The UHD-SDI core can automatically determine the SDI mode (SD, HD, 3G, 6G, or 12G-SDI) of the SDI signal coming into the GTX RX. When it is not locked to the current SDI input signal, the UHD-SDI core sequences the GTX RX through the five different SDI modes until it detects recognizably good SDI data on the rxdata output port of the GTX. At that point, the UHD-SDI core indicates that the GTX CDR is locked to the SDI signal by asserting its rx_mode_locked_out port. And, it indicates which SDI mode the RX is locked to on its sdi_mode_out port.
It is important to understand that the rx_mode_locked signal is an indication of whether or not the UHD-SDI core thinks that the GTX RX is locked to the SDI signal, and nothing more. It is really just an indication of whether the UHD-SDI core's mode search state machine is still searching for the correct SDI mode or not. Because of this, rx_mode_locked should not be considered as an absolute indicator as to the locked status of the UHD-SDI RX.
When the GTX RX is not locked to the input SDI signal and the UHD-SDI core is actively controlling GTX RX in an effort to determine the correct SDI mode, the rx_mode_locked signal might briefly become asserted. This happens if the incoming data randomly appears to be a valid SAV sequence. If an SAV sequence is detected, the UHD-SDI core asserts rx_mode_locked and pause the mode search expecting more good data to be received. If, however, good data is not received within a specific timeout period, the rx_mode_locked signal is negated and the SDI mode search resumes.
The SDI mode search algorithm attempts only to lock to SDI modes that are enabled by the rx_mode_en_in port of the UHD-SDI wrapper. This 6-bit port has unary bits that enable HD-SDI (bit 0), SD-SDI (bit 1), 3G-SDI (bit 2), 6G-SDI (bit 3), 12G-SDI at 11.88 Gb/s (bit 4), and 12G-SDI at 11.88/1.001 Gb/s (bit 5). Because the GTX RX must be configured with different reference clock frequencies for the two 12G-SDI line rates, the two 12G-SDI line rates are treated as different SDI modes by the mode search algorithm. And, because there are separate enable bits on the rx_mode_en_in port, it is possible to specify that only one of the two 12G-SDI line rates should be included in the mode search. This is useful for those applications where it is undesirable to have the reference clock frequency of the QPLL frequently changed as the GTX RX scans through the two 12G-SDI line rates. For example, the use case shown in Figure 1 only supports the 11.88 Gb/s 12G-SDI line rate because only the 148.5 MHz reference clock is supplied to the QPLL. In this case, the application should set bit 5 of rx_mode_en_in Low to prevent the mode search algorithm from trying to lock in the 11.88/1.001 Gb/s 12G-SDI mode. If rx_mode_en_in is driven with a value of 6'b011111 in the use case shown in Figure 1, the SDI mode search algorithm searches for a lock in all SDI modes except the 11.88/1.001 Gb/s 12G-SDI mode.
The rx_mode_en_in port can be changed dynamically. However, if the UHD-SDI RX is already locked to a mode that becomes disabled by dynamically clearing its bit on the rx_mode_en_in port, this does not automatically kick the UHD-SDI RX out of that mode. The UHD-SDI RX remains locked in the SDI mode until the input SDI signal changes or the UHD-SDI RX is reset, forcing the SDI mode search algorithm to try and identify the SDI mode using the new settings of the rx_mode_en_in port.
It is possible to disable the automatic SDI mode search algorithm of the UHD-SDI core. The mode search algorithm is enabled only when the rx_mode_detect_en_in port is High. If this port is Low, then the UHD-SDI RX must be told what SDI mode to operate in through the rx_forced_mode_in port. When rx_mode_detect_en_in is Low and the SDI mode search algorithm is disabled, the SDI RX is in the mode specified by the rx_forced_mode_in port and the rx_mode_locked output is always High. Thus, rx_mode_locked cannot be used as a locked indicator or a data valid indicator in this mode. When the mode search algorithm is disabled, dynamic changes on rx_forced_mode_in causes the SDI control logic to dynamically change the settings of the GTX RX as necessary for the new SDI mode.