void DS3231::begin()
{
	if ((_sda_pin == SDA) and (_scl_pin == SCL))
	{
		_use_hw = true;
		// activate internal pullups for twi.
		digitalWrite(SDA, HIGH);
		digitalWrite(SCL, HIGH);
		//delay(1);  // Workaround for a linker bug

		// initialize twi prescaler and bit rate
		cbi(TWSR, TWPS0);
		cbi(TWSR, TWPS1);
		TWBR = ((F_CPU / TWI_FREQ) - 16) / 2;

		// enable twi module, acks, and twi interrupt
		TWCR = _BV(TWEN) | _BV(TWIE)/* | _BV(TWEA)*/;
	}
	else
	{
		_use_hw = false;
		pinMode(_scl_pin, OUTPUT);
	}
}

void DS3231::_burstRead()
{
	if (_use_hw)
	{
		// Send start address
		TWCR = _BV(TWEN) | _BV(TWEA) | _BV(TWINT) | _BV(TWSTA);						// Send START
		while ((TWCR & _BV(TWINT)) == 0) {};										// Wait for TWI to be ready
		TWDR = DS3231_ADDR_W;
		TWCR = _BV(TWEN) | _BV(TWINT) | _BV(TWEA);									// Clear TWINT to proceed
		while ((TWCR & _BV(TWINT)) == 0) {};										// Wait for TWI to be ready
		TWDR = 0;
		TWCR = _BV(TWEN) | _BV(TWINT) | _BV(TWEA);									// Clear TWINT to proceed
		while ((TWCR & _BV(TWINT)) == 0) {};										// Wait for TWI to be ready

		// Read data starting from start address
		TWCR = _BV(TWEN) | _BV(TWEA) | _BV(TWINT) | _BV(TWSTA);						// Send rep. START
		while ((TWCR & _BV(TWINT)) == 0) {};										// Wait for TWI to be ready
		TWDR = DS3231_ADDR_R;
		TWCR = _BV(TWEN) | _BV(TWINT) | _BV(TWEA);									// Clear TWINT to proceed
		while ((TWCR & _BV(TWINT)) == 0) {};										// Wait for TWI to be ready
		for (int i=0; i<7; i++)
		{
			TWCR = _BV(TWEN) | _BV(TWINT) | _BV(TWEA);								// Send ACK and clear TWINT to proceed
			while ((TWCR & _BV(TWINT)) == 0) {};									// Wait for TWI to be ready
			_burstArray[i] = TWDR;
		}
		TWCR = _BV(TWEN) | _BV(TWINT);												// Send NACK and clear TWINT to proceed
		while ((TWCR & _BV(TWINT)) == 0) {};										// Wait for TWI to be ready

		TWCR = _BV(TWEN)| _BV(TWINT) | _BV(TWSTO);									// Send STOP
	}
	else
	{
		_sendStart(DS3231_ADDR_W);
		_waitForAck();
		_writeByte(0);
		_waitForAck();
		_sendStart(DS3231_ADDR_R);
		_waitForAck();

		for (int i=0; i<7; i++)
		{
			_burstArray[i] = _readByte();
			if (i<6)
				_sendAck();
			else
				_sendNack();
		}
		_sendStop();
	}
}

uint8_t DS3231::_readRegister(uint8_t reg)
{
	uint8_t	readValue=0;

	if (_use_hw)
	{
		// Send start address
		TWCR = _BV(TWEN) | _BV(TWEA) | _BV(TWINT) | _BV(TWSTA);						// Send START
		while ((TWCR & _BV(TWINT)) == 0) {};										// Wait for TWI to be ready
		TWDR = DS3231_ADDR_W;
		TWCR = _BV(TWEN) | _BV(TWINT) | _BV(TWEA);									// Clear TWINT to proceed
		while ((TWCR & _BV(TWINT)) == 0) {};										// Wait for TWI to be ready
		TWDR = reg;
		TWCR = _BV(TWEN) | _BV(TWINT) | _BV(TWEA);									// Clear TWINT to proceed
		while ((TWCR & _BV(TWINT)) == 0) {};										// Wait for TWI to be ready

		// Read data starting from start address
		TWCR = _BV(TWEN) | _BV(TWEA) | _BV(TWINT) | _BV(TWSTA);						// Send rep. START
		while ((TWCR & _BV(TWINT)) == 0) {};										// Wait for TWI to be ready
		TWDR = DS3231_ADDR_R;
		TWCR = _BV(TWEN) | _BV(TWINT) | _BV(TWEA);									// Clear TWINT to proceed
		while ((TWCR & _BV(TWINT)) == 0) {};										// Wait for TWI to be ready
		TWCR = _BV(TWEN) | _BV(TWINT) | _BV(TWEA);									// Send ACK and clear TWINT to proceed
		while ((TWCR & _BV(TWINT)) == 0) {};										// Wait for TWI to be ready
		readValue = TWDR;
		TWCR = _BV(TWEN) | _BV(TWINT);												// Send NACK and clear TWINT to proceed
		while ((TWCR & _BV(TWINT)) == 0) {};										// Wait for TWI to be ready

		TWCR = _BV(TWEN)| _BV(TWINT) | _BV(TWSTO);									// Send STOP
	}
	else
	{
		_sendStart(DS3231_ADDR_W);
		_waitForAck();
		_writeByte(reg);
		_waitForAck();
		_sendStart(DS3231_ADDR_R);
		_waitForAck();
		readValue = _readByte();
		_sendNack();
		_sendStop();
	}
	return readValue;
}

void DS3231::_writeRegister(uint8_t reg, uint8_t value)
{
	if (_use_hw)
	{
		// Send start address
		TWCR = _BV(TWEN) | _BV(TWEA) | _BV(TWINT) | _BV(TWSTA);						// Send START
		while ((TWCR & _BV(TWINT)) == 0) {};										// Wait for TWI to be ready
		TWDR = DS3231_ADDR_W;
		TWCR = _BV(TWEN) | _BV(TWINT) | _BV(TWEA);									// Clear TWINT to proceed
		while ((TWCR & _BV(TWINT)) == 0) {};										// Wait for TWI to be ready
		TWDR = reg;
		TWCR = _BV(TWEN) | _BV(TWINT) | _BV(TWEA);									// Clear TWINT to proceed
		while ((TWCR & _BV(TWINT)) == 0) {};										// Wait for TWI to be ready
		TWDR = value;
		TWCR = _BV(TWEN) | _BV(TWINT) | _BV(TWEA);									// Clear TWINT to proceed
		while ((TWCR & _BV(TWINT)) == 0) {};										// Wait for TWI to be ready

		TWCR = _BV(TWEN)| _BV(TWINT) | _BV(TWSTO);									// Send STOP
	}
	else
	{
		_sendStart(DS3231_ADDR_W);
		_waitForAck();
		_writeByte(reg);
		_waitForAck();
		_writeByte(value);
		_waitForAck();
		_sendStop();
	}
}