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How to disable the micromirrors' refresh (which is about 10 kHz)?

user4724985
Prodigy 140 points
Other Parts Discussed in Thread: DLPC900, TPS65145

Hello,

I am playing with the DLP 6500 EVM and have a question about the micromirros' refresh.

I think the micromirrors continuously refresh themselves at around 10 kHz rate (which I can recognize with the high-pitch sound and with the light intensity), even when I project an image for 0.5 s in the 'pattern on the fly' mode. 

Is there a way to disable this refresh? I got some tips from my friend to manually turn off part of the signal going from the DLPC900 to the DMD by adding a MOSFET between TP14 and the ground of C70. When the MOSFET is closed, the strobe doesn't reach to the DMD. However, when I did the same thing, the mirrors don't freeze themselves. 

Thanks in advance,

Eunmi

  • 0
    TI__Mastermind 30600 points
    Eunmi,

    The mirrors do not have a continuous refresh of 10kHz. The high pitch noise you're hearing probably is coming from the TPS65145 power regulator on the board - be careful not to touch that, otherwise you'll damage the board and DMD.

    We don't recommend putting components on the EVM either - it is hard to say how that will impact performance.

    What issue are you trying to solve? Your light intensity isn't high enough? You're seeing flickering? If you can detail for me the symptoms of your issue perhaps we can get you a real solution.

    Thanks!
    Paul
  • 0 in reply to Paul Goedeke
    Prodigy 140 points

    Hi Paul,

    Thanks for your answer! 

    I see. I first heard from my friend who is using the same evaluation module (DLP 6500EVM) that it has a 105us 'blinking'. I also saw that by monitoring the light from the DMD using a photo diode (picture attached). It looks light the mirrors are slightly vibrating at around 10kHz which is not good for my application. 

     

    Thanks!!

    Eunmi

  • 0 in reply to user4724985
    TI__Mastermind 30600 points
    Hi Eunmi,

    I'm not sure what would be causing the behavior that you're seeing. The mirrors on this device can only be driven up to ~9000Hz by the controller, so those vibrations are too fast. It is interesting that your friend is seeing the same behavior.

    Can you describe to me what pattern you are displaying to the photo diode?

    Paul
  • 0 in reply to Paul Goedeke
    Prodigy 140 points

    Hi Paul,

    I think the period of the vibration ~ 110 us is close to 9000Hz. I guess that is the reason why we think the controller is causing this vibration.

    I was displaying just a circle on the DMD, with 'pattern on the fly' mode, exposure time of 0.5s, no turn-off time, in repeat. 

    Thanks a lot!

    Eunmi

  • 0 in reply to user4724985
    TI__Mastermind 30600 points
    Eunmi,

    Your pattern is binary? One single pattern repeated over and over again?

    Does anything change if you use Pre-stored Pattern Mode?

    -Paul
  • 0 in reply to Paul Goedeke
    TI__Mastermind 30600 points
    Can you please post your pattern? And a screen shot of your GUI?

    What is your illumination source?

    Paul
  • 0 in reply to Paul Goedeke
    Prodigy 140 points

    Hi Paul,

    I'm using a python code to communicate with the DMD. The image is binary, and is a circle at the center of the DMD with the radius of 100 pixels which is also generated in the code.

    The setting is exposure time of 0.5 s and no dark time. This single pattern was repeated.

    A 532nm laser from Thorlabs (https://www.thorlabs.com/thorproduct.cfm?partnumber=CPS532

  • 0 in reply to user4724985
    TI__Mastermind 30600 points
    Is your friend who is seeing the same issue using the same light source?

    Before I feel comfortable concluding that what you're seeing has to do with the mirrors, I'd like to eliminate any other sources of noise in your system. Could your laser have small fluctuations that are causing what you're seeing? Is there noise outside of the system - lights, etc that could have an impact?

    In ideal operation, the mirrors should not move once they have transitioned into their pattern. If the next pattern is the same as the previous one, the mirrors will move slightly as the DMD is reset, but that "stay" transition is on the order of nanoseconds and only occurs at the beginning of each pattern.

    Paul
  • 0 in reply to Paul Goedeke
    Prodigy 140 points

    Hi Paul,

    I don't think my friend uses the same light source. I also checked with a different light source and still observed the same behavior.

    The intensity signal from the DMD was about 1.2V on a photodiode and the ~ 10kHz intensity fluctuation was about 50 mV, which means about 4 % of the light intensity is fluctuating.  

    thanks,

    Eunmi

  • 0 in reply to user4724985
    TI__Mastermind 30600 points
    Eunmi,

    Does this issue occur if you use the GUI? I would like to try using the pattern you created and upload it into the Lightcrafter GUI and display the pattern with that.

    www.ti.com/dlpc900ref-sw-v300

    Thanks,
    Paul
  • 0 in reply to Paul Goedeke
    Prodigy 140 points

    Hi Paul,

    Yes, it occurred with the GUI too. Please let me know your result. 

    Thanks!

    Eunmi

  • 0 in reply to user4724985
    TI__Mastermind 30600 points

    Eunmi,

    Please assist me by posting a screenshot of your GUI with the pattern you are displaying as well as the image you are uploading to the device.

    Thank you,

    Paul

  • 0 in reply to Paul Goedeke
    Prodigy 140 points

    Hi Paul,

    Below are the screenshot and the figure for the DMD.

    Also, my friend wrote a paper regarding the 10kHz refresh. ( https://arxiv.org/abs/1611.03397 ).

    thanks,

    Eunmi

  • 0 in reply to user4724985
    TI__Mastermind 30600 points
    Eunmi,

    Thank you. Please give a little time to get a test set-up so that we can see if we're able to replicate your results and the ones from the paper. I'll post back when I know more.

    Paul
  • 0 in reply to Paul Goedeke
    TI__Mastermind 30600 points
    Eunmi,

    I am able to verify the behavior that you are seeing. The mirrors are receiving a continuous refresh signal every 105us. You can verify this on your own EVM by putting an oscilloscope on TP0 on the DMD board.

    Thank you for bringing this to our attention. I will be investigating this further. As I learn more and if there is any potential work around/fix, I will let you know.

    Best,
    -Paul
  • 0 in reply to Paul Goedeke
    Prodigy 140 points

    Hi Paul,

    Thank you very much for the update! Please let me know when there is a progress.

    Is TP0 where I can reach the mirror clocking pulses? What is the signal on TP14 that my friend is using in the paper that I sent you before? I also saw 105 us pulses on that pin too. 

    Thanks,

    Eunmi

  • 0 in reply to user4724985
    TI__Mastermind 30600 points

    TP0 through TP14 are all mirror reset signals. This 105us behavior is built into the FW, there is no getting around this sadly.

    Thanks,
    Paul

  • 0 in reply to Paul Goedeke
    Prodigy 70 points

    Hi Paul

    I'm in the same situation; I would really like to disable the 10 kHz mirror reset signals.

    I tried to set the exposure time to 105µs and the Dark Time to 0 in the GUI, as shown below.

     

    I then hooked the DMD up to a frequency generator that produced a 1kHz signal, which would correspond to a period of 1ms. If I understand your previous reply correctly, I would expect 1ms exposure on the DMD and no 10kHz mirror reset signals.
    Looking at a by the DMD reflected laser beam, this is unfortunately not the case. I don't see any 10kHz signal but instead of seeing a 1ms exposure, I see a 105µs exposure every 1ms. This is shown below:


    For my experiment this outcome is unfortunately even worse than having the 10kHz mirror reset clock signal.

    Have I understood your previous solution correctly and/or do you have a workaround so that can show a truly static image on the DMD for more than 105µs?


    Thanks,

    Nicolai

  • 0 in reply to NicolaI
    TI__Mastermind 30600 points
    Hi Nicolai,

    Thanks for your post - just to make sure, the laser is being shown on the DMD continuously or also with a 1ms period?

    I'll investigate this further and let you know what I learn - I'll try to post back in the next day or so.

    -Paul
  • 0 in reply to Paul Goedeke
    Prodigy 70 points
    Thanks for your quick reply. The laser is being shown on the DMD continuously. I look forward to hear the results of your investigation.

    Nicolai
  • 0 in reply to NicolaI
    TI__Mastermind 30600 points
    Hi Nicolai,

    I confirmed that what you are seeing is correct. Unfortunately, after consulting further with the experts, the fix that I suggested above is incorrect and will not work.

    It is built into the FW of the DLPC900 to have the 105us reset pulses and there is no other work around short of modifying the board or the FW. Please accept my apologies on this.

    -Paul
  • 0 in reply to Paul Goedeke
    Prodigy 90 points

    I understand the result of these posts that there is no good way to disable the reset pulses.  Please add our group to the list of those who would like to do so. 

    Thanks.

    William A (Bill) Monette

    Indiana University, School of Optometry

    Bloomington, Indiana

  • 0 in reply to William Monette
    TI__Mastermind 30600 points
    Bill,

    Thank you for your input - it is valuable for us to hear from other folks. At this point, I am unable to offer any update other than to say that your request has been noted.

    Perhaps you'd be able to describe your application in a little detail? So far it seems that folks who need the refresh disabled are those who have laser based applications. General projection optics aren't typically sensitive to this DMD behavior.

    Happy New Year,
    Paul
  • 0 in reply to Paul Goedeke
    Prodigy 140 points

    Hello,

    I succeeded to disable the refresh by following this arXiv paper (arxiv:1611.03397).

    One of the important keys is to make the wires between the board and the MOSFET as short as possible since the signals are very fast.

    This method is a hardware solution and one should take full responsibility for modifying the board by himself/herself.

    Happy new year!