Key Digital ES381 - Manual (Page 20)

2.5.3 Period Mode
Period mode uses an internally generated reference clock to time the
period between two edges on the input waveform. It is selected as
shown.
The reference clock can be selected from 100MHz, 10MHz or
1MHz. The pair of edges that is timing is measured can be selected
from:
FALLING -> FALLING
RISING -> FALLING
FALLING -> RISING
RISING -> RISING
Note that for all practical purposes the first and last listed settings
will yield the same answer, but there may be cases where being able
to switch between these two settings is useful, in certain PWM
waveforms for example.
For most input periods, the 100MHz setting will suffice. It offers a
range of 0ns to 2.684s in steps of 10ns ! If you have a very slow
input period, then selecting the 10MHz or 1MHz can increase this
maximum period to 26.84s and 268.4s respectively, at the expense of
timing resolution (down to 100ns and 1us respectively).
Elan Digital Systems Ltd. 20 USBcount50 USER’S GUIDE Iss1