Square waveform generator

Hi,

I want to make a continuous square waveform. there are some samples and files online to do so, but I'd like to be able to change the frequency of waveform continuously, I mean something like a function generator. I try to use PFI channel in NI 6221, but It just provides waveform with constant frequency. I am wondering if it is a good idea to use PFI channel? I want to provide 100 to kHz square waveform.

Thanks,