The following applies to the ADV7280-M, ADV7281-M, ADV7281-MA and ADV7282-M.
If the below sequence of writes are performed then the crystal clock input to the XTALN pin of the ADV728x is fed out on the GPO1 pin. GPO1 pin is pin 7 on the ADV7280-M, ADV7281-M, ADV7281-MA and ADV7282-M.
This can be a useful method of checking if the crystal clock input to the ADV728x is correct as placing probes (even high quality probes) directly to the crystal can affect its performance.
This can also be a way to reduce the cost of a board. For example if a board is designed with two ADV728x devices then only a single crystal is needed. One crystal to clock one of the ADV728x devices and the GPO1 output from that device clocking the other ADV728x device.
See this post on how an additional write is needed if an external oscillator (e.g. the GP01 pin acts as an external oscillator) instead of a crystal is used to clock an ADV728x.
How Do I Use an External Oscillator to Clock the ADV728x ?
Step 1: write an ADI recommended script
Step 2: perform the following writes
0x42 0x0E 0x00 ; enter user map
0x42 0x59 0x10 ; enable GPIOs
0x42 0x0E 0x80 ; enter engineering map
0x42 0x57 0x19 ; Test signal muxing
0x42 0x58 0x04 ; Feed Xtal signal out to GPO1
A 28.63636 MHz clock signal will now be output from the GP01 pin of the ADV728x.
Note if this signal is being used to clock another device ensure that the PCB traces are as short as possible and are not going through vias etc.
Analog Devices Inc.