I have two AD9915 chips with a common 32-bit data bus but each with their own addressable function pins F[3:0].
I am using them in parallel mode with streaming disabled (0x00[17] = 0).
I want to get different frequency outputs on each DDS that update together. In order to get a synchronised update to both chips, I have setup multichip synchronisation and have a common IO-update signal.
My plan is to set the FTW for each DDS by individually setting the function pins while setting the appropriate FTW value on the common bus.
My question is, how can I tell one chip to ignore a value on the common bus while I program the other?
The datasheet reports that function pin values of 1110 and 1111 are "not used". Does this mean the value on the data bus is essentially ignored?
If this is the case, my approach would be:
Step 1:
DDS[0] F[3:0] = 0010
DDS[1] F[3:0] = 111x
Data bus [31:0] = FTW for DDS[0]
Step 2:
DDS[0] F[3:0] = 111x
DDS[1] F[3:0] = 0010
Data bus [31:0] = FTW for DDS[1]
Step 3:
DDS[0] F[3:0] = 111x
DDS[1] F[3:0] = 111x
Assert IO_UPDATE
Will this approach work as I intend? Thanks for any help!