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Period Mode
TIP: When using a counter for a trigger source, it is a good idea to use a pre-trigger with a value of at least 1.
The reason is that all counters start at zero with the initial scan; and there will be no valid reference in regard to
rising or falling edge. Setting a pre-trigger to 1 or more ensures that a valid reference value is present, and that
the first trigger will be legitimate.
This mode allows for period measurement of the channel input. The measurement period is the time from
edge-to-edge, either both rising or both falling. Period data is latched as it becomes available and the data
is logged by the /3000 Series board at the scan rate. Therefore, if the scan period is much faster than the
input waveform, there will be a great deal of repetition in the period values. This repetition is due to the
fact that updates take place only when another full period becomes available.
Period Mode
*There is one asynchronous read strobe for each of the four counter channels.
Note 1: Tick-sizes are: 20.83ns, 208.3ns, 2083ns, and 20833ns, derived from the 48 MHz system clock.
An example: One channel’s acquired data might be: 0,0,0,0,80,80,80,80,79,79,79,79,81,81,81,81,…..
This data represents the number of ticksize intervals counted during the period measurement. The first
value(s) returned will be zero since the counters are cleared at the beginning of the acquisition. The data
comes in sets of four since the scan period is about one-fourth as long as the input channel’s period. Every
time the period measurement is latched from the counter, the counter is immediately cleared and begins to
count the time for the subsequent period.
If the scan period is a lot slower than the input period, the acquired data will be missing some periods.
To obtain greater resolution, you can increase the scan period, or use an averaging option (see OPT[1:0]).
The data returned is interpreted as time measured in ticks. There are four timebase settings: 20.83 ns,
208.3 ns, 2083 ns, and 20833 ns. These are often referred to as tick-sizes. The /3000 Series board uses a
48 MHz, 30 ppm oscillator as a timing source. The tick sizes are derived from 1 period, 10 periods,
100 periods, or 1000 periods of the 48 MHz clock.
5-8 Counter Input Modes 918494 DaqBoard/3000 Series User’s Manual