Measurement Computing Personal Daq rev.6.0 - Manual (Page 55)

Debounce
⎯ Debouncing is a process of ignoring signals which are considered as too short in duration to
be real events. Personal Daq's debounce circuit ignores two types of edge signal events: 1) rising edges
that are not preceded by a sustained low signal, and 2) falling edges that are not preceded by a sustained
high signal. The interval can be independently set for each channel to a value of 0, 0.8, 3.2, or 13 ms. You
can select 0 ms to disable debouncing for clean high-frequency signals. Note that long debounce times will
limit high-frequency response. For example, a 13 ms debounce will limit frequency to about 50 Hz.
The following figure shows the effect of 13 ms debouncing on a noisy signal. To be counted, a rising edge
must be preceded by a sustained low signal for at least 13 ms without any other edges [a falling edge must
be preceded by a sustained high signal for at least 13 ms without any other edges]. (See following figure).
Sustained Lows and Rising Edges… "Bouncy" Input compared with Debounced Output
In the this example of “bouncy” input, edge was selected for rising and debounce was selected for 13 ms.
Rising edges a and f are counted because they are preceded by low signal levels sustained for at least 13
ms (the debounce time). All other rising edges (b, c, d, and e) are ignored.
Min Value
⎯ For Frequency, Pulses/scan, and Totalize, defines the minimum range value (lower range
limit) for the charted signal. The value is applicable to charts in Personal DaqView and PostView.
Max Value
⎯ For Frequency, Pulses/scan, and Totalize, defines the maximum range value (upper range
limit) for the charted signal. The value is applicable to charts in Personal DaqView and PostView.
Resolution and Settling Time
Frequency measurements on the Personal Daq are achieved by querying the unit for a current time and
pulse count on a repetitive basis. Aside from edge and debounce settings, two values are used to set up
frequency input; Maximum Frequency (
Fmax ) and Measurement Resolution ( Mreso ). Both of these
values have units of Hertz.
The Acquisition Period (AcqPeriod) is calculated from these values. AcqPeriod is the minimum length of
time needed to get a reading at the desired resolution.
AcqPeriod (in seconds) = Fmax * (0.0000004 / Mreso)
However, if the requested scan period for the entire scan (of which the frequency channels are a part) is
longer than the calculated AcqPeriod, then the scan period becomes the acquisition period.
To put bounding limits on values so measurements can be done in a timely fashion, the
AcqPeriod must be
<10 seconds. This means: Fmax / Mreso <25,000,000 .
Frequency measurement on the Personal Daq is only done when collecting scans. Values are updated on
every scan even if the
AcqPeriod has not been reached. During each scan a current pulse count and timer
count is read from the Personal Daq.
Personal Daq User’s Manual 989394 Personal DaqView 4-13