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LM98725: Registers and gpio functionality

Part Number: LM98725

Hi,

We need to interface the LM98725 with a Toshiba tcd2918bfg linear CCD color (RGB) sensor.  Our circuit is as attached (CCD_board.pdf).  I'm also attaching a file (tcd2918bfg_timing.pdf) that shows the timing diagram for "bit clamp mode" and the timing diagram for the "line clamp mode".  The base conditions are: f(phi)=2.5 MHz, f(RS)=5.0 MHz, tINT (integration time)=11 ms

Q1.  I'm having trouble setting up the registers to get the correct timing for them.  Can you provide the minimal register setup/values for these two modes (bit clamp and line clamp) based on our circuit and the timing diagram?

Q2.  The LM98725 documentation lists SH1 as a "low speed sensor timing output".  Toshiba uses "SH" as a label for their shift gate.  I assume I can use SH1 for the CCD sensor's shift gate, is that okay? 

Q3.  There is an external calibration-request pin on the LM98725.  Is it necessary to initiate the calibration through this pin or can we initiate it through the registers/spi as well?

Q4.  Can the unused clock pins be used as GPIO (output-only is okay)?

Thanks in advance.


CCD_board.pdf

tcd2918bfg_timing.pdf

  • Hi Robert,

    Thank you for your question. Let me contact the expert for this part. You can expect a response by 2/4/20.

    Best,

    Grant

  • Hi Grant,

    Okay.  Thank you for looking into this.  I will wait to hear back from your expert.

    Best regards,
    Robert

  • Hi Robert,

    Q1. Attached is LM98725 programing manual, you'll find needed firmware sequence.  System delays has to be aligned and black and white should be calibrated

    5621.5618.LM9872X_Prog_Man_1.5.pdf

    Q2. SH is same signal should be connected similar to LM98725 example.

    Q3. Calibration can be hardware initiated - through pin or by firmware- calibration bit

    Q4. All CMOS outputs can be used as GPIO

    Regards,

    Costin

  • Hi Robert,

    Q1.  Please check programing manual page64 for high level programing sequence. Based on the timing diagram Toshiba CCD sensor can be operate in S/H or CDS mode  All sensor control signal timing  and system delays can be programmed, best is to scope check them for right system alignment.  That's no need to enable SSCG function until the system is fully operational.

    Q2. SH1 needs to be programmed to meet the CCD sensor shift gate timing

    Q3. Have no other clarifications

    Q4. I assume you're referring to the unused SH and PHI outputs, if that's the case can be used as GPIO

    Regards,

    Costin

      

  • Hi,

    Thank you for your additional comments.  Sorry, I should have mentioned that I had already read the same manuals you provided.    


    In response to your replies, here are my comments:
    > Q1.  I understand the basic programming sequence.  I guess I'm not clear how the delays and periods tie together.  I'll have go over it some more it seems.  Ultimately, I was hoping to get an example that correlated to the timing diagram I provided (or even one of the timing diagrams provided in the LM98725 datasheet).  Understandably there would need to be some adjustments that needed to be made but such an example would be illustrative and helpful.  Adding something like that to the datasheet and/or programming manual might help other customers.  I had been wondering if I could wait to implement the SSCG function.  You clarified that, thanks.

    >Q2.  Understood.  I will set the SH1 to match the shift register timing.

    >Q3.  Understood.  Calibration can be set externally or by setting the register.  Thanks for that clarification.

    >Q4.  Yes, I am referring to unused SH and PHI pins with respect to CMOS outputs.  I assume as long as their respective timing registers aren't set, their pin outputs will correspond to the pins' registers?  (Meaning if I set the pin value of the register as "1", the value will pin output will be guaranteed to stay high until the register is set to "0"?)

    Q5 (new). Our sensor board and main control board will be separated over a relatively long distance, far enough that we will need to transmit the SPI signals via LVDS.  Because of the distance and distance-related issues such as noise, etc, we want to minimize the number of lines (within reason) between the main control board and the sensor board.  If we use a CMOS clock for the timing (not currently shown in the schematic I attached earlier), the SPI and the LVDS sensor data lines are sufficient for basic operation, is that correct?  Any operation such as sample and hold requests (SH_R) can be done through internal registers set through SPI? 

    Note: Someone was asking on the e2e forum what CCD sensor (U5) was being used in the example application schematic.  One of the answers from TI was that it's a "generic" CCD.  However, it seems that the IC is a (now-obsolete) NEC μPD8873.  Since the question was closed, I couldn't add a comment so I mention it here in case someone else is looking for the same information.  I think the same basic sample application schematic is used in some of your other AFE chip datasheets.

  • Hi Robert,

    First of all sorry for late reply.

    Q1. TI doesn't provide firmware example, a generic example may cause more complications since the firmware needs to match customer application.

    Q4. The simple answer is yes.  SH  has two periods:

    a)SH interval( between lines) ; up to 31 states, each state length and polarity programable

    b) Line interval : "0" or 1" , or  programable "0" and 1"duration

    Basically you can use the non sensor SH as timing markers

    Q5. SPI and LVDS is enough if LM98725 is set to master mode. The length of the cable will affect SPI signal integrity due prop delay and capacitive loading, SPI signals should meet datasheet requirements at LM98725 pins.

    Regards,

    Costin

  • Ok, thanks.

    Best regards.