You'll need to use the "Implicit" version of DAQmx Timing rather than the "Sample Clock" version.
The idea is that the timing information is "implicit" in the signal being measured. The timing intervals that define a period are defined by the signal itself, that's why counter period measurement can't use a "regular" sample clock.
-Kevin P
P.S. Newer boards such as the X-series MIO *do* offer frequency measurement modes that involve constant-rate sampling. I don't they offer similar constant-rate sampling for period measurement, maybe because freq measurement is much more common and the conversion to periods is trivial.
CAUTION! New LabVIEW adopters -- it's too late for me, but you *can* save yourself. The new subscription policy for LabVIEW puts NI's hand in your wallet for the rest of your working life. Are you sure you're *that* dedicated to LabVIEW? (Summary of my reasons in this post, part of a voluminous thread of mostly complaints starting here).
