In this FAQ the ADV728x refers to the ADV7280, ADV7280-M, ADV7281, ADV7281-M, ADV7281-MA, ADV7282, ADV7282-M and ADV7283 models.
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 clock oscillator to the XTALN pin of the ADV728x. Leave the XTALP pin of the ADV728x floating.
The ADV728x normally drives a voltage out on the XTALN and XTALP pins. This voltage is used to power the external crystal. However if a clock 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 Map. This write sets the XTAL_TTL_SEL bit to 1.
When XTAL_TTL_SEL bit is set to 0 (default) the ADV728x will drive out 1.8 V on its XTALN and XTALP pins.
When XTAL_TTL_SEL bit is set to 1 the ADV728x will not drive out a voltage on its XTALN and XTALP pins.
The ADV728x documentation 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. The internal control register is write only and contains the XTAL_TTL_SEL bit .