Measurement Computing WBK Options - Manual (Page 95)

WBK17, Counter/Encoder Module 987996 WBK17, pg. 17
Pulsewidth Mode
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.
Pulsewidth Mode
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 ns, 200 ns,
2 µs, and 20 µs. These are often referred to as tick-sizes. The WBK17 uses a 50 MHz, 10 ppm oscillator
as a timing source.
If the input signal has a poor slew rate the pulsewidth mode will provide variant results,
dependant upon the comparator threshold..
PULSEWIDTH: OPT2: Determines whether the pulsewidth is to be measured with a 16-bit (Counter
Low), or 32-bit (counter High) counter. Since pulsewidth measurements always have the “stop at the top”
option enabled, this option dictates whether the measurement has a range of 0 to 65535 ticks, or 0 to
4,294,967,295 ticks.
PULSEWIDTH: OPT4: Allows the mapped channel to gate the counter. When the mapped channel is
high, the counter is enabled to count. When the mapped channel is low, the counter is disabled, but holds
the count value. The mapped channel can be any other input channel, or one of the detection signals.