Most of the documentation for the ADV7180 assumes that you are using a external 28.63636 MHz crystal to clock the ADV7180. How do you use an external oscillator instead?
Firstly you must use an oscillator with an output less than 1.8Vpp. The oscillator must output a 28.63636 MHz clock with a tolerance of +/- 50ppm.
Connect the output of the oscillator to the XTAL pin of the ADV7180. Leave the XTAL1 pin of the ADV7180 floating.
The ADV7180 normally drives a voltage out on the XTAL and XTAL1 pins. This voltage is used to power the external crystal. However if an oscillator is used instead of a crystal, then this voltage on the XTAL pins can affect the oscillator performance.
To prevent this you should perform the following I2C write. Write 0x04 to register 0x13 in the User Main Map. This will stop the ADV7180 driving out a voltage on its XTAL pins.
The ADV7180 states that register 0x13 is a read only register (status register 3). Actually two registers share the register address 0x13.
When you read from register 0x13, you read back the Status Register 3 data ( this is read only) . When you write to register 0x13 you write to an internal control register (which is write only).