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Loading the FTW on the AD9957

Question asked by creed on Oct 14, 2011

I am able to initialize the device in single-tone mode and load a FTW and see the desired signal

at J4 on the evaluation board.


My question is related to how I'm writing the FTW to the profile register (x0e).

With Fs = 500MHz, I need to write x33333333 for the FTW to see a tone at

100MHz.  This happens to be convenient since I write the same hex value to

each register.   When I tried to generate a 60MHz tone, I calculated the FTW

to be x1eb851ec, but I did not see a signal at the output when I loaded


[31:24] = 1e

[23:16] = b8

[15:8]  = 51

[7:0]    = ec


for the FTW.  After playing around with the different combinations, I found that writing


[31:24] = e1

[23:16] = 8b

[15:7]   = 15

[7:0]     = ce


generated the desired tone.  This reversed on a nibble boundary, but from the description

on page 51 of the datasheet (rev B.)  there's no indication that this type of swap is needed.


Is there something I'm missing?   Are there other registers that need to written in this manner?

I'm using the on-board 25MHz xtal, so given that I need to enable the pll and provide a multiplier

and FTW - x33333333 generates the right tone, I don't think that there is an issue with order

in which I write the bits to the register, but I'd like to confirm that the behavior I'm seeing is

to be expected.