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AD9523-1 reference selection

Question asked by EdCooper on Sep 6, 2017
Latest reply on Dec 29, 2017 by hakancylk

I am working on a design with the AD9523-1 clock generator. In my design, there is:

an off board connector wired to the REFA input, which allows an external 10MHz reference signal to be fed into the chip;

an on board 10MHz TCXO, wired to the REFB input.


I want the AD9523-1 to use the external reference (e.g. REFA) whenever it is available. The TCXO (e.g. REFB) should only be used when the external reference is not connected. I configured the 'reference selection mode' bits in register 0x01C to 'Revert to REFA', thinking that this would give the desired behaviour.


My problem is start up. If REFA is not present when I configure the AD9523-1, PLL1 does not automatically switch to REFB, even though read back register 0x22C bit 3 indicates that REFB is ok. Read back 0x22D bit 2 reports that the device is trying to use REFA, even though register 0x22C bit 2 indicates that REFA is missing. As a result, PLL1 does not lock. If I force the device to switch to REFB, by reprogramming 0x01C to the 'Select REFB' state, then PLL1 locks. Once that has happened, I can set 0x01C back to 'Revert to REFA' and the AD9523-1 automatically switches between sources as expected.


My question: does REFA have to present initially for the automatic PLL1 reference switchover to function? If that is the case, is there any other way I can achieve the behaviour I want? I'm hoping to load register values into the internal EEPROM once on the production line and won't normally have SPI communication with the device.