Arduino and R-2R DAC working as a signal generator. Check this link for detailed info. I went a different way with the coding inorder to add a delay function between individual pulses and not cycles so i could influence the frequency. The code is big and it took sometime to write but it worth it not only for the delay but for the resolution too. Here you can download my Arduino sketch (R_2R_DAC). The DIY Arduino GLCD Oscilloscope by Filear.com.
Important note: I use the digital pins (3-10) and the sketch works only for these pins because i initialized them using DDRD and DDRB