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DLPC3478: LED_SEL timing and LED turn-on time

Part Number: DLPC3478
Other Parts Discussed in Thread: DLPA2005, , DLPC3470

I am using DLPC3478 together with DLPA2005. I am currently using internal pattern mode, though I will likely also be using the pixel-parallel streaming mode.

We are not using the DLPA2005 to drive the R/G/B LEDs directly, but have our own LED drivers implemented. We were planning on using the LED_SEL signals from DLPC3478 to turn on / off LEDs, though we have the flexibility in our design to use other signals.

From the DLPC3478 documentation, it seems to me that the LED_SEL_0 and LED_SEL_1 signals are active during the actual exposure of the image. However, the LEDs will have a certain turn-on-time, such that I would have liked to have the LED_SEL signals go active some time before the image exposure starts.

For TRIG_OUT_2 it is possible to configure a negative delay relative to the exposure time, such that the signal goes active prior to the image exposure.

Is it possible to do something similar to adjust the timing of the LED_SEL_0 and LED_SEL_1 signals? If not, how would you ensure that the LEDs are enabled early enough to stabilize prior to image exposure?

  • Hello,

    Welcome to DLP forum and thank you for your interest in DLP technology. Please find my response to your questions :

    Your question: From the DLPC3478 documentation, it seems to me that the LED_SEL_0 and LED_SEL_1 signals are active during the actual exposure of the image. However, the LEDs will have a certain turn-on-time, such that I would have liked to have the LED_SEL signals go active some time before the image exposure starts.

    The firmware deos not support control of LED select signal by application/user. This issue is critical only for 8 bit patterns. It is incorporated in the firmware and DMD mirror switching for 8-bit patterns. For a 1-bit pattern, it has uniform impact across all patterns , hence no adjustment is required. 

    Your question: For TRIG_OUT_2 it is possible to configure a negative delay relative to the exposure time, such that the signal goes active prior to the image exposure.

    Yes, Please refer to page 66 section 3.4.3.1 of DLPC3470 Software Programmer guide

    http://www.ti.com/lit/ug/dlpu075a/dlpu075a.pdf

    Your question: Is it possible to do something similar to adjust the timing of the LED_SEL_0 and LED_SEL_1 signals? If not, how would you ensure that the LEDs are enabled early enough to stabilize prior to image exposure?

    It is incorporated in the firmware and DMD mirror switching. It is applicable only for 8-bit patterns.

    regards,

    Vivek

  • Hi Vivek,

    Thank you for your answer, it was most helpful.

    For our application, the actual timing of the displayed patterns is important, both for 1-bit, 7-bit and 8-bit patterns. (From documentation, I understand that 8-bit patterns are actually displayed as 7-bit for the shortest exposure times.)

    So can you provide the actual timing of the pattern display within the documented exposure time? 

    Is the timing always a fixed offset within the exposure time, or is there a variable delay from the documented exposure start until the actual pattern projection?

    Is the actual exposure period (where mirrors may be in the on-position) available on an output pin somehow?

    You did not explicitly say that it is not possible to adjust the timing of the LED_SEL pins  -  but I assume it might not be?

  • Hello,

    The actual timing of the display pattern is exactly same as programmed exposure time. The pattern trigger will be active during exposure period with a flexibility to program in delay. 

    There is minimum dark time requirement of 200 uS (170 pre and 30 post). This minimum requirement to address any delay in switching of illumination source or circuits.

    We do support zero dark time for 1 -bit pattern. In this mode the LEDs are not switched off.

       You wrote - (From documentation, I understand that 8-bit patterns are actually displayed as 7-bit for the shortest exposure times.)

     This is special case to support a multi-bit pattern with exposure time lower than minimum required for a 8-bit pattern. 

    Regards,

    Vivek