How to filter starttrigger on counter output precisely

Question

HW PCI6602
Measurement studio 2008 (C#)NI-DAQmx 9.02I’m trying to trig a camera using the output from a counter.
The camera should be trigged when the counter input pulse width is larger than approx (filterPulseWidth 10us).To do this a have set up the following tasktask.COChannels.CreatePulseChannelTime(counter,                "TriggerTaskChannel", COPulseTimeUnits.Seconds, COPulseIdleState.Low, 0, 80E-6, 0.007);             task.Triggers.StartTrigger.Type = StartTriggerType.DigitalEdge;            task.Triggers.StartTrigger.DigitalEdge.Edge = DigitalEdgeStartTriggerEdge.Rising;             task.Triggers.StartTrigger.Retriggerable = true;task.Triggers.StartTrigger.DigitalEdge.DigitalFilterMinimumPulseWidth = filterPulseWidth;            task.Triggers.StartTrigger.DigitalEdge.DigitalFilterEnable = true;Unfortunatly this filter has the following behaviour”If the period of the filter clock timebase is tfltrclk, this filter guarantees topass pulse widths that are 2*tfltrclk or longer and to block pulse widths thatare tfltrclk or shorter. A pulse with a width between these two ranges may ormay not pass, depending on the phase of the pulse with respect to the filterclock timebase”.It means that I have no sharp distinction on my filter as one would when applying a filter to an ordinary pulse width measuring task. Implementing this with via the software in a callback is to slow.The bottom line is that I would like to generate a pulse on my counter output when the trigger/counter input is greater than say 10us. The output pulse could be predetermind as in the sample code above or as long as the filtered input (I.e counter just pass filtered input to my output).How can this be done? BR
Jongas
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Answer 1

Hi Jongas,
Is it OK if the trigger is sent once the pulse hits 10 us, rather than
on the exact falling edge?  I'll assume the exact timing isn't as important, but you would like the trigger to occur very close to the falling edge (within a couple of us).  The important thing is that we trigger as close as possible to when the PWM has hit 50% duty cycle.
Some brainstorming:
Digital Filtering on the 660x Isn't the Best for This:
Digital filtering might not be as practical here on your 6602 due to the region of uncertainty between 5 and 10 (or 10 and 20) us pulses.  The TIO boards count two consecutive edges of a filter clock to determine when to pass a signal through so the guaranteed rejected pulse width is always half of your guaranteed passed pulse width (providing an external filter clock timebase that is synchronized with your external signal could potentially reduce this uncertainty but I honestly haven't tried this before and I would imagine it is not going to be very straightforward).
X Series Alternative:
Our X Series devices use a different method of digital filtering that would work better for you.  If a hardware change is an option (and you can use PCIe) then you might consider this.  You could use the 20 MHz timebase as your filter clock timebase and could guarantee to pass 10 us (200/20M) and reject 9.95 us (201/20M).  The 6320 is currently our lowest cost X Series board.  A couple of points about this solution:
1.  To configure the PFI filter, you need to use some sort of dummy task to
access the property nodes.  Here is an
example of this (although it is written in LabVIEW).
2.  You can route the filtered PFI signal to be exported on another PFI line, but this will reserve Counter 3.  This is documented in the Device Routes tab of Measurement and Automation Explorer.
3.  The filtered output will be 9.95-10 us delayed from the input signal, so you could trigger the camera off of the rising edge of the filtered output directly and be fairly close to the actual falling edge.
External AND Gate Alternative:
You could configure a Counter Output to generate a re-triggerable pulse with a 10 us initial delay (to be triggered off of the rising edge of your PWM signal).  Assuming the pulse is short enough to complete before the next period of your PWM signal, the counter output would only be high at the same time as the PWM signal if the signal was longer than 10 us.  Use the external AND gate to combine these two signals and the result would be the trigger for your camera (the rising edge would correspond to 10 us after the PWM signal first goes high).
If you wanted to you could make the counter output pulse a little longer (say 8 us) and trigger the camera off of the falling edge out of the AND gate (a.k.a. rising edge out of a NAND gate) which would line up with the exact falling edge of the PWM signal.  Don't make the CO pulse too long or it will overlap with the next period of the PWM.
Another Idea:
One idea that I keep coming back to is to use the internal rollover event of a counter input task (a pulse is generated whenever a counter rolls over on its Internal Output which can be routed to a PFI line).  I don't think this will work, but the idea in theory would be to:
1.  Set Default state to known value (e.g. 2^32-200).
2.  Gate the Counter so it only counts during the time PWM is high.  Have it count the 20 MHz timebase.
3.  Reset the Counter to default state on the falling edge of the PWM signal.
4.  The counter would rollover if 200 pulses occurred of the 20 MHz timebase (10 us), and the Counter Output could be routed to a PFI line to trigger the camera.
The problem is that there is no good way to reset the counter except for an encoder measurement (Kevin has already made a nice suggestion about this).  A Pulse Width Measurement would technically reset the counter, but you cannot currently set the default value of a Pulse Width Measurement task so there is no way to make the rollover happen prematurely).
Configuring an encoder measurement and working with the multiple counters on the 6602 to produce appropriate A,B, and Z signals might be a method to look into further, but at this point I think you'd be better off with an external AND gate.
I don't want to say it's impossible with just the 6602, but I can't think of a straightforward way to go about it without external hardware (although maybe I can sleep on it and think of something later... how many counters do you have to work with?). 
With your current NI Hardware, I think your best bet is to go with an external AND gate.  If you're planning on purchasing an X Series card the digital filtering idea is actually not a bad way to go.
I hope this is helpful!
Best Regards,
John Passiak