You have a couple options. One is a resistor network, which should work pretty well at relatively high frequencies. The other is PWM waveform synthesis, which (after some configuration) will allow you to produce low-frequency waveforms with a constant 62500hz background frequency (this is not so good). PWM wave synthesis will also mess with your timers and stuff because you have to speed up the timer that millis() and delay() use (if you want 62500hz). Personally, I would recommend the resistor network. Resistor ladder - Wikipedia