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measure the falling time of a TTL signal



I would like to measure the pulse width of a TTL signal. Ive tried using a counter to do so but keep getting errorneous valued even when I've tried using NI example "Meas Pulse Width" (see pict pulse width -> for a pulse duration of more than a sec, im getting getting really tiny values 0.00000022500 sec).


I've wired my TTL signal to an AI and tried to measure the pulse width using the pulse measurement vi, but keep getting error message :"the waveform did not cross the mid-ref level, check ref levels and units..."


Would there be a way to trigger the pulse duration measurement as soon as the TTL starts going low or just before





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Hello Dan,


I'm not exactly sure what measurement you're trying to make; are you looking to measure falling time (in your post's title) or pulse width?


From the looks of your screenshots you may be trying to make both of these measurements.  Without more details on the measurement and issue you're having it will be difficult to provide specific advice, but I'll do my best.


If you want to measure falling time:


You will need to use an analog input to sample the signal and then compute the fall time from that measurement.  I would recommend using a reference trigger to only capture the falling edge of your signal.  If you open the LabVIEW Example finder (Open LabVIEW and go to Help» Find Examples) then navigate to Hardware Input and Output» DAQmx» Analog Measurements» Voltage» Acq&Graph Voltage-Int Clk-Dig


If you want to measure pulse width:


1.  Again use the analog reference trigger to sample the pulse and measure the pulse width.

2.  (Preferred)  Use a counter to make a pulse width measurement Hardware Input and Output» DAQmx» Counter Measurements» Period and Pulse Width» Meas Pulse  It looks like you're already using this VI, but make sure you're connecting your signal to the gate of counter zero and also make sure to connect a ground reference.


Possible causes for the small pulse width measurement:


1.  It is fall time and not pulse width.

2.  Your signal is not ground referenced or rings significantly and the input detects a falling edge right after the rising edge.


I hope this helps and if you're still having trouble please clarify what measurement you're trying to make, what VI you're using, and what the specific issue is.



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