Why does the Serial Digital Interface (SDI) II MegaCore’s rx_pll_locked signal occasionally toggle?

Description

For HD-SDI and 3G-SDI video standards, the CDR mode of the transceiver is lock to data (LTD) mode.

Hence the rx_pll_locked signal of Serial Digital Interface (SDI) II MegaCore® may oscillate when the transceiver is correctly locked to the incoming data in HD-SDI or 3G-SDI standard.

However, for SD-SDI video standard, the CDR mode is lock to reference clock (LTR) mode, the rx_pll_locked signal of SDI II MegaCore remain locked at all times.