Pulsewidth Mode - Omega OMB-DAQBOARD-3000 Series User Manual

Pulsewidth 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 provides a means to measure a channel's pulsewidth. The measurement is the time from the
rising edge to the falling edge, or visa versa. The measurement will be either pulsewidth low, or
pulsewidth high, depending upon the edge polarity set in the debounce module.
Every time the pulsewidth measurement is latched from the counter, the counter is immediately cleared and
enabled to count the time for the next pulsewidth. The pulsewidth measurements are latched as they
become available.
*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 pulsewidth measurement. The first
value(s) returned will be zero since the counters are cleared at the beginning of the acquisition. In this
example the data comes in sets of four because the scan period is about one-fourth as long as the input
channel's period. Every time the pulsewidth measurement is latched from the counter, the counter is
immediately cleared and enabled to count time for the next pulsewidth.
If the scan period is much slower than the input period, then the acquisitions will miss some pulsewidths.
Decreasing the scan period will increase the number of different pulsewidths received.
The data returned is interpreted as time measured in ticks. There are four timebase settings: 20.833 ns,
208.33 ns, 2.083 µs, and 20.83 µs. These are often referred to as tick-sizes. The 3000 Series board uses a
48 MHz, 30 ppm oscillator as a timing source.
DaqBoard/3000 Series User's Manual
If the input signal has a poor slew rate the pulsewidth mode will provide variant results.
918494

Pulsewidth Mode

Counter Input Modes 5-11