When I enable the Interlaced Video Support and Field ID Generation options in the GUI, the optional field id signal (field_id_out) is available.
The field_id_out signal should toggle each field, but it stays high (active polarity field ID is set to high) in constant mode (AXI-Lite interface is disabled).
What is the cause of this behavior?
This is a known issue discovered in version 6.1 of core which is fixed in the 2015.3 release (v6.1 (Rev. 6)).
Users can work around this issue by including the AXI-lite interface.