In semi-period measurements, the counter measures a semi-period on its Gate input signal after the counter is armed. A semi-period is the time between any two consecutive edges on the Gate input.
You can route an internal or external periodic clock signal (with a known period) to the Source input of the counter. The counter counts the number of rising (or falling) edges occurring on the Source input between two edges of the Gate signal.
You can calculate the semi-period of the Gate input by multiplying the period of the Source signal by the number of edges returned by the counter.
Refer to the Pulse versus Semi-Period Measurements section for information about the differences between semi-period measurement and pulse measurement.