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Sequences without light emission stop between patterns

Julien Tanteri
Prodigy 50 points

Hello,

First of all, sorry if this question already answered, but I didn't find the answer on TI forums or with google. 

I'm using the LCR4500 in pattern sequence mode (8bits patterns, internal trigger). For now, I use trigger out #1 connected to the still capture pin of my camera. The camera is in global shutter mode and expect trigger out #1 to be high during all the exposure time (not just a pulse to release the image grab).

Since my camera is in global shutter, I need a dead time (20 to 60ms) between patterns to read out the pixels value. So, black periods (no projection) between patterns are clearly visible and generate annoying flashes.

I'm trying to find a way to keep my LEDs ON between patterns display.

My first attempt was to continue to display last image (or even last bit-plane) between patterns (but trigger out #1 must fall after exposure). I didn't find a way to do so.
Did I miss something ?

My second attempt was fill black periods by displaying an additional pattern. Basically, if pattern n has an exposure of 20ms and a period of 60ms, I set pattern n for an exposure & period of 20ms, add a n+1 pattern with an exposure & period of 40ms.
I tried several configuration, like do not generate internal trigger and/or clear DMD, but I was unable to make trigger out #1 low on my additional/black filler pattern.
Did I miss something ?

Finally, I tried to use trigger out #2, since it is possible to generate the trig each N patterns. Unfortunately, I didn't find a way to keep trigger out #2 high during all the exposure time. It just pulses during 500ns.
Did I miss something ?

More generally, what would be the best solution to avoid those flashes ?

Thanks for your help.

  • 0
    TI__Genius 13235 points
    Hi Julien,

    Thanks for posting. The different options you tried all are working as expected from the LightCrafter point of view. Does your camera have any other triggering options?

    With the given constraints you mentioned in your post, some external logic would most likely be required. For example, you could create a circuit that pulls the trigger line low 20 - 60 ms before the exposure ends and overshoot the exposure time by that amount (like your 1st attempt).

    There would also be more options if you had an external input trigger to use in the system. If you provide some more information on your camera and use case requirements (number of patterns, application, etc), we could provide some more insight.

    Thanks,
    Clinton
  • 0 in reply to Clinton Rodgers
    Prodigy 50 points

    Hello Clinton,


    Thanks for your reply. I expected such an answer.

    For now, this is the only trigger option we have if we want the camera to be slave (trigger in). Since we have 2 cameras in our system, this is the simplest solution.

    For example, you could create a circuit that pulls the trigger line low 20 - 60 ms before the exposure ends and overshoot the exposure time by that amount (like your 1st attempt).

    But, if I do so, I'll miss some bitplanes no ?

    There would also be more options if you had an external input trigger to use in the system.

    Sure, but I'll try to work around camera's firmware first. A trigger mode accepting pulses for a fixed exposure should work using trigger #2.