We have DVI into the system and we pass it through SDRAM and out the other end. If the user pushes a button and locks the display, the data that was passing through SDRAM is now used to refresh the display until the user “unlocks” and then the live data picks back up. Now, during this lock event, we need the display to keep refreshing if the user pulls the cable, so we need the clock to continue as if it was there even though it is no longer present. So, we need a PLL to learn the frequency, then if programmed to lock, we need it to continue the output even though the input is gone. When the input clock comes back, a seemless transition occurs back to the “real” clock. Let me know if you think the block diagram is correct. I searched and searched for a part and then found the LMH1983. It seems like it will do what I need, but would like confirmation.
The LMH1983 will do what you are looking for - it generates all of it's output clocks from a PLL based on a 27MHz VCXO. When the reference is lost, the control voltage to the VCXO is frozen, and the device moves into 'holdover' mode until the reference is reapplied.
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