer_in buffer.  

Functions  
i2c_init_slave  
	Initialize the TWI hardware to support I2C Slave interface. This needs to  
    be called once.  
	input reg:	none  
	output reg:	none  

i2c_slave_init  
	Initialize the TWI for SLAVE I2C interface to receive data. This is  
    called to prime the Slave to receive data.  
	The interface will be disabled after receiving a message to prevent  
    overflow.  
	input reg:	none  
	output reg:	none  

i2c_iter  
	This is the I2C Slave interrupt service routine. It is enabled by the  
    i2c_slave_init() call and disabled when a message is received.  
    i2c_status indicates the state of the function.