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ADF4350/4351

I am using these parts in a new design.

I know the SPI is working, the parts tune to the right frequency.

However, for production relaibility purposes, I want to use muxout to let the how know it is indeed communications qith these chips.

So I set reg 2 appropriately and well, muxout is always high, even if i select muxout = low or muxout  = high

I checked register contents with the scope, I get what I expect to select the proper muxot, but to no avail.

I have 3 AFD4351 chips on my board, all 3 do the same thing.

What could I be doing wrong?