DaqBook/3000USB User’s Manual
988391
Counter Input Modes 5-13
Timing 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 of measuring time between two subsequent events, i.e., the edge of one
channel with respect to the edge of another channel. The edge selection is done in each channel’s debounce
setup. Whenever the time measurement is latched from the counter, the counter is immediately cleared and
enabled for accepting the subsequent time period, which starts with the next edge on the main channel.
Timing 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 of Timing Mode
The following example represents one channel in timing mode. The time desired is between the
rising
edge
on the
input channel
and the
falling edge
on the
mapped channel
. Zeroes are returned, in the scan,
until one complete time measurement has been taken. At that point, the value (time in ticks) is latched and
logged by the DaqBook/3000USB until the next time measurement has been completed. Rising edges on
the input channel will clear the counter and falling edges on the mapped channel will latch the output of the
counter at that time. If the scan period is much slower than the rate of time-frames coming [available on the
two channels] then the data will miss some time-frames. The scan period can be decreased to capture more
time-frames.
The data returned is interpreted as time measured in ticks. This data represents the number of ticksize
intervals counted during the timing measurement. 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 DaqBook/3000USB uses a
48 MHz, 50 ppm oscillator as a timing source.
If the input signal has a poor slew rate the timing mode will provide variant results,
dependant upon the input switching threshold.
