Determine fundamental period from zero crossing signal
In general, you can not determine period of a signal from just the zero crossings.  If the signal has harmonic content, then a pitch detection/estimation algorithm, such as autocorrelation, might be one solution.  If the signal is more spectrally pure (e.g. close to sinusoidal fundamental, little overtone or harmonic energy), then a suitable DSP band-pass filter applied before locating the zero-crossings may help get rid of "noisy" samples.