The behavior of the frame_locked output signal for the Serial Digital Interface (SDI) MegaCore® to assert and de-assert the frame_locked signal, also known as rx_status [4], has the role to detect TRS (EAV), inspect transition of F & V sync and count line number. The frame timing is worked out from inspecting transitions on the F (field) and V (Vertical) syncs. And the line count values is stored if there is a rising or falling edge on the F or V syncs through the frame. To assert the frame_locked signal, these count values should be stable when they are compared over multiple frames. This frame locked output signal gets de-asserted if there is bad F or V fall or rise position from frame to frame. The de-assertion of trs_locked signal will also cause the frame_locked signal to be low.
Also, if frame_locked signal is de-asserted, the receiver is considered not receiving reliable video data.