In pulse measurements, the counter measures the high and low time of a pulse on its Gate input signal after the counter is armed. A pulse is defined in terms of its high and low time, high and low ticks or frequency and duty cycle. This is similar to the pulse-width measurement, except that the inactive pulse is measured as well.
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 high and low time of the Gate input by multiplying the period of the Source signal by the number of edges returned by the counter.