I'm a student and I'm working on my first project using a DDS (I'm trying to control the board with a MBED microcontroler). As I have had no success making the communication, I tried to just turn the DDS on and observe the SCLK output. I'm using the board's crystal as the system clock reference, and have made all the changes described at the User Guide available on the Analog Devices' website (meaning I have set all the jumpers correctly, removed the two capacitors and placed zero ohms impedance at the two required terminals). What I expected to see at the SCLK output was a crystal's quarter frequency, but I have had no output (the datasheet says that the this output is active by default). Can I really have a "default response" from the DDS in the external control mode without sending any commands as I have thought? Do I have to make any others modifications?
PS.: I have tried to ground the EXT_PWR_DWN pin as well as to keep it opened, because I don't understand its function.
I tried to verify this by powering up the AD9910 evaluation boards and supplying a REF CLK to the part. Upon checking SYNC CLK, there is no output. When I connected the evaluation board to the evaluation software, output from the SYNC CLK can now be seen. Thus, communication with the part should be established first before a response on the SYNC CLK can be observed.
In fact I had no success in observing a "default SYNC_CLK output". But after sending a pulse to MASTER_RESET the output showed up.
Thanks for the help,