===== I2C Bus =====
The I2C clock line (SCL) going to the DS1307 chip has a 2k7 resistor in series with it. Additionally port D0 is connected via solder jumper SJ1 to the SCL pin of the DS1307 chip. This is a circuit for enabling or disabling the DS1307 chip. Because the DS1307 chip only works up to 100kbit/sec I2C speed, it means no other I2C devices can be accessed at full speed (when the DS1307 is present), like the 24LC512 for example. That is why the DS1307 has this selection circuit. The DS1307 chip does not have any other means of disabling it, like a CS pin for example.
To enable the DS1307 chip, port D0 should be configured as an input pin. When D0 is configured as an input, the DS1307 will be connected to the bus, and work as normal.
To disable the DS1307, D0 should be set to +5V (port pin set). This will disable the I2C clock line going into the DS1307, allowing the I2C bus to be used at higher speeds. This will however add an additional 2k7 pull-up resistor to the I2C line. This should be no problem for most I2C devices, but might have an affect if there are many I2C devices on the network.
