I don't know about your part but the ones I have used work like this.
Inside there is a "adder" that adds two numbers every 125mhz.
W1,W2,W3, and W4 is the inputs to the adder.
Every time the adder caries out the output completes one cycle.
so:
If the internal adder has a 0 and the W1-W4 has a 80,0,0,0. Every clock we add 80,0,0,0 hex.
0,0,0,0 +80,0,0,0=80,0,0,0 MSB=0
80,0,0,0,+80,0,0,0=0,0,0,0 MSB=1
0,0,0,0 +80,0,0,0=80,0,0,0 MSB=0
80,0,0,0,+80,0,0,0=0,0,0,0 MSB=1
0,0,0,0 +80,0,0,0=80,0,0,0 MSB=0
80,0,0,0,+80,0,0,0=0,0,0,0 MSB=1
The output frequency is 62.5mhz.
This can be tested by going to
http://designtools.analog.com/dt/dds/ad9850.html and put in 80 in w1 and 0 in w2-w4.
If you enter a number 1/2 as big then:
0,0,0,0,+40,0,0,0,=40,0,0,0 MSB=0
40,0,0,0+40,0,0,0=80,0,0,0 MSB=0
80,0,0,0+40,0,0,0=c0,0,0,0 MSB=1
c0,0,0,0+40,0,0,0=0,0,0,0 MSB=1
0,0,0,0,+40,0,0,0,=40,0,0,0 MSB=0
40,0,0,0+40,0,0,0=80,0,0,0 MSB=0
80,0,0,0+40,0,0,0=c0,0,0,0 MSB=1
c0,0,0,0+40,0,0,0=0,0,0,0 MSB=1
The output is now 31.25mhz
If W1-4=0 then 0+0=0 and the frequency =0
To further test this enter 0,0,0,1 which is the smallest number.
The F=.029hz T=34.5 seconds
This can be worked out in frequency or time. I usually use time.
Tout=34.5seconds/N Where N is the number you enter.
Hope this helps. Ron