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DLPC3478: Not guaranteed behaviour and how to calculate minimum pre- and post-exposure time for longer exposure times

Part Number: DLPC3478

I'm reading the FAQ posted here: https://e2e.ti.com/support/dlp/f/94/t/729968  regarding calculation of the minimum pre and post exposure times and have a few questions:

  1. In our system we need longer exposure times than what is given in the tables, but the FAQ states that for longer exposure behavior is not guaranteed, what is meant by this?
  2. How do we calculate minimum pre/post exposure times for longer exposures than given in the table? Do we need to scale up the pre/post darktimes? (leads to very long darktimes for longer exposures)

  • Anders,

    Can you share specifically which section of the FAQ you are referring to when you state "behavior is not guaranteed"?

    Regards,
    Philippe Dollo

  • I'm sorry, the FAQ link I posted didn't contain this quote, it is from the release-notes for the firmware. Release notes for FW 8.0.0. It has this line: "Longer exposure times will operate with decreased timing accuracy and behavior is not guaranteed."

  • Anders,

    The answers to your queries are as follows -

    1. In our system, we need longer exposure times than what is given in the tables, but the FAQ states that for longer exposure behavior is not guaranteed, what is meant by this? 

    >> With longer exposure times that are out of the supported range, there can be deviation in the timing behavior of trigger out 1 and trigger out 2 signals. You may not get the exact expected output for these signals. And there can be deviation in the pattern exposure time as well.

    2. How do we calculate minimum pre/post exposure times for longer exposures than given in the table? Do we need to scale up the pre/post darktimes? (leads to very long darktimes for longer exposures)

    >> You can use the "Read Validate Exposure Time" command to get the required pre and post-exposure time for a given input exposure time. 

    This command is available in the Advance mode of the GUI, under Light Control section.

    Regards,

    Mayank

  • Hi again Mayank!

    Thank you for your answer, using the "Read Validate Exposure Time" function outside the supported range gives us some numbers that work, but the timing of this function is quite unpredictable and often takes  a long time (i2c clock stretching for 10+ ms before it responds) which is difficult to work with in our application. It looks like we'll have to make some equations ourselves that matches what the firmware says. 

    Can you go into a bit more detail about the comment: "You may not get the exact expected output for these signals. And there can be deviation in the pattern exposure time as well" 

    • How big are the deviations?  some microseconds off, or a percentage of the exposure time?
    • Will the trigger signals be correct for the projected light, even if the actual exposure time is not inaccurate? (if we use the trigger signal to frame a camera exposure, the result will be correct?)

  • Anders,

    I believe the team is still working this issue and should get back to you within a few days. Thanks for your patience.

    Regards,

    Philippe Dollo

  • Hello Anders,

    1. Please refer to the following example from E2E FAQ ( https://e2e.ti.com/support/dlp/f/94/t/778576?tisearch=e2e-sitesearch&keymatch=faq%3Atrue ) as a reference. It lists down all the supported values of the exposure times for different kinds of internal pattern sequences.

     

    Let say you want to display 1bit patterns via mono illumination for a exposure time of 2000us. This particular exposure time is supported via sequence of index 15. The minimum pre and post exposure time will be calculated as follows –

    Min pre-exposure dark time = 171*(2000/1800) + 1(to round up if needed)

    Min post-exposure dark time = 31*(2000/1800) + 1(to round up if needed)

    In this way, you can use the sequence database from the firmware release notes to calculate the pre and post dark time requirements and hence skip the use of “Read Validate Exposure Time” command.

    2. Can you go into a bit more detail about the comment: "You may not get the exact expected output for these signals. And there can be deviation in the pattern exposure time as well" 

    Unfortunately, we do not have the data related to this. It would be recommended that you look for the inaccuracies at your end by observing the trigger out signals and the light output, and see if it meets the requirements or not.

    What I can say is that the behavior of trigger-out signals will match the actual light output behavior i.e. It will have similar deviations like the deviations in the actual exposure time (if there are any).

    Regards,

    Mayank