Frequency Measurement - Advantech PCI-1716 User Manual

The advantage of instant event counting is low latency. It is typically used for reading
a single sample of counter value.
3.5.2

Frequency Measurement

In frequency measurement mode, the frequency of the counter clock signal is mea-
sured by one of the two measuring methods: Period inversion or counting number of
pulses in fixed duration.
3.5.2.1 Period Inversion
In this method, the period of the counter clock signal is first measured by an internal
high frequency clock. The frequency of the signal is then calculated by inverting the
period value. This is shown in Figure 3.36 and by the following equation.
Figure 3.36 Frequency measurement by period inversion.
This method is suitable if the counter clock signal frequency is much smaller (< 0.1%)
than the internal clock frequency. Measuring accuracy degrades as the counter clock
signal frequency increases.
3.5.2.2 Counting Number of Pulses in Fixed Duration
In this method, the pulse number of the counter clock signal is measured in a fixed
time duration. The frequency of the signal is then calculated by dividing this number
by the time duration. This is shown in Figure 3.37 and by the following equation.
Figure 3.35 Instant (software-timed) event counting.
39 PCI-1716_Series User Manual